Discover the world of microcontrollers through Rust!

This book is an introductory course on microcontroller-based embedded systems that uses Rust as the teaching language rather than the usual C/C++.

The following topics will be covered (eventually, I hope):

• How to write, build, flash and debug an "embedded" (Rust) program.

• Functionality ("peripherals") commonly found in microcontrollers: Digital input and output, Pulse Width Modulation (PWM), Analog to Digital Converters (ADC), common communication protocols like Serial, I2C and SPI, etc.

• Multitasking concepts: cooperative vs preemptive multitasking, interrupts, schedulers, etc.

• Control systems concepts: sensors, calibration, digital filters, actuators, open loop control, closed loop control, etc.

• Beginner friendly. No previous experience with microcontrollers or embedded systems is required.

• Hands on. Plenty of exercises to put the theory into practice. You will be doing most of the work here.

• Tool centered. We'll make plenty use of tooling to ease development. "Real" debugging, with GDB, and logging will be introduced early on. Using LEDs as a debugging mechanism has no place here.

What's out of scope for this book:

• Teaching Rust. There's plenty of material on that topic already. We'll focus on microcontrollers and embedded systems.

• Being a comprehensive text about electric circuit theory or electronics. We'll just cover the minimum required to understand how some devices work.

• Covering details such as linker scripts and the boot process. For example, we'll use existing tools to help get your code onto your board, but not go into detail about how those tools work.

Also I don't intend to port this material to other development boards; this book will make exclusive use of the STM32F3DISCOVERY development board.

The source of this book is in this repository. If you encounter any typo or problem with the code report it on the issue tracker.

This Discovery book is just one of several embedded Rust resources provided by the Embedded Working Group. The full selection can be found at The Embedded Rust Bookshelf. This includes the list of Frequently Asked Questions.

Many thanks to integer 32 for sponsoring me to work on this book! Please give them lots of work (they do Rust consulting!) so they'll have no choice but to hire more Rustaceans <3.

A microcontroller is a system on a chip. Whereas your computer is made up of several discrete components: a processor, RAM sticks, a hard drive, an ethernet port, etc.; a microcontroller has all those components built into a single "chip" or package. This makes it possible to build systems with minimal part count.

Lots of things! Microcontrollers are the central part of systems known as embedded systems. These systems are everywhere but you don't usually notice them. These systems control the brakes of your car, wash your clothes, print your documents, keep you warm, keep you cool, optimize the fuel consumption of your car, etc.

The main trait of these systems is that they operate without user intervention even if they expose a user interface like a washing machine does; most of their operation is done on their own.

The other common trait of these systems is that they control a process. And for that these systems usually have one or more sensors and one or more actuators. For example, an HVAC system has several sensors, thermometers and humidity sensors spread across some area, and several actuators as well, heating elements and fans connected to ducts.

All these application I've mentioned, you can probably implement with a Raspberry Pi, a computer that runs Linux. Why should I bother with a microcontroller that operates without an OS? Sounds like it would be harder to develop a program.

The main reason is cost. A microcontroller is much cheaper than a general purpose computer. Not only the microcontroller is cheaper; it also requires many fewer external electrical components to operate. This makes Printed Circuit Boards (PCB) smaller and cheaper to design and manufacture.

The other big reason is power consumption. A microcontroller consumes orders of magnitude less power than a full blown processor. If your application will run on batteries that makes a huge difference.

And last but not least: (hard) real time constraints. Some processes require their controllers to respond to some events within some time interval (e.g. a quadcopter/drone hit by a wind gust). If this deadline is not met, the process could end in catastrophic failure (e.g. the drone crashes to the ground). A general purpose computer running a general purpose OS has many services running in the background. This makes it hard to guarantee execution of a program within tight time constraints.

Where heavy computations are involved. To keep their power consumption low, microcontrollers have very limited computational resources available to them. For example, some microcontrollers don't even have hardware support for floating point operations. On those devices, performing a simple addition of single precision numbers can take hundreds of CPU cycles.

Hopefully, I don't need to convince you here as you are probably familiar with the language differences between Rust and C. One point I do want to bring up is package management. C lacks an official, widely accepted package management solution whereas Rust has Cargo. This makes development much easier. And, IMO, easy package management encourages code reuse because libraries can be easily integrated into an application which is also a good thing as libraries get more "battle testing".

Or why should I prefer C over Rust?

The C ecosystem is way more mature. Off the shelf solution for several problems already exist. If you need to control a time sensitive process, you can grab one of the existing commercial Real Time Operating Systems (RTOS) out there and solve your problem. There are no commercial, production-grade RTOSes in Rust yet so you would have to either create one yourself or try one of the ones that are in development.

The primary knowledge requirement to read this book is to know some Rust. It's hard for me to quantify some but at least I can tell you that you don't need to fully grok generics but you do need to know how to use closures. You also need to be familiar with the idioms of the 2018 edition, in particular with the fact that extern crate is not necessary in the 2018 edition.

Due to the nature of embedded programming, it will also be extremely helpful to understand how binary and hexadecimal representations of values work, as well as the use of some bitwise operators. For example, it would be useful to understand how the following program produces its output.

fn main() {
    let a = 0x4000_0000 + 0xa2;

    // Use of the bit shift "<<" operation.
    let b = 1 << 5;

    // {:X} will format values as hexadecimal
    println!("{:X}: {:X}", a, b);
}

Also, to follow this material you'll need the following hardware:

(Some components are optional but recommended)

• A STM32F3DISCOVERY board.

(You can purchase this board from "big" electronics suppliers or from e-commerce sites)

• OPTIONAL. A 3.3V USB <-> Serial module. To elaborate: if you have one of the latest revisions of the discovery board (which is usually the case given the first revision was released years ago) then you do not need this module because the board includes this functionality on-board. If you have an older revision of the board then you'll need this module for chapters 10 and 11. For completeness, we'll include instructions for using a Serial module. The book will use this particular model but you can use any other model as long as it operates at 3.3V. The CH340G module, which you can buy from e-commerce sites works too and it's probably cheaper for you to get.

• OPTIONAL. A HC-05 Bluetooth module (with headers!). A HC-06 would work too.

(As with other Chinese parts, you pretty much can only find these on e-commerce sites. (US) Electronics suppliers don't usually stock these for some reason)

• Two mini-B USB cables. One is required to make the STM32F3DISCOVERY board work. The other is only required if you have the Serial <-> USB module. Make sure that the cables both support data transfer as some cables only support charging devices.

NOTE These are not the USB cables that ship with pretty much every Android phone; those are micro USB cables. Make sure you have the right thing!

• MOSTLY OPTIONAL. 5 female to female, 4 male to female and 1 Male to Male jumper (AKA Dupont) wires. You'll very likely need one female to female to get ITM working. The other wires are only needed if you'll be using the USB <-> Serial and Bluetooth modules.

(You can get these from electronics suppliers or from e-commerce sites)

FAQ: Wait, why do I need this specific hardware?

It makes my life and yours much easier.

The material is much, much more approachable if we don't have to worry about hardware differences. Trust me on this one.

FAQ: Can I follow this material with a different development board?

Maybe? It depends mainly on two things: your previous experience with microcontrollers and/or whether there already exists a high level crate, like the f3, for your development board somewhere.

With a different development board, this text would lose most if not all its beginner friendliness and "easy to follow"-ness, IMO.

If you have a different development board and you don't consider yourself a total beginner, you are better off starting with the quickstart project template.

Dealing with microcontrollers involves several tools as we'll be dealing with an architecture different than your computer's and we'll have to run and debug programs on a "remote" device.

Tooling is not everything though. Without documentation it is pretty much impossible to work with microcontrollers.

We'll be referring to all these documents throughout this book:

HEADS UP All these links point to PDF files and some of them are hundreds of pages long and several MBs in size.

• STM32F3DISCOVERY User Manual
• STM32F303VC Datasheet
• STM32F303VC Reference Manual
• LSM303DLHC
• L3GD20

We'll use all the tools listed below. Where a minimum version is not specified, any recent version should work but we have listed the version we have tested.

• Rust 1.31 or a newer toolchain.

• itmdump v0.3.1 (cargo install itm)

• OpenOCD >=0.8. Tested versions: v0.9.0 and v0.10.0

• arm-none-eabi-gdb. Version 7.12 or newer highly recommended. Tested versions: 7.10, 7.11, 7.12 and 8.1

• cargo-binutils. Version 0.1.4 or newer.

• minicom on Linux and macOS. Tested version: 2.7. Readers report that picocom also works but we'll use minicom in this text.

• PuTTY on Windows.

If your computer has Bluetooth functionality and you have the Bluetooth module, you can additionally install these tools to play with the Bluetooth module. All these are optional:

• Linux, only if you don't have a Bluetooth manager application like Blueman.
  • bluez
  • hcitool
  • rfcomm
  • rfkill

macOS / OSX / Windows users only need the default bluetooth manager that ships with their OS.

Next, follow OS-agnostic installation instructions for a few of the tools:

Install rustup by following the instructions at https://rustup.rs.

If you already have rustup installed double check that you are on the stable channel and your stable toolchain is up to date. rustc -V should return a date newer than the one shown below:

$ rustc -V
rustc 1.31.0 (abe02cefd 2018-12-04)

$ cargo install itm --vers 0.3.1

$ itmdump -V
itmdump 0.3.1

$ rustup component add llvm-tools-preview

$ cargo install cargo-binutils --vers 0.1.4

$ cargo size -- -version
LLVM (http://llvm.org/):
  LLVM version 8.0.0svn
  Optimized build.
  Default target: x86_64-unknown-linux-gnu
  Host CPU: skylake

Now follow the instructions specific to the OS you are using:

• Linux
• Windows
• macOS

Here are the installation commands for a few Linux distributions.

NOTE gdb-multiarch is the GDB command you'll use to debug your ARM Cortex-M programs

$ sudo apt-get install \
  gdb-multiarch \
  minicom \
  openocd

NOTE arm-none-eabi-gdb is the GDB command you'll use to debug your ARM Cortex-M programs

$ sudo apt-get install \
  gdb-arm-none-eabi \
  minicom \
  openocd

NOTE arm-none-eabi-gdb is the GDB command you'll use to debug your ARM Cortex-M programs

$ sudo dnf install \
  arm-none-eabi-gdb \
  minicom \
  openocd

NOTE arm-none-eabi-gdb is the GDB command you'll use to debug your ARM Cortex-M programs

$ sudo pacman -S \
  arm-none-eabi-gdb \
  minicom \
  openocd

NOTE arm-none-eabi-gdb is the GDB command you'll use to debug your ARM Cortex-M programs

For distros that don't have packages for ARM's pre-built toolchain, download the "Linux 64-bit" file and put its bin directory on your path. Here's one way to do it:

$ mkdir -p ~/local && cd ~/local
$ tar xjf /path/to/downloaded/file/gcc-arm-none-eabi-7-2017-q4-major-linux.tar.bz2.tbz

Then, use your editor of choice to append to your PATH in the appropriate shell init file (e.g. ~/.zshrc or ~/.bashrc):

PATH=$PATH:$HOME/local/gcc-arm-none-eabi-7-2017-q4-major/bin

$ sudo apt-get install \
  bluez \
  rfkill

$ sudo dnf install \
  bluez \
  rfkill

$ sudo pacman -S \
  bluez \
  bluez-utils \
  rfkill

These rules let you use USB devices like the F3 and the Serial module without root privilege, i.e. sudo.

Create these two files in /etc/udev/rules.d with the contents shown below.

$ cat /etc/udev/rules.d/99-ftdi.rules

# FT232 - USB <-> Serial Converter
ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", MODE:="0666"

If you have a different USB <-> Serial converter, get its vendor and product ids from lsusb output.

$ cat /etc/udev/rules.d/99-openocd.rules

# STM32F3DISCOVERY rev A/B - ST-LINK/V2
ATTRS{idVendor}=="0483", ATTRS{idProduct}=="3748", MODE:="0666"

# STM32F3DISCOVERY rev C+ - ST-LINK/V2-1
ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", MODE:="0666"

Then reload the udev rules with:

$ sudo udevadm control --reload-rules

If you had any board plugged to your computer, unplug them and then plug them in again.

Now, go to the next section.

ARM provides .exe installers for Windows. Grab one from here, and follow the instructions. Just before the installation process finishes tick/select the "Add path to environment variable" option. Then verify that the tools are in your %PATH%:

$ arm-none-eabi-gcc -v
(..)
gcc version 5.4.1 20160919 (release) (..)

There's no official binary release of OpenOCD for Windows but there are unofficial releases available here. Grab the 0.10.x zipfile and extract it somewhere in your drive (I recommend C:\OpenOCD but with the drive letter that makes sense to you) then update your %PATH% environment variable to include the following path: C:\OpenOCD\bin (or the path that you used before).

Verify that OpenOCD is in yout %PATH% with:

$ openocd -v
Open On-Chip Debugger 0.10.0
(..)

Download the latest putty.exe from this site and place it somewhere in your %PATH%.

You'll also need to install this USB driver or OpenOCD won't work. Follow the installer instructions and make sure you install the right (32-bit or 64-bit) version of the driver.

That's all! Go to the next section.

All the tools can be install using Homebrew:

$ # Arm GCC toolchain
$ brew tap ArmMbed/homebrew-formulae
$ brew install arm-none-eabi-gcc

$ # Minicom and OpenOCD
$ brew install minicom openocd

That's all! Go to the next section.

Let's verify that all the tools were installed correctly.

Connect the F3 to your computer using an USB cable. Be sure to connect the cable to the "USB ST-LINK" port, the USB port in the center of the edge of the board.

The F3 should now appear as a USB device (file) in /dev/bus/usb. Let's find out how it got enumerated:

$ lsusb | grep -i stm
Bus 003 Device 004: ID 0483:374b STMicroelectronics ST-LINK/V2.1
$ # ^^^        ^^^

In my case, the F3 got connected to the bus #3 and got enumerated as the device #4. This means the file /dev/bus/usb/003/004 is the F3. Let's check its permissions:

$ ls -l /dev/bus/usb/003/004
crw-rw-rw- 1 root root 189, 20 Sep 13 00:00 /dev/bus/usb/003/004

The permissions should be crw-rw-rw-. If it's not ... then check your udev rules and try re-loading them with:

$ sudo udevadm control --reload-rules

Now let's repeat the procedure for the Serial module.

Unplug the F3 and plug the Serial module. Now, figure out what's its associated file:

$ lsusb | grep -i ft232
Bus 003 Device 005: ID 0403:6001 Future Technology Devices International, Ltd FT232 Serial (UART) IC

In my case, it's the /dev/bus/usb/003/005. Now, check its permissions:

$ ls -l /dev/bus/usb/003/005
crw-rw-rw- 1 root root 189, 21 Sep 13 00:00 /dev/bus/usb/003/005

As before, the permissions should be crw-rw-rw-.

First, connect the F3 to your computer using an USB cable. Connect the cable to the USB port in the center of edge of the board, the one that's labeled "USB ST-LINK".

Two red LEDs should turn on right after connecting the USB cable to the board.

Next, run this command:

$ # *nix
$ openocd -f interface/stlink-v2-1.cfg -f target/stm32f3x.cfg

$ # Windows
$ # NOTE cygwin users have reported problems with the -s flag. If you run into
$ # that you can call openocd from the `C:\OpenOCD\share\scripts` directory
$ openocd -s C:\OpenOCD\share\scripts -f interface/stlink-v2-1.cfg -f target/stm32f3x.cfg

NOTE Windows users: C:\OpenOCD is the directory where you installed OpenOCD to.

IMPORTANT There is more than one hardware revision of the STM32F3DISCOVERY board. For older revisions, you'll need to change the "interface" argument to -f interface/stlink-v2.cfg (note: no -1 at the end). Alternatively, older revisions can use -f board/stm32f3discovery.cfg instead of -f interface/stlink-v2-1.cfg -f target/stm32f3x.cfg.

You should see output like this:

Open On-Chip Debugger 0.10.0
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
none separate
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : clock speed 950 kHz
Info : STLINK v2 JTAG v27 API v2 SWIM v15 VID 0x0483 PID 0x374B
Info : using stlink api v2
Info : Target voltage: 2.915608
Info : stm32f3x.cpu: hardware has 6 breakpoints, 4 watchpoints

(If you don't ... then check the general troubleshooting instructions.)

openocd will block the terminal. That's fine.

Also, one of the red LEDs, the one closest to the USB port, should start oscillating between red light and green light.

That's it! It works. You can now close/kill openocd.

Let's get familiar with the hardware we'll be working with.

We'll refer to this board as "F3" throughout this book. Here are some of the many components on the board:

• A microcontroller.
• A number of LEDs, including the eight aligned in a "compass" formation.
• Two buttons.
• Two USB ports.
• An accelerometer.
• A magnetometer.
• A gyroscope.

Of these components, the most important is the microcontroller (sometimes shortened to "MCU" for "microcontroller unit"), which is the large black square sitting in the center of your board. The MCU is what runs your code. You might sometimes read about "programming a board", when in reality what we are doing is programming the MCU that is installed on the board.

Since the MCU is so important, let's take a closer look at the one sitting on our board.

Our MCU is surrounded by 100 tiny metal pins. These pins are connected to traces, the little "roads" that act as the wires connecting components together on the board. The MCU can dynamically alter the electrical properties of the pins. This works similar to a light switch altering how electrical current flows through a circuit. By enabling or disabling electrical current to flow through a specific pin, an LED attached to that pin (via the traces) can be turned on and off.

Each manufacturer uses a different part numbering scheme, but many will allow you to determine information about a component simply by looking at the part number. Looking at our MCU's part number (STM32F303VCT6), the ST at the front hints to us that this is a part manufactured by ST Microelectronics. Searching through ST's marketing materials we can also learn the following:

• The M32 represents that this is an Arm®-based 32-bit microcontroller.
• The F3 represents that the MCU is from ST's "STM32F3" series. This is a series of MCUs based on the Cortex®-M4 processor design.
• The remainder of the part number goes into more details about things like extra features and RAM size, which at this point we're less concerned about.

Arm? Cortex-M4?

If our chip is manufactured by ST, then who is Arm? And if our chip is the STM32F3, what is the Cortex-M4?

You might be surprised to hear that while "Arm-based" chips are quite popular, the company behind the "Arm" trademark (Arm Holdings) doesn't actually manufacture chips for purchase. Instead, their primary business model is to just design parts of chips. They will then license those designs to manufacturers, who will in turn implement the designs (perhaps with some of their own tweaks) in the form of physical hardware that can then be sold. Arm's strategy here is different from companies like Intel, which both designs and manufactures their chips.

Arm licenses a bunch of different designs. Their "Cortex-M" family of designs are mainly used as the core in microcontrollers. For example, the Cortex-M0 is designed for low cost and low power usage. The Cortex-M7 is higher cost, but with more features and performance. The core of our STM32F3 is based on the Cortex-M4, which is in the middle: more features and performance than the Cortex-M0, but less expensive than the Cortex-M7.

Luckily, you don't need to know too much about different types of processors or Cortex designs for the sake of this book. However, you are hopefully now a bit more knowledgeable about the terminology of your device. While you are working specifically with an STM32F3, you might find yourself reading documentation and using tools for Cortex-M-based chips, as the STM32F3 is based on a Cortex-M design.

If you have an older revision of the discovery board, you can use this module to exchange data between the microcontroller in the F3 and your computer. This module will be connected to your computer using an USB cable. I won't say more at this point.

If you have a newer release of the board then you don't need this module. The ST-LINK will double as a USB<->serial converter connected to the microcontroller USART1 at pins PC4 and PC5.

This module has the exact same purpose as the serial module but it sends the data over Bluetooth instead of over USB.

Alright, let's start by building the following application:

I'm going to give you a high level API to implement this app but don't worry we'll do low level stuff later on. The main goal of this chapter is to get familiar with the flashing and debugging process.

Throughout this text we'll be using the starter code that's in the discovery repository. Make sure you always have the latest version of the master branch because this website tracks that branch.

The starter code is in the src directory of that repository. Inside that directory there are more directories named after each chapter of this book. Most of those directories are starter Cargo projects.

Now, jump into the src/05-led-roulette directory. Check the src/main.rs file:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux5::entry;

#[entry]
fn main() -> ! {
    let _y;
    let x = 42;
    _y = x;

    // infinite loop; just so we don't leave this stack frame
    loop {}
}

Microcontroller programs are different from standard programs in two aspects: #![no_std] and #![no_main].

The no_std attribute says that this program won't use the std crate, which assumes an underlying OS; the program will instead use the core crate, a subset of std that can run on bare metal systems (i.e., systems without OS abstractions like files and sockets).

The no_main attribute says that this program won't use the standard main interface, which is tailored for command line applications that receive arguments. Instead of the standard main we'll use the entry attribute from the cortex-m-rt crate to define a custom entry point. In this program we have named the entry point "main", but any other name could have been used. The entry point function must have signature fn() -> !; this type indicates that the function can't return -- this means that the program never terminates.

If you are a careful observer, you'll also notice there is a .cargo directory in the Cargo project as well. This directory contains a Cargo configuration file (.cargo/config) that tweaks the linking process to tailor the memory layout of the program to the requirements of the target device. This modified linking process is a requirement of the cortex-m-rt crate.

Alright, let's start by building this program.

The first step is to build our "binary" crate. Because the microcontroller has a different architecture than your computer we'll have to cross compile. Cross compiling in Rust land is as simple as passing an extra --target flag to rustcor Cargo. The complicated part is figuring out the argument of that flag: the name of the target.

The microcontroller in the F3 has a Cortex-M4F processor in it. rustc knows how to cross compile to the Cortex-M architecture and provides 4 different targets that cover the different processor families within that architecture:

• thumbv6m-none-eabi, for the Cortex-M0 and Cortex-M1 processors
• thumbv7m-none-eabi, for the Cortex-M3 processor
• thumbv7em-none-eabi, for the Cortex-M4 and Cortex-M7 processors
• thumbv7em-none-eabihf, for the Cortex-M4F and Cortex-M7F processors

For the F3, we'll use the thumbv7em-none-eabihf target. Before cross compiling you have to download pre-compiled version of the standard library (a reduced version of it actually) for your target. That's done using rustup:

$ rustup target add thumbv7em-none-eabihf

You only need to do the above step once; rustup will re-install a new standard library (rust-std component) whenever you update your toolchain.

With the rust-std component in place you can now cross compile the program using Cargo:

$ # make sure you are in the `src/05-led-roulette` directory

$ cargo build --target thumbv7em-none-eabihf
   Compiling semver-parser v0.7.0
   Compiling aligned v0.1.1
   Compiling libc v0.2.35
   Compiling bare-metal v0.1.1
   Compiling cast v0.2.2
   Compiling cortex-m v0.4.3
   (..)
   Compiling stm32f30x v0.6.0
   Compiling stm32f30x-hal v0.1.2
   Compiling aux5 v0.1.0 (file://$PWD/aux)
   Compiling led-roulette v0.1.0 (file://$PWD)
    Finished dev [unoptimized + debuginfo] target(s) in 35.84 secs

NOTE Be sure to compile this crate without optimizations. The provided Cargo.toml file and build command above will ensure optimizations are off.

OK, now we have produced an executable. This executable won't blink any leds, it's just a simplified version that we will build upon later in the chapter. As a sanity check, let's verify that the produced executable is actually an ARM binary:

$ # equivalent to `readelf -h target/thumbv7em-none-eabihf/debug/led-roulette`
$ cargo readobj --target thumbv7em-none-eabihf --bin led-roulette -- -file-headers
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0x0
  Type:                              EXEC (Executable file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0x8000197
  Start of program headers:          52 (bytes into file)
  Start of section headers:          740788 (bytes into file)
  Flags:                             0x5000400
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           40 (bytes)
  Number of section headers:         20
  Section header string table index: 18

Next, we'll flash the program into our microcontroller.

Flashing is the process of moving our program into the microcontroller's (persistent) memory. Once flashed, the microcontroller will execute the flashed program every time it is powered on.

In this case, our led-roulette program will be the only program in the microcontroller memory. By this I mean that there's nothing else running on the microcontroller: no OS, no "daemon", nothing. led-roulette has full control over the device.

Onto the actual flashing. First thing we need is to do is launch OpenOCD. We did that in the previous section but this time we'll run the command inside a temporary directory (/tmp on *nix; %TEMP% on Windows).

Make sure the F3 is connected to your computer and run the following commands on a new terminal.

$ # *nix
$ cd /tmp

$ # Windows
$ cd %TEMP%

$ # Windows: remember that you need an extra `-s %PATH_TO_OPENOCD%\share\scripts`
$ openocd \
  -f interface/stlink-v2-1.cfg \
  -f target/stm32f3x.cfg

NOTE Older revisions of the board need to pass slightly different arguments to openocd. Review this section for the details.

The program will block; leave that terminal open.

Now it's a good time to explain what this command is actually doing.

I mentioned that the F3 actually has two microcontrollers. One of them is used as a programmer/debugger. The part of the board that's used as a programmer is called ST-LINK (that's what STMicroelectronics decided to call it). This ST-LINK is connected to the target microcontroller using a Serial Wire Debug (SWD) interface (this interface is an ARM standard so you'll run into it when dealing with other Cortex-M based microcontrollers). This SWD interface can be used to flash and debug a microcontroller. The ST-LINK is connected to the "USB ST-LINK" port and will appear as a USB device when you connect the F3 to your computer.

As for OpenOCD, it's software that provides some services like a GDB server on top of USB devices that expose a debugging protocol like SWD or JTAG.

Onto the actual command: those .cfg files we are using instruct OpenOCD to look for a ST-LINK USB device (interface/stlink-v2-1.cfg) and to expect a STM32F3XX microcontroller (target/stm32f3x.cfg) to be connected to the ST-LINK.

The OpenOCD output looks like this:

Open On-Chip Debugger 0.9.0 (2016-04-27-23:18)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
none separate
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : clock speed 950 kHz
Info : STLINK v2 JTAG v27 API v2 SWIM v15 VID 0x0483 PID 0x374B
Info : using stlink api v2
Info : Target voltage: 2.919073
Info : stm32f3x.cpu: hardware has 6 breakpoints, 4 watchpoints

The "6 breakpoints, 4 watchpoints" part indicates the debugging features the processor has available.

Leave that openocd process running, and open a new terminal. Make sure that you are inside the project's src/05-led-roulette/ directory.

I mentioned that OpenOCD provides a GDB server so let's connect to that right now:

$ <gdb> -q target/thumbv7em-none-eabihf/debug/led-roulette
Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done.
(gdb)

NOTE: <gdb> represents a GDB program capable of debugging ARM binaries. This could be arm-none-eabi-gdb, gdb-multiarch or gdb depending on your system -- you may have to try all three.

This only opens a GDB shell. To actually connect to the OpenOCD GDB server, use the following command within the GDB shell:

(gdb) target remote :3333
Remote debugging using :3333
0x00000000 in ?? ()

NOTE: If you are getting errors like undefined debug reason 7 - target needs reset, you can try running monitor reset halt as described here.

NOTE: If the debugger is still not connecting to the OpenOCD server, then you may need to try using arm-none-eabi-gdb instead of the gdb command, as described above.

By default OpenOCD's GDB server listens on TCP port 3333 (localhost). This command is connecting to that port.

After entering this command, you'll see new output in the OpenOCD terminal:

 Info : stm32f3x.cpu: hardware has 6 breakpoints, 4 watchpoints
+Info : accepting 'gdb' connection on tcp/3333
+Info : device id = 0x10036422
+Info : flash size = 256kbytes

Almost there. To flash the device, we'll use the load command inside the GDB shell:

(gdb) load
Loading section .vector_table, size 0x188 lma 0x8000000
Loading section .text, size 0x38a lma 0x8000188
Loading section .rodata, size 0x8 lma 0x8000514
Start address 0x8000188, load size 1306
Transfer rate: 6 KB/sec, 435 bytes/write.

And that's it. You'll also see new output in the OpenOCD terminal.

 Info : flash size = 256kbytes
+Info : Unable to match requested speed 1000 kHz, using 950 kHz
+Info : Unable to match requested speed 1000 kHz, using 950 kHz
+adapter speed: 950 kHz
+target state: halted
+target halted due to debug-request, current mode: Thread
+xPSR: 0x01000000 pc: 0x08000194 msp: 0x2000a000
+Info : Unable to match requested speed 8000 kHz, using 4000 kHz
+Info : Unable to match requested speed 8000 kHz, using 4000 kHz
+adapter speed: 4000 kHz
+target state: halted
+target halted due to breakpoint, current mode: Thread
+xPSR: 0x61000000 pc: 0x2000003a msp: 0x2000a000
+Info : Unable to match requested speed 1000 kHz, using 950 kHz
+Info : Unable to match requested speed 1000 kHz, using 950 kHz
+adapter speed: 950 kHz
+target state: halted
+target halted due to debug-request, current mode: Thread
+xPSR: 0x01000000 pc: 0x08000194 msp: 0x2000a000

Our program is loaded, let's debug it!

We are already inside a debugging session so let's debug our program.

After the load command, our program is stopped at its entry point. This is indicated by the "Start address 0x8000XXX" part of GDB's output. The entry point is the part of a program that a processor / CPU will execute first.

The starter project I've provided to you has some extra code that runs before the main function. At this time, we are not interested in that "pre-main" part so let's skip right to the beginning of the main function. We'll do that using a breakpoint:

(gdb) break main
Breakpoint 1 at 0x800018c: file src/05-led-roulette/src/main.rs, line 10.

(gdb) continue
Continuing.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, main () at src/05-led-roulette/src/main.rs:10
10          let x = 42;

Breakpoints can be used to stop the normal flow of a program. The continue command will let the program run freely until it reaches a breakpoint. In this case, until it reaches the main function because there's a breakpoint there.

Note that GDB output says "Breakpoint 1". Remember that our processor can only use six of these breakpoints so it's a good idea to pay attention to these messages.

For a nicer debugging experience, we'll be using GDB's Text User Interface (TUI). To enter into that mode, on the GDB shell enter the following command:

(gdb) layout src

NOTE Apologies Windows users. The GDB shipped with the GNU ARM Embedded Toolchain doesn't support this TUI mode :-(.

At any point you can leave the TUI mode using the following command:

(gdb) tui disable

OK. We are now at the beginning of main. We can advance the program statement by statement using the step command. So let's use that twice to reach the _y = x statement. Once you've typed step once you can just hit enter to run it again.

(gdb) step
14           _y = x;

If you are not using the TUI mode, on each step call GDB will print back the current statement along with its line number.

We are now "on" the _y = x statement; that statement hasn't been executed yet. This means that x is initialized but _y is not. Let's inspect those stack/local variables using the print command:

(gdb) print x
$1 = 42

(gdb) print &x
$2 = (i32 *) 0x10001ff4

(gdb) print _y
$3 = -536810104

(gdb) print &_y
$4 = (i32 *) 0x10001ff0

As expected, x contains the value 42. _y, however, contains the value -536810104 (?). Because _y has not been initialized yet, it contains some garbage value.

The command print &x prints the address of the variable x. The interesting bit here is that GDB output shows the type of the reference: i32*, a pointer to an i32 value. Another interesting thing is that the addresses of x and _y are very close to each other: their addresses are just 4 bytes apart.

Instead of printing the local variables one by one, you can also use the info locals command:

(gdb) info locals
x = 42
_y = -536810104

OK. With another step, we'll be on top of the loop {} statement:

(gdb) step
17          loop {}

And _y should now be initialized.

(gdb) print _y
$5 = 42

If we use step again on top of the loop {} statement, we'll get stuck because the program will never pass that statement. Instead, we'll switch to the disassemble view with the layout asm command and advance one instruction at a time using stepi. You can always switch back into Rust source code view later by issuing the layout src command again.

NOTE If you used the step command by mistake and GDB got stuck, you can get unstuck by hitting Ctrl+C.

(gdb) layout asm

If you are not using the TUI mode, you can use the disassemble /m command to disassemble the program around the line you are currently at.

(gdb) disassemble /m
Dump of assembler code for function main:
7       #[entry]
   0x08000188 <+0>:     sub     sp, #8
   0x0800018a <+2>:     movs    r0, #42 ; 0x2a

8       fn main() -> ! {
9           let _y;
10          let x = 42;
   0x0800018c <+4>:     str     r0, [sp, #4]

11          _y = x;
   0x0800018e <+6>:     ldr     r0, [sp, #4]
   0x08000190 <+8>:     str     r0, [sp, #0]

12
13          // infinite loop; just so we don't leave this stack frame
14          loop {}
=> 0x08000192 <+10>:    b.n     0x8000194 <main+12>
   0x08000194 <+12>:    b.n     0x8000194 <main+12>

End of assembler dump.

See the fat arrow => on the left side? It shows the instruction the processor will execute next.

If not inside the TUI mode on each stepi command GDB will print the statement, the line number and the address of the instruction the processor will execute next.

(gdb) stepi
0x08000194      14          loop {}

(gdb) stepi
0x08000194      14          loop {}

One last trick before we move to something more interesting. Enter the following commands into GDB:

(gdb) monitor reset halt
Unable to match requested speed 1000 kHz, using 950 kHz
Unable to match requested speed 1000 kHz, using 950 kHz
adapter speed: 950 kHz
target halted due to debug-request, current mode: Thread
xPSR: 0x01000000 pc: 0x08000196 msp: 0x10002000

(gdb) continue
Continuing.

Breakpoint 1, main () at src/05-led-roulette/src/main.rs:10
10          let x = 42;

We are now back at the beginning of main!

monitor reset halt will reset the microcontroller and stop it right at the program entry point. The following continue command will let the program run freely until it reaches the main function that has a breakpoint on it.

This combo is handy when you, by mistake, skipped over a part of the program that you were interested in inspecting. You can easily roll back the state of your program back to its very beginning.

The fine print: This reset command doesn't clear or touch RAM. That memory will retain its values from the previous run. That shouldn't be a problem though, unless your program behavior depends of the value of uninitialized variables but that's the definition of Undefined Behavior (UB).

We are done with this debug session. You can end it with the quit command.

(gdb) quit
A debugging session is active.

        Inferior 1 [Remote target] will be detached.

Quit anyway? (y or n) y
Detaching from program: $PWD/target/thumbv7em-none-eabihf/debug/led-roulette, Remote target
Ending remote debugging.

NOTE If the default GDB CLI is not to your liking check out gdb-dashboard. It uses Python to turn the default GDB CLI into a dashboard that shows registers, the source view, the assembly view and other things.

Don't close OpenOCD though! We'll use it again and again later on. It's better just to leave it running.

What's next? The high level API I promised.

Now, I'm going to introduce two high level abstractions that we'll use to implement the LED roulette application.

The auxiliary crate, aux5, exposes an initialization function called init. When called this function returns two values packed in a tuple: a Delay value and a Leds value.

Delay can be used to block your program for a specified amount of milliseconds.

Leds is actually an array of eight Leds. Each Led represents one of the LEDs on the F3 board, and exposes two methods: on and off which can be used to turn the LED on or off, respectively.

Let's try out these two abstractions by modifying the starter code to look like this:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux5::{entry, prelude::*, Delay, Leds};

#[entry]
fn main() -> ! {
    let (mut delay, mut leds): (Delay, Leds) = aux5::init();

    let half_period = 500_u16;

    loop {
        leds[0].on();
        delay.delay_ms(half_period);

        leds[0].off();
        delay.delay_ms(half_period);
    }
}

Now build it:

$ cargo build --target thumbv7em-none-eabihf

NOTE It's possible to forget to rebuild the program before starting a GDB session; this omission can lead to very confusing debug sessions. To avoid this problem you can call cargo run instead of cargo build; cargo run will build and start a debug session ensuring you never forget to recompile your program.

Now, we'll repeat the flashing procedure that we did in the previous section:

$ # this starts a GDB session of the program; no need to specify the path to the binary
$ arm-none-eabi-gdb -q target/thumbv7em-none-eabihf/debug/led-roulette
Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done.
(gdb) target remote :3333
Remote debugging using :3333
(..)

(gdb) load
Loading section .vector_table, size 0x188 lma 0x8000000
Loading section .text, size 0x3fc6 lma 0x8000188
Loading section .rodata, size 0xa0c lma 0x8004150
Start address 0x8000188, load size 19290
Transfer rate: 19 KB/sec, 4822 bytes/write.

(gdb) break main
Breakpoint 1 at 0x800018c: file src/05-led-roulette/src/main.rs, line 9.

(gdb) continue
Continuing.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, main () at src/05-led-roulette/src/main.rs:9
9           let (mut delay, mut leds): (Delay, Leds) = aux5::init();

OK. Let's step through the code. This time, we'll use the next command instead of step. The difference is that the next command will step over function calls instead of going inside them.

(gdb) next
11          let half_period = 500_u16;

(gdb) next
13          loop {

(gdb) next
14              leds[0].on();

(gdb) next
15              delay.delay_ms(half_period);

After executing the leds[0].on() statement, you should see a red LED, the one pointing North, turn on.

Let's continue stepping over the program:

(gdb) next
17              leds[0].off();

(gdb) next
18              delay.delay_ms(half_period);

The delay_ms call will block the program for half a second but you may not notice because the next command also takes some time to execute. However, after stepping over the leds[0].off() statement you should see the red LED turn off.

You can already guess what this program does. Let it run uninterrupted using the continue command.

(gdb) continue
Continuing.

Now, let's do something more interesting. We are going to modify the behavior of our program using GDB.

First, let's stop the infinite loop by hitting Ctrl+C. You'll probably end up somewhere inside Led::on, Led::off or delay_ms:

Program received signal SIGINT, Interrupt.
0x080033f6 in core::ptr::read_volatile (src=0xe000e010) at /checkout/src/libcore/ptr.rs:472
472     /checkout/src/libcore/ptr.rs: No such file or directory.

In my case, the program stopped its execution inside a read_volatile function. GDB output shows some interesting information about that: core::ptr::read_volatile (src=0xe000e010). This means that the function comes from the core crate and that it was called with argument src = 0xe000e010.

Just so you know, a more explicit way to show the arguments of a function is to use the info args command:

(gdb) info args
src = 0xe000e010

Regardless of where your program may have stopped you can always look at the output of the backtrace command (bt for short) to learn how it got there:

(gdb) backtrace
#0  0x080033f6 in core::ptr::read_volatile (src=0xe000e010)
    at /checkout/src/libcore/ptr.rs:472
#1  0x08003248 in <vcell::VolatileCell<T>>::get (self=0xe000e010)
    at $REGISTRY/vcell-0.1.0/src/lib.rs:43
#2  <volatile_register::RW<T>>::read (self=0xe000e010)
    at $REGISTRY/volatile-register-0.2.0/src/lib.rs:75
#3  cortex_m::peripheral::syst::<impl cortex_m::peripheral::SYST>::has_wrapped (self=0x10001fbc)
    at $REGISTRY/cortex-m-0.5.7/src/peripheral/syst.rs:124
#4  0x08002d9c in <stm32f30x_hal::delay::Delay as embedded_hal::blocking::delay::DelayUs<u32>>::delay_us (self=0x10001fbc, us=500000)
    at $REGISTRY/stm32f30x-hal-0.2.0/src/delay.rs:58
#5  0x08002cce in <stm32f30x_hal::delay::Delay as embedded_hal::blocking::delay::DelayMs<u32>>::delay_ms (self=0x10001fbc, ms=500)
    at $REGISTRY/stm32f30x-hal-0.2.0/src/delay.rs:32
#6  0x08002d0e in <stm32f30x_hal::delay::Delay as embedded_hal::blocking::delay::DelayMs<u16>>::delay_ms (self=0x10001fbc, ms=500)
    at $REGISTRY/stm32f30x-hal-0.2.0/src/delay.rs:38
#7  0x080001ee in main () at src/05-led-roulette/src/main.rs:18

backtrace will print a trace of function calls from the current function down to main.

Back to our topic. To do what we are after, first, we have to return to the main function. We can do that using the finish command. This command resumes the program execution and stops it again right after the program returns from the current function. We'll have to call it several times.

(gdb) finish
cortex_m::peripheral::syst::<impl cortex_m::peripheral::SYST>::has_wrapped (self=0x10001fbc)
    at $REGISTRY/cortex-m-0.5.7/src/peripheral/syst.rs:124
124             self.csr.read() & SYST_CSR_COUNTFLAG != 0
Value returned is $1 = 5

(gdb) finish
Run till exit from #0  cortex_m::peripheral::syst::<impl cortex_m::peripheral::SYST>::has_wrapped (
    self=0x10001fbc)
    at $REGISTRY/cortex-m-0.5.7/src/peripheral/syst.rs:124
0x08002d9c in <stm32f30x_hal::delay::Delay as embedded_hal::blocking::delay::DelayUs<u32>>::delay_us (
    self=0x10001fbc, us=500000)
    at $REGISTRY/stm32f30x-hal-0.2.0/src/delay.rs:58
58              while !self.syst.has_wrapped() {}
Value returned is $2 = false

(..)

(gdb) finish
Run till exit from #0  0x08002d0e in <stm32f30x_hal::delay::Delay as embedded_hal::blocking::delay::DelayMs<u16>>::delay_ms (self=0x10001fbc, ms=500)
    at $REGISTRY/stm32f30x-hal-0.2.0/src/delay.rs:38
0x080001ee in main () at src/05-led-roulette/src/main.rs:18
18              delay.delay_ms(half_period);

We are back in main. We have a local variable in here: half_period

(gdb) info locals
half_period = 500
delay = (..)
leds = (..)

Now, we are going to modify this variable using the set command:

(gdb) set half_period = 100

(gdb) print half_period
$1 = 100

If you let program run free again using the continue command, you should see that the LED will blink at a much faster rate now!

Question! What happens if you keep lowering the value of half_period? At what value of half_period you can no longer see the LED blink?

Now, it's your turn to write a program.

You are now well armed to face a challenge! Your task will be to implement the application I showed you at the beginning of this chapter.

Here's the GIF again:

Also, this may help:

This is a timing diagram. It indicates which LED is on at any given instant of time and for how long each LED should be on. On the X axis we have the time in milliseconds. The timing diagram shows a single period. This pattern will repeat itself every 800 ms. The Y axis labels each LED with a cardinal point: North, East, etc. As part of the challenge you'll have to figure out how each element in the Leds array maps to these cardinal points (hint: cargo doc --open ;-)).

Before you attempt this challenge, let me give you one last tip. Our GDB sessions always involve entering the same commands at the beginning. We can use a .gdb file to execute some commands right after GDB is started. This way you can save yourself the effort of having to enter them manually on each GDB session.

Place this openocd.gdb file in the root of the Cargo project, right next to the Cargo.toml:

$ cat openocd.gdb

target remote :3333
load
break main
continue

Then modify the second line of the .cargo/config file:

$ cat .cargo/config

[target.thumbv7em-none-eabihf]
runner = "arm-none-eabi-gdb -q -x openocd.gdb" # <-
rustflags = [
  "-C", "link-arg=-Tlink.x",
]

With that in place, you should now be able to start a gdb session that will automatically flash the program and jump to the beginning of main:

$ cargo run --target thumbv7em-none-eabihf
     Running `arm-none-eabi-gdb -q -x openocd.gdb target/thumbv7em-none-eabihf/debug/led-roulette`
Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done.
(..)
Loading section .vector_table, size 0x188 lma 0x8000000
Loading section .text, size 0x3b20 lma 0x8000188
Loading section .rodata, size 0xb0c lma 0x8003cc0
Start address 0x8003b1c, load size 18356
Transfer rate: 20 KB/sec, 6118 bytes/write.
Breakpoint 1 at 0x800018c: file src/05-led-roulette/src/main.rs, line 9.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, main () at src/05-led-roulette/src/main.rs:9
9           let (mut delay, mut leds): (Delay, Leds) = aux5::init();
(gdb)

What solution did you come up with?

Here's mine:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux5::{entry, prelude::*, Delay, Leds};

#[entry]
fn main() -> ! {
    let (mut delay, mut leds): (Delay, Leds) = aux5::init();

    let ms = 50_u8;
    loop {
        for curr in 0..8 {
            let next = (curr + 1) % 8;

            leds[next].on();
            delay.delay_ms(ms);
            leds[curr].off();
            delay.delay_ms(ms);
        }
    }
}

One more thing! Check that your solution also works when compiled in "release" mode:

$ cargo build --target thumbv7em-none-eabihf --release

You can test it with this gdb command:

$ # or, you could simply call `cargo run --target thumbv7em-none-eabihf --release`
$ arm-none-eabi-gdb target/thumbv7em-none-eabihf/release/led-roulette
$ #                                              ~~~~~~~

Binary size is something we should always keep an eye on! How big is your solution? You can check that using the size command on the release binary:

$ # equivalent to size target/thumbv7em-none-eabihf/debug/led-roulette
$ cargo size --target thumbv7em-none-eabihf --bin led-roulette -- -A
led-roulette  :
section               size        addr
.vector_table          392   0x8000000
.text                16404   0x8000188
.rodata               2924   0x80041a0
.data                    0  0x20000000
.bss                     4  0x20000000
.debug_str          602185         0x0
.debug_abbrev        24134         0x0
.debug_info         553143         0x0
.debug_ranges       112744         0x0
.debug_macinfo          86         0x0
.debug_pubnames      56467         0x0
.debug_pubtypes      94866         0x0
.ARM.attributes         58         0x0
.debug_frame        174812         0x0
.debug_line         354866         0x0
.debug_loc             534         0x0
.comment                75         0x0
Total              1993694

$ cargo size --target thumbv7em-none-eabihf --bin led-roulette --release -- -A
led-roulette  :
section              size        addr
.vector_table         392   0x8000000
.text                1826   0x8000188
.rodata                84   0x80008ac
.data                   0  0x20000000
.bss                    4  0x20000000
.debug_str          23334         0x0
.debug_loc           6964         0x0
.debug_abbrev        1337         0x0
.debug_info         40582         0x0
.debug_ranges        2936         0x0
.debug_macinfo          1         0x0
.debug_pubnames      5470         0x0
.debug_pubtypes     10016         0x0
.ARM.attributes        58         0x0
.debug_frame          164         0x0
.debug_line          9081         0x0
.comment               18         0x0
Total              102267

NOTE The Cargo project is already configured to build the release binary using LTO.

Know how to read this output? The text section contains the program instructions. It's around 2KB in my case. On the other hand, the data and bss sections contain variables statically allocated in RAM (static variables). A static variable is being used in aux5::init; that's why it shows 4 bytes of bss.

One final thing! We have been running our programs from within GDB but our programs don't depend on GDB at all. You can confirm this be closing both GDB and OpenOCD and then resetting the board by pressing the black button on the board. The LED roulette application will run without intervention of GDB.

HEADS UP Several readers have reported that the "solder bridge" SB10 (see back of the board) on the STM32F3DISCOVERY, which is required to use the ITM and the iprint! macros shown below, is not soldered even though the User Manual (page 21) says that it should be.

TL;DR You have two options to fix this: Either solder the solder bridge SB10 or connect a female to female jumper wire between SWO and PB3 as shown in the picture below.

Just a little more of helpful magic before we start doing low level stuff.

Blinking an LED is like the "Hello, world" of the embedded world.

But in this section, we'll run a proper "Hello, world" program that prints stuff to your computer console.

Go to the 06-hello-world directory. There's some starter code in it:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux6::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let mut itm = aux6::init();

    iprintln!(&mut itm.stim[0], "Hello, world!");

    loop {}
}

The iprintln macro will format messages and output them to the microcontroller's ITM. ITM stands for Instrumentation Trace Macrocell and it's a communication protocol on top of SWD (Serial Wire Debug) which can be used to send messages from the microcontroller to the debugging host. This communication is only one way: the debugging host can't send data to the microcontroller.

OpenOCD, which is managing the debug session, can receive data sent through this ITM channel and redirect it to a file.

The ITM protocol works with frames (you can think of them as Ethernet frames). Each frame has a header and a variable length payload. OpenOCD will receive these frames and write them directly to a file without parsing them. So, if the microntroller sends the string "Hello, world!" using the iprintln macro, OpenOCD's output file won't exactly contain that string.

To retrieve the original string, OpenOCD's output file will have to be parsed. We'll use the itmdump program to perform the parsing as new data arrives.

You should have already installed the itmdump program during the installation chapter.

In a new terminal, run this command inside the /tmp directory, if you are using a *nix OS, or from within the %TEMP% directory, if you are running Windows. This should be the same directory from where you are running OpenOCD.

NOTE It's very important that both itmdump and openocd are running from the same directory!

$ # itmdump terminal

$ # *nix
$ cd /tmp && touch itm.txt

$ # Windows
$ cd %TEMP% && type nul >> itm.txt

$ # both
$ itmdump -F -f itm.txt

This command will block as itmdump is now watching the itm.txt file. Leave this terminal open.

Make sure that F3 is connected to your computer. Open another terminal from /tmp directory (on Windows %TEMP%) to launch OpenOCD similar as described in chapter First OpenOCD connection.

Alright. Now, let's build the starter code and flash it into the microcontroller.

To avoid passing the --target thumbv7em-none-eabihf flag to every Cargo invocation we can set a default target in .cargo/config:

 [target.thumbv7em-none-eabihf]
 runner = "arm-none-eabi-gdb -q -x openocd.gdb"
 rustflags = [
   "-C", "link-arg=-Tlink.x",
 ]

+[build]
+target = "thumbv7em-none-eabihf"

Now if --target is not specified Cargo will assume that the target is thumbv7em-none-eabihf.

$ cargo run
Reading symbols from target/thumbv7em-none-eabihf/debug/hello-world...done.
(..)
Loading section .vector_table, size 0x400 lma 0x8000000
Loading section .text, size 0x27c4 lma 0x8000400
Loading section .rodata, size 0x744 lma 0x8002be0
Start address 0x8002980, load size 13064
Transfer rate: 18 KB/sec, 4354 bytes/write.
Breakpoint 1 at 0x8000402: file src/06-hello-world/src/main.rs, line 10.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, main () at src/06-hello-world/src/main.rs:10
10          let mut itm = aux6::init();

Note that there's a openocd.gdb at the root of the Cargo project. It's pretty similar to the one we used in the previous section.

Before we execute the iprintln! statement. We have to instruct OpenOCD to redirect the ITM output into the same file that itmdump is watching.

(gdb) # globally enable the ITM and redirect all output to itm.txt
(gdb) monitor tpiu config internal itm.txt uart off 8000000

(gdb) # enable the ITM port 0
(gdb) monitor itm port 0 on

All should be ready! Now execute the iprintln! statement.

(gdb) next
12          iprintln!(&mut itm.stim[0], "Hello, world!");

(gdb) next
14          loop {}

You should see some output in the itmdump terminal:

$ itmdump -F -f itm.txt
(..)
Hello, world!

Awesome, right? Feel free to use iprintln as a logging tool in the coming sections.

Next: That's not all! The iprint! macros are not the only thing that uses the ITM. :-)

The panic! macro also sends its output to the ITM!

Change the main function to look like this:

#[entry]
fn main() -> ! {
    panic!("Hello, world!");
}

Let's try this program. But before that let's update openocd.gdb to run that monitor stuff for us during GDB startup:

 target remote :3333
 set print asm-demangle on
 set print pretty on
 load
+monitor tpiu config internal itm.txt uart off 8000000
+monitor itm port 0 on
 break main
 continue

OK, now run it.

$ cargo run
(..)
Breakpoint 1, main () at src/06-hello-world/src/main.rs:10
10          panic!("Hello, world!");

(gdb) next

You'll see some new output in the itmdump terminal.

$ # itmdump terminal
(..)
panicked at 'Hello, world!', src/06-hello-world/src/main.rs:10:5

Another thing you can do is catch the panic before it does the logging by putting a breakpoint on the rust_begin_unwind symbol.

(gdb) monitor reset halt
(..)
target halted due to debug-request, current mode: Thread
xPSR: 0x01000000 pc: 0x080026ba msp: 0x10002000

(gdb) break rust_begin_unwind
Breakpoint 2 at 0x80011d2: file $REGISTRY/panic-itm-0.4.0/src/lib.rs, line 46.

(gdb) continue
Continuing.

Breakpoint 2, rust_begin_unwind (info=0x10001fac) at $REGISTRY/panic-itm-0.4.0/src/lib.rs:46
46          interrupt::disable();

You'll notice that nothing got printed on the itmdump console this time. If you resume the program using continue then a new line will be printed.

In a later section we'll look into other simpler communication protocols.

It's time to explore what the Led API does under the hood.

In a nutshell, it just writes to some special memory regions. Go into the 07-registers directory and let's run the starter code statement by statement.

#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    aux7::init();

    unsafe {
        // A magic address!
        const GPIOE_BSRR: u32 = 0x48001018;

        // Turn on the "North" LED (red)
        *(GPIOE_BSRR as *mut u32) = 1 << 9;

        // Turn on the "East" LED (green)
        *(GPIOE_BSRR as *mut u32) = 1 << 11;

        // Turn off the "North" LED
        *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);

        // Turn off the "East" LED
        *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
    }

    loop {}
}

What's this magic?

The address 0x48001018 points to a register. A register is a special region of memory that controls a peripheral. A peripheral is a piece of electronics that sits right next to the processor within the microcontroller package and provides the processor with extra functionality. After all, the processor, on its own, can only do math and logic.

This particular register controls General Purpose Input/Output (GPIO) pins (GPIO is a peripheral) and can be used to drive each of those pins low or high.

Drive? Pin? Low? High?

A pin is a electrical contact. Our microcontroller has several of them and some of them are connected to LEDs. An LED, a Light Emitting Diode, will only emit light when voltage is applied to it with a certain polarity.

Luckily for us, the microcontroller's pins are connected to the LEDs with the right polarity. All that we have to do is output some non-zero voltage through the pin to turn the LED on. The pins attached to the LEDs are configured as digital outputs and can only output two different voltage levels: "low", 0 Volts, or "high", 3 Volts. A "high" (voltage) level will turn the LED on whereas a "low" (voltage) level will turn it off.

These "low" and "high" states map directly to the concept of digital logic. "low" is 0 or false and "high" is 1 or true. This is why this pin configuration is known as digital output.

OK. But how can one find out what this register does? Time to RTRM (Read the Reference Manual)!

I mentioned that the microcontroller has several pins. For convenience, these pins are grouped in ports of 16 pins. Each port is named with a letter: Port A, Port B, etc. and the pins within each port are named with numbers from 0 to 15.

The first thing we have to find out is which pin is connected to which LED. This information is in the STM32F3DISCOVERY User Manual (You downloaded a copy, right?). In this particular section:

Section 6.4 LEDs - Page 18

The manual says:

• LD3, the North LED, is connected to the pin PE9. PE9 is the short form of: Pin 9 on Port E.
• LD7, the East LED, is connected to the pin PE11.

Up to this point, we know that we want to change the state of the pins PE9 and PE11 to turn the North/East LEDs on/off. These pins are part of Port E so we'll have to deal with the GPIOE peripheral.

Each peripheral has a register block associated to it. A register block is a collection of registers allocated in contiguous memory. The address at which the register block starts is known as its base address. We need to figure out what's the base address of the GPIOE peripheral. That information is in the following section of the microcontroller Reference Manual:

Section 3.2.2 Memory map and register boundary addresses - Page 51

The table says that base address of the GPIOE register block is 0x4800_1000.

Each peripheral also has its own section in the documentation. Each of these sections ends with a table of the registers that the peripheral's register block contains. For the GPIO family of peripheral, that table is in:

Section 11.4.12 GPIO register map - Page 243

We are interested in the register that's at an offset of 0x18 from the base address of the GPIOE peripheral. According to the table, that would be the register BSRR.

Now we need to jump to the documentation of that particular register. It's a few pages above in:

Section 11.4.7 GPIO port bit set/reset register (GPIOx_BSRR) - Page 240

Finally!

This is the register we were writing to. The documentation says some interesting things. First, this register is write only ... so let's try reading its value :-).

We'll use GDB's examine command: x.

(gdb) next
16              *(GPIOE_BSRR as *mut u32) = 1 << 9;

(gdb) x 0x48001018
0x48001018:     0x00000000

(gdb) # the next command will turn the North LED on
(gdb) next
19              *(GPIOE_BSRR as *mut u32) = 1 << 11;

(gdb) x 0x48001018
0x48001018:     0x00000000

Reading the register returns 0. That matches what the documentation says.

The other thing that the documentation says is that the bits 0 to 15 can be used to set the corresponding pin. That is bit 0 sets the pin 0. Here, set means outputting a high value on the pin.

The documentation also says that bits 16 to 31 can be used to reset the corresponding pin. In this case, the bit 16 resets the pin number 0. As you may guess, reset means outputting a low value on the pin.

Correlating that information with our program, all seems to be in agreement:

• Writing 1 << 9 (BS9 = 1) to BSRR sets PE9 high. That turns the North LED on.

• Writing 1 << 11 (BS11 = 1) to BSRR sets PE11 high. That turns the East LED on.

• Writing 1 << 25 (BR9 = 1) to BSRR sets PE9 low. That turns the North LED off.

• Finally, writing 1 << 27 (BR11 = 1) to BSRR sets PE11 low. That turns the East LED off.

Reads/writes to registers are quite special. I may even dare to say that they are embodiment of side effects. In the previous example we wrote four different values to the same register. If you didn't know that address was a register, you may have simplified the logic to just write the final value 1 << (11 + 16) into the register.

Actually, LLVM, the compiler's backend / optimizer, does not know we are dealing with a register and will merge the writes thus changing the behavior of our program. Let's check that really quick.

$ cargo run --release
(..)
Breakpoint 1, main () at src/07-registers/src/main.rs:9
9           aux7::init();

(gdb) next
25              *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);

(gdb) disassemble /m
Dump of assembler code for function main:
7       #[entry]

8       fn main() -> ! {
9           aux7::init();
   0x08000188 <+0>:     bl      0x800019c <aux7::init>
   0x0800018c <+4>:     movw    r0, #4120       ; 0x1018
   0x08000190 <+8>:     mov.w   r1, #134217728  ; 0x8000000
   0x08000194 <+12>:    movt    r0, #18432      ; 0x4800

10
11          unsafe {
12              // A magic address!
13              const GPIOE_BSRR: u32 = 0x48001018;
14
15              // Turn on the "North" LED (red)
16              *(GPIOE_BSRR as *mut u32) = 1 << 9;
17
18              // Turn on the "East" LED (green)
19              *(GPIOE_BSRR as *mut u32) = 1 << 11;
20
21              // Turn off the "North" LED
22              *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);
23
24              // Turn off the "East" LED
25              *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
=> 0x08000198 <+16>:    str     r1, [r0, #0]

26          }
27
28          loop {}
   0x0800019a <+18>:    b.n     0x800019a <main+18>

End of assembler dump.

The state of the LEDs didn't change this time! The str instruction is the one that writes a value to the register. Our debug (unoptimized) program had four of them, one for each write to the register, but the release (optimized) program only has one.

We can check that using objdump:

$ # same as cargo objdump -- -d -no-show-raw-insn -print-imm-hex -source target/thumbv7em-none-eabihf/debug/registers
$ cargo objdump --bin registers -- -d -no-show-raw-insn -print-imm-hex -source
registers:      file format ELF32-arm-little

Disassembly of section .text:
main:
; #[entry]
 8000188:       sub     sp, #0x18
; aux7::init();
 800018a:       bl      #0xbc
 800018e:       str     r0, [sp, #0x14]
 8000190:       b       #-0x2 <main+0xa>
; *(GPIOE_BSRR as *mut u32) = 1 << 9;
 8000192:       b       #-0x2 <main+0xc>
 8000194:       movw    r0, #0x1018
 8000198:       movt    r0, #0x4800
 800019c:       mov.w   r1, #0x200
 80001a0:       str     r1, [r0]
; *(GPIOE_BSRR as *mut u32) = 1 << 11;
 80001a2:       b       #-0x2 <main+0x1c>
 80001a4:       movw    r0, #0x1018
 80001a8:       movt    r0, #0x4800
 80001ac:       mov.w   r1, #0x800
 80001b0:       str     r1, [r0]
 80001b2:       movs    r0, #0x19
; *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);
 80001b4:       mov     r1, r0
 80001b6:       cmp     r0, #0x9
 80001b8:       str     r1, [sp, #0x10]
 80001ba:       bvs     #0x54 <main+0x8a>
 80001bc:       b       #-0x2 <main+0x36>
 80001be:       ldr     r0, [sp, #0x10]
 80001c0:       and     r1, r0, #0x1f
 80001c4:       movs    r2, #0x1
 80001c6:       lsl.w   r1, r2, r1
 80001ca:       lsrs    r2, r0, #0x5
 80001cc:       cmp     r2, #0x0
 80001ce:       str     r1, [sp, #0xc]
 80001d0:       bne     #0x4c <main+0x98>
 80001d2:       b       #-0x2 <main+0x4c>
 80001d4:       movw    r0, #0x1018
 80001d8:       movt    r0, #0x4800
 80001dc:       ldr     r1, [sp, #0xc]
 80001de:       str     r1, [r0]
 80001e0:       movs    r0, #0x1b
; *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
 80001e2:       mov     r2, r0
 80001e4:       cmp     r0, #0xb
 80001e6:       str     r2, [sp, #0x8]
 80001e8:       bvs     #0x42 <main+0xa6>
 80001ea:       b       #-0x2 <main+0x64>
 80001ec:       ldr     r0, [sp, #0x8]
 80001ee:       and     r1, r0, #0x1f
 80001f2:       movs    r2, #0x1
 80001f4:       lsl.w   r1, r2, r1
 80001f8:       lsrs    r2, r0, #0x5
 80001fa:       cmp     r2, #0x0
 80001fc:       str     r1, [sp, #0x4]
 80001fe:       bne     #0x3a <main+0xb4>
 8000200:       b       #-0x2 <main+0x7a>
 8000202:       movw    r0, #0x1018
 8000206:       movt    r0, #0x4800
 800020a:       ldr     r1, [sp, #0x4]
 800020c:       str     r1, [r0]
; loop {}
 800020e:       b       #-0x2 <main+0x88>
 8000210:       b       #-0x4 <main+0x88>
; *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);
 8000212:       movw    r0, #0x41bc
 8000216:       movt    r0, #0x800
 800021a:       bl      #0x3b28
 800021e:       trap
 8000220:       movw    r0, #0x4204
 8000224:       movt    r0, #0x800
 8000228:       bl      #0x3b1a
 800022c:       trap
; *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
 800022e:       movw    r0, #0x421c
 8000232:       movt    r0, #0x800
 8000236:       bl      #0x3b0c
 800023a:       trap
 800023c:       movw    r0, #0x4234
 8000240:       movt    r0, #0x800
 8000244:       bl      #0x3afe
 8000248:       trap

How do we prevent LLVM from misoptimizing our program? We use volatile operations instead of plain reads/writes:

#![no_main]
#![no_std]

use core::ptr;

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    aux7::init();

    unsafe {
        // A magic address!
        const GPIOE_BSRR: u32 = 0x48001018;

        // Turn on the "North" LED (red)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 9);

        // Turn on the "East" LED (green)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 11);

        // Turn off the "North" LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (9 + 16));

        // Turn off the "East" LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (11 + 16));
    }

    loop {}
}

If we look at the disassembly of this new program compiled in release mode:

$ cargo objdump --bin registers --release -- -d -no-show-raw-insn -print-imm-hex -source
registers:      file format ELF32-arm-little

Disassembly of section .text:
main:
; #[entry]
 8000188:       bl      #0x22
; aux7::init();
 800018c:       movw    r0, #0x1018
 8000190:       mov.w   r1, #0x200
 8000194:       movt    r0, #0x4800
 8000198:       str     r1, [r0]
 800019a:       mov.w   r1, #0x800
 800019e:       str     r1, [r0]
 80001a0:       mov.w   r1, #0x2000000
 80001a4:       str     r1, [r0]
 80001a6:       mov.w   r1, #0x8000000
 80001aa:       str     r1, [r0]
; loop {}
 80001ac:       b       #-0x4 <main+0x24>

We see that the four writes (str instructions) are preserved. If you run it (use stepi), you'll also see that behavior of the program is preserved.

Not all the peripheral memory can be accessed. Look at this program.

#![no_main]
#![no_std]

use core::ptr;

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    aux7::init();

    unsafe {
        ptr::read_volatile(0x4800_1800 as *const u32);
    }

    loop {}
}

This address is close to the GPIOE_BSRR address we used before but this address is invalid. Invalid in the sense that there's no register at this address.

Now, let's try it.

$ cargo run
Breakpoint 3, main () at src/07-registers/src/main.rs:9
9           aux7::init();

(gdb) continue
Continuing.

Breakpoint 2, UserHardFault_ (ef=0x10001fc0)
    at $REGISTRY/cortex-m-rt-0.6.3/src/lib.rs:535
535         loop {

We tried to do an invalid operation, reading memory that doesn't exist, so the processor raised an exception, a hardware exception.

In most cases, exceptions are raised when the processor attempts to perform an invalid operation. Exceptions break the normal flow of a program and force the processor to execute an exception handler, which is just a function/subroutine.

There are different kind of exceptions. Each kind of exception is raised by different conditions and each one is handled by a different exception handler.

The aux7 crate depends on the cortex-m-rt crate which defines a default hard fault handler, named UserHardFault, that handles the "invalid memory address" exception. openocd.gdb placed a breakpoint on HardFault; that's why the debugger halted your program while it was executing the exception handler. We can get more information about the exception from the debugger. Let's see:

(gdb) list
530
531     #[allow(unused_variables)]
532     #[doc(hidden)]
533     #[no_mangle]
534     pub unsafe extern "C" fn UserHardFault_(ef: &ExceptionFrame) -> ! {
535         loop {
536             // add some side effect to prevent this from turning into a UDF instruction
537             // see rust-lang/rust#28728 for details
538             atomic::compiler_fence(Ordering::SeqCst);
539         }

ef is a snapshot of the program state right before the exception occurred. Let's inspect it:

(gdb) print/x *ef
$1 = cortex_m_rt::ExceptionFrame {
  r0: 0x48001800,
  r1: 0x48001800,
  r2: 0xb,
  r3: 0xc,
  r12: 0xd,
  lr: 0x800019f,
  pc: 0x80028d6,
  xpsr: 0x1000000
}

There are several fields here but the most important one is pc, the Program Counter register. The address in this register points to the instruction that generated the exception. Let's disassemble the program around the bad instruction.

(gdb) disassemble /m ef.pc
Dump of assembler code for function core::ptr::read_volatile:
471     /checkout/src/libcore/ptr.rs: No such file or directory.
   0x080028ce <+0>:     sub     sp, #16
   0x080028d0 <+2>:     mov     r1, r0
   0x080028d2 <+4>:     str     r0, [sp, #8]

472     in /checkout/src/libcore/ptr.rs
   0x080028d4 <+6>:     ldr     r0, [sp, #8]
   0x080028d6 <+8>:     ldr     r0, [r0, #0]
   0x080028d8 <+10>:    str     r0, [sp, #12]
   0x080028da <+12>:    ldr     r0, [sp, #12]
   0x080028dc <+14>:    str     r1, [sp, #4]
   0x080028de <+16>:    str     r0, [sp, #0]
   0x080028e0 <+18>:    b.n     0x80028e2 <core::ptr::read_volatile+20>

473     in /checkout/src/libcore/ptr.rs
   0x080028e2 <+20>:    ldr     r0, [sp, #0]
   0x080028e4 <+22>:    add     sp, #16
   0x080028e6 <+24>:    bx      lr

End of assembler dump.

The exception was caused by the ldr r0, [r0, #0] instruction, a read instruction. The instruction tried to read the memory at the address indicated by the r0 register. By the way, r0 is a CPU (processor) register not a memory mapped register; it doesn't have an associated address like, say, GPIO_BSRR.

Wouldn't it be nice if we could check what the value of the r0 register was right at the instant when the exception was raised? Well, we already did! The r0 field in the ef value we printed before is the value of r0 register had when the exception was raised. Here it is again:

(gdb) p/x *ef
$1 = cortex_m_rt::ExceptionFrame {
  r0: 0x48001800,
  r1: 0x48001800,
  r2: 0xb,
  r3: 0xc,
  r12: 0xd,
  lr: 0x800019f,
  pc: 0x80028d6,
  xpsr: 0x1000000
}

r0 contains the value 0x4800_1800 which is the invalid address we called the read_volatile function with.

BSRR is not the only register that can control the pins of Port E. The ODR register also lets you change the value of the pins. Furthermore, ODR also lets you retrieve the current output status of Port E.

ODR is documented in:

Section 11.4.6 GPIO port output data register - Page 239

Let's try this program:

#![no_main]
#![no_std]

use core::ptr;

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let mut itm = aux7::init().0;

    unsafe {
        const GPIOE_BSRR: u32 = 0x4800_1018;
        const GPIOE_ODR: u32 = 0x4800_1014;

        iprintln!(
            &mut itm.stim[0],
            "ODR = 0x{:04x}",
            ptr::read_volatile(GPIOE_ODR as *const u16)
        );

        // Turn on the NORTH LED (red)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 9);

        iprintln!(
            &mut itm.stim[0],
            "ODR = 0x{:04x}",
            ptr::read_volatile(GPIOE_ODR as *const u16)
        );

        // Turn on the EAST LED (green)
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 11);

        iprintln!(
            &mut itm.stim[0],
            "ODR = 0x{:04x}",
            ptr::read_volatile(GPIOE_ODR as *const u16)
        );

        // Turn off the NORTH LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (9 + 16));

        iprintln!(
            &mut itm.stim[0],
            "ODR = 0x{:04x}",
            ptr::read_volatile(GPIOE_ODR as *const u16)
        );

        // Turn off the EAST LED
        ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (11 + 16));
    }

    loop {}
}

If you run this program, you'll see:

$ # itmdump's console
(..)
ODR = 0x0000
ODR = 0x0200
ODR = 0x0a00
ODR = 0x0800

Side effects! Although we are reading the same address multiple times without actually modifying it, we still see its value change every time BSRR is written to.

The last register we were working with, ODR, had this in its documentation:

Bits 31:16 Reserved, must be kept at reset value

We are not supposed to write to those bits of the register or Bad Stuff May Happen.

There's also the fact the registers have different read/write permissions. Some of them are write only, others can be read and written to and there must be others that are read only.

Finally, directly working with hexadecimal addresses is error prone. You already saw that trying to access an invalid memory address causes an exception which disrupts the execution of our program.

Wouldn't it be nice if we had an API to manipulate registers in a "safe" manner? Ideally, the API should encode these three points I've mentioned: No messing around with the actual addresses, should respect read/write permissions and should prevent modification of the reserved parts of a register.

Well, we do! aux7::init() actually returns a value that provides a type safe API to manipulate the registers of the GPIOE peripheral.

As you may remember: a group of registers associated to a peripheral is called register block, and it's located in a contiguous region of memory. In this type safe API each register block is modeled as a struct where each of its fields represents a register. Each register field is a different newtype over e.g. u32 that exposes a combination of the following methods: read, write or modify according to its read/write permissions. Finally, these methods don't take primitive values like u32, instead they take yet another newtype that can be constructed using the builder pattern and that prevent the modification of the reserved parts of the register.

The best way to get familiar with this API is to port our running example to it.

#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let gpioe = aux7::init().1;

    // Turn on the North LED
    gpioe.bsrr.write(|w| w.bs9().set_bit());

    // Turn on the East LED
    gpioe.bsrr.write(|w| w.bs11().set_bit());

    // Turn off the North LED
    gpioe.bsrr.write(|w| w.br9().set_bit());

    // Turn off the East LED
    gpioe.bsrr.write(|w| w.br11().set_bit());

    loop {}
}

First thing you notice: There are no magic addresses involved. Instead we use a more human friendly way, for example gpioe.bsrr, to refer to the BSRR register in the GPIOE register block.

Then we have this write method that takes a closure. If the identity closure (|w| w) is used, this method will set the register to its default (reset) value, the value it had right after the microcontroller was powered on / reset. That value is 0x0 for the BSRR register. Since we want to write a non-zero value to the register, we use builder methods like bs9 and br9 to set some of the bits of the default value.

Let's run this program! There's some interesting stuff we can do while debugging the program.

gpioe is a reference to the GPIOE register block. print gpioe will return the base address of the register block.

$ cargo run
Breakpoint 3, main () at src/07-registers/src/main.rs:9
9           let gpioe = aux7::init().1;

(gdb) next
12          gpioe.bsrr.write(|w| w.bs9().set_bit());

(gdb) print gpioe
$1 = (stm32f30x::gpioc::RegisterBlock *) 0x48001000

But if we instead print *gpioe, we'll get a full view of the register block: the value of each of its registers will be printed.

(gdb) print *gpioe
$2 = stm32f30x::gpioc::RegisterBlock {
  moder: stm32f30x::gpioc::MODER {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x55550000
      }
    }
  },
  otyper: stm32f30x::gpioc::OTYPER {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  ospeedr: stm32f30x::gpioc::OSPEEDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  pupdr: stm32f30x::gpioc::PUPDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  idr: stm32f30x::gpioc::IDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0xcc
      }
    }
  },
  odr: stm32f30x::gpioc::ODR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  bsrr: stm32f30x::gpioc::BSRR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  lckr: stm32f30x::gpioc::LCKR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  afrl: stm32f30x::gpioc::AFRL {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  afrh: stm32f30x::gpioc::AFRH {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  brr: stm32f30x::gpioc::BRR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  }
}

All these newtypes and closures sound like they'd generate large, bloated programs but, if you actually compile the program in release mode with LTO enabled, you'll see that it produces exactly the same instructions that the "unsafe" version that used write_volatile and hexadecimal addresses did!

$ cargo objdump --bin registers --release -- -d -no-show-raw-insn -print-imm-hex
registers:      file format ELF32-arm-little

Disassembly of section .text:
main:
 8000188:       bl      #0x22
 800018c:       movw    r0, #0x1018
 8000190:       mov.w   r1, #0x200
 8000194:       movt    r0, #0x4800
 8000198:       str     r1, [r0]
 800019a:       mov.w   r1, #0x800
 800019e:       str     r1, [r0]
 80001a0:       mov.w   r1, #0x2000000
 80001a4:       str     r1, [r0]
 80001a6:       mov.w   r1, #0x8000000
 80001aa:       str     r1, [r0]
 80001ac:       b       #-0x4 <main+0x24>

The best part of all this is that I didn't have to write a single line of code to implement the GPIOE API. All was automatically generated from a System View Description (SVD) file using the svd2rust tool. This SVD file is actually an XML file that microcontroller vendors provide and that contains the register maps of their microcontrollers. The file contains the layout of register blocks, the base addresses, the read/write permissions of each register, the layout of the registers, whether a register has reserved bits and lots of other useful information.

In the last section, I gave you initialized (configured) peripherals (I initialized them in aux7::init). That's why just writing to BSRR was enough to control the LEDs. But, peripherals are not initialized right after the microcontroller boots.

In this section, you'll have more fun with registers. I won't do any initialization and you'll have to initialize configure GPIOE pins as digital outputs pins so that you'll be able to drive LEDs again.

This is the starter code.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux8::entry;

#[entry]
fn main() -> ! {
    let (gpioe, rcc) = aux8::init();

    // TODO initialize GPIOE

    // Turn on all the LEDs in the compass
    gpioe.odr.write(|w| {
        w.odr8().set_bit();
        w.odr9().set_bit();
        w.odr10().set_bit();
        w.odr11().set_bit();
        w.odr12().set_bit();
        w.odr13().set_bit();
        w.odr14().set_bit();
        w.odr15().set_bit()
    });

    aux8::bkpt();

    loop {}
}

If you run the starter code, you'll see that nothing happens this time. Furthermore, if you print the GPIOE register block, you'll see that every register reads as zero even after the gpioe.odr.write statement was executed!

$ cargo run
Breakpoint 1, main () at src/08-leds-again/src/main.rs:9
9           let (gpioe, rcc) = aux8::init();

(gdb) continue
Continuing.

Program received signal SIGTRAP, Trace/breakpoint trap.
0x08000f3c in __bkpt ()

(gdb) finish
Run till exit from #0  0x08000f3c in __bkpt ()
main () at src/08-leds-again/src/main.rs:25
25          aux8::bkpt();

(gdb) p/x *gpioe
$1 = stm32f30x::gpioc::RegisterBlock {
  moder: stm32f30x::gpioc::MODER {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  otyper: stm32f30x::gpioc::OTYPER {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  ospeedr: stm32f30x::gpioc::OSPEEDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  pupdr: stm32f30x::gpioc::PUPDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  idr: stm32f30x::gpioc::IDR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  odr: stm32f30x::gpioc::ODR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  bsrr: stm32f30x::gpioc::BSRR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  lckr: stm32f30x::gpioc::LCKR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  afrl: stm32f30x::gpioc::AFRL {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  afrh: stm32f30x::gpioc::AFRH {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  },
  brr: stm32f30x::gpioc::BRR {
    register: vcell::VolatileCell<u32> {
      value: core::cell::UnsafeCell<u32> {
        value: 0x0
      }
    }
  }
}

Turns out that, to save power, most peripherals start in a powered off state -- that's their state right after the microcontroller boots.

The Reset and Clock Control (RCC) peripheral can be used to power on or off every other peripheral.

You can find the list of registers in the RCC register block in:

Section 9.4.14 - RCC register map - Page 166 - Reference Manual

The registers that control the power status of other peripherals are:

• AHBENR
• APB1ENR
• APB2ENR

Each bit in these registers controls the power status of a single peripheral, including GPIOE.

Your task in this section is to power on the GPIOE peripheral. You'll have to:

• Figure out which of the three registers I mentioned before has the bit that controls the power status.
• Figure out what value that bit must be set to,0 or 1, to power on the GPIOE peripheral.
• Finally, you'll have to change the starter code to modify the right register to turn on the GPIOE peripheral.

If you are successful, you'll see that the gpioe.odr.write statement will now be able to modify the value of the ODR register.

Note that this won't be enough to actually turn on the LEDs.

After turning on the GPIOE peripheral, it still needs to be configured. In this case, we want the pins to be configured as digital outputs so they can drive the LEDs; by default, most pins are configured as digital inputs.

You can find the list of registers in the GPIOE register block in:

Section 11.4.12 - GPIO registers - Page 243 - Reference Manual

The register we'll have to deal with is: MODER.

Your task for this section is to further update the starter code to configure the right GPIOE pins as digital outputs. You'll have to:

• Figure out which pins you need to configure as digital outputs. (hint: check Section 6.4 LEDs of the User Manual (page 18)).
• Read the documentation to understand what the bits in the MODER register do.
• Modify the MODER register to configure the pins as digital outputs.

If successful, you'll see the 8 LEDs turn on when you run the program.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux8::entry;

#[entry]
fn main() -> ! {
    let (gpioe, rcc) = aux8::init();

    // enable the GPIOE peripheral
    rcc.ahbenr.modify(|_, w| w.iopeen().set_bit());

    // configure the pins as outputs
    gpioe.moder.modify(|_, w| {
        w.moder8().output();
        w.moder9().output();
        w.moder10().output();
        w.moder11().output();
        w.moder12().output();
        w.moder13().output();
        w.moder14().output();
        w.moder15().output()
    });

    // Turn on all the LEDs in the compass
    gpioe.odr.write(|w| {
        w.odr8().set_bit();
        w.odr9().set_bit();
        w.odr10().set_bit();
        w.odr11().set_bit();
        w.odr12().set_bit();
        w.odr13().set_bit();
        w.odr14().set_bit();
        w.odr15().set_bit()
    });

    aux8::bkpt();

    loop {}
}

In this section, we'll re-implement the LED roulette application. I'm going to give you back the Led abstraction but this time I'm going to take away the Delay abstraction :-).

Here's the starter code. The delay function is unimplemented so if you run this program the LEDs will blink so fast that they'll appear to always be on.

#![no_main]
#![no_std]

use aux9::{entry, tim6};

#[inline(never)]
fn delay(tim6: &tim6::RegisterBlock, ms: u16) {
    // TODO implement this
}

#[entry]
fn main() -> ! {
    let (mut leds, rcc, tim6) = aux9::init();

    // TODO initialize TIM6

    let ms = 50;
    loop {
        for curr in 0..8 {
            let next = (curr + 1) % 8;

            leds[next].on();
            delay(tim6, ms);
            leds[curr].off();
            delay(tim6, ms);
        }
    }
}

The first challenge is to implement the delay function without using any peripheral and the obvious solution is to implement it as a for loop delay:

# #![allow(unused_variables)]
#fn main() {
#[inline(never)]
fn delay(tim6: &tim6::RegisterBlock, ms: u16) {
    for _ in 0..1_000 {}
}
#}

Of course, the above implementation is wrong because it always generates the same delay for any value of ms.

In this section, you'll have to:

• Fix the delay function to generate delays proportional to its input ms.
• Tweak the delay function to make the LED roulette spin at a rate of approximately 5 cycles in 4 seconds (800 milliseconds period).
• The processor inside the microcontroller is clocked at 72 MHz and executes most instructions in one "tick", a cycle of its clock. How many (for) loops do you think the delay function must do to generate a delay of 1 second?
• How many for loops does delay(1000) actually do?
• What happens if compile your program in release mode and run it?

If in the previous section you compiled the program in release mode and actually looked at the disassembly, you probably noticed that the delay function is optimized away and never gets called from within main.

LLVM decided that the function wasn't doing anything worthwhile and just removed it.

There is a way to prevent LLVM from optimizing the for loop delay: add a volatile assembly instruction. Any instruction will do but NOP (No OPeration) is a particular good choice in this case because it has no side effect.

Your for loop delay would become:

# #![allow(unused_variables)]
#fn main() {
#[inline(never)]
fn delay(_tim6: &tim6::RegisterBlock, ms: u16) {
    const K: u16 = 3; // this value needs to be tweaked
    for _ in 0..(K * ms) {
        aux9::nop()
    }
}
#}

And this time delay won't be compiled away by LLVM when you compile your program in release mode:

$ cargo objdump --bin clocks-and-timers --release -- -d -no-show-raw-insn
clocks-and-timers:      file format ELF32-arm-little

Disassembly of section .text:
clocks_and_timers::delay::h711ce9bd68a6328f:
 8000188:       push    {r4, r5, r7, lr}
 800018a:       movs    r4, #0
 800018c:       adds    r4, #1
 800018e:       uxth    r5, r4
 8000190:       bl      #4666
 8000194:       cmp     r5, #150
 8000196:       blo     #-14 <clocks_and_timers::delay::h711ce9bd68a6328f+0x4>
 8000198:       pop     {r4, r5, r7, pc}

Now, test this: Compile the program in debug mode and run it, then compile the program in release mode and run it. What's the difference between them? What do you think is the main cause of the difference? Can you think of a way to make them equivalent or at least more similar again?

I hope that, by now, I have convinced you that for loop delays are a poor way to implement delays.

Now, we'll implement delays using a hardware timer. The basic function of a (hardware) timer is ... to keep precise track of time. A timer is yet another peripheral that's available to the microcontroller; thus it can be controlled using registers.

The microcontroller we are using has several (in fact, more than 10) timers of different kinds (basic, general purpose, and advanced timers) available to it. Some timers have more resolution (number of bits) than others and some can be used for more than just keeping track of time.

We'll be using one of the basic timers: TIM6. This is one of the simplest timers available in our microcontroller. The documentation for basic timers is in the following section:

Section 22 Timers - Page 670 - Reference Manual

Its registers are documented in:

Section 22.4.9 TIM6/TIM7 register map - Page 682 - Reference Manual

The registers we'll be using in this section are:

• SR, the status register.
• EGR, the event generation register.
• CNT, the counter register.
• PSC, the prescaler register.
• ARR, the autoreload register.

We'll be using the timer as a one-shot timer. It will sort of work like an alarm clock. We'll set the timer to go off after some amount of time and then we'll wait until the timer goes off. The documentation refers to this mode of operation as one pulse mode.

Here's a description of how a basic timer works when configured in one pulse mode:

• The counter is enabled by the user (CR1.CEN = 1).
• The CNT register resets its value to zero and, on each tick, its value gets incremented by one.
• Once the CNT register has reached the value of the ARR register, the counter will be disabled by hardware (CR1.CEN = 0) and an update event will be raised (SR.UIF = 1).

TIM6 is driven by the APB1 clock, whose frequency doesn't have to necessarily match the processor frequency. That is, the APB1 clock could be running faster or slower. The default, however, is that both APB1 and the processor are clocked at 8 MHz.

The tick mentioned in the functional description of the one pulse mode is not the same as one tick of the APB1 clock. The CNT register increases at a frequency of apb1 / (psc + 1) times per second, where apb1 is the frequency of the APB1 clock and psc is the value of the prescaler register, PSC.

As with every other peripheral, we'll have to initialize this timer before we can use it. And just as in the previous section, initialization is going to involve two steps: powering up the timer and then configuring it.

Powering up the timer is easy: We just have to set TIM6EN bit to 1. This bit is in the APB1ENR register of the RCC register block.

# #![allow(unused_variables)]
#fn main() {
    // Power on the TIM6 timer
    rcc.apb1enr.modify(|_, w| w.tim6en().set_bit());
#}

The configuration part is slightly more elaborate.

First, we'll have to configure the timer to operate in one pulse mode.

# #![allow(unused_variables)]
#fn main() {
    // OPM Select one pulse mode
    // CEN Keep the counter disabled for now
    tim6.cr1.write(|w| w.opm().set_bit().cen().clear_bit());
#}

Then, we'll like to have the CNT counter operate at a frequency of 1 KHz because our delay function takes a number of milliseconds as arguments and 1 KHz produces a 1 millisecond period. For that we'll have to configure the prescaler.

# #![allow(unused_variables)]
#fn main() {
    // Configure the prescaler to have the counter operate at 1 KHz
    tim6.psc.write(|w| w.psc().bits(psc));
#}

I'm going to let you figure out the value of the prescaler, psc. Remember that the frequency of the counter is apb1 / (psc + 1) and that apb1 is 8 MHz.

The timer should now be properly initialized. All that's left is to implement the delay function using the timer.

First thing we have to do is set the autoreload register (ARR) to make the timer go off in ms milliseconds. Because the counter operates at 1 KHz, the autoreload value will be the same as ms.

# #![allow(unused_variables)]
#fn main() {
    // Set the timer to go off in `ms` ticks
    // 1 tick = 1 ms
    tim6.arr.write(|w| w.arr().bits(ms));
#}

Next, we need to enable the counter. It will immediately start counting.

# #![allow(unused_variables)]
#fn main() {
    // CEN: Enable the counter
    tim6.cr1.modify(|_, w| w.cen().set_bit());
#}

Now we need to wait until the counter reaches the value of the autoreload register, ms, then we'll know that ms milliseconds have passed. That condition is known as an update event and its indicated by the UIF bit of the status register (SR).

# #![allow(unused_variables)]
#fn main() {
    // Wait until the alarm goes off (until the update event occurs)
    while !tim6.sr.read().uif().bit_is_set() {}
#}

This pattern of just waiting until some condition is met, in this case that UIF becomes 1, is known as busy waiting and you'll see it a few more times in this text :-).

Finally, we must clear (set to 0) this UIF bit. If we don't, next time we enter the delay function we'll think the update event has already happened and skip over the busy waiting part.

# #![allow(unused_variables)]
#fn main() {
    // Clear the update event flag
    tim6.sr.modify(|_, w| w.uif().clear_bit());
#}

Now, put this all together and check if it works as expected.

#![no_main]
#![no_std]

use aux9::{entry, tim6};

#[inline(never)]
fn delay(tim6: &tim6::RegisterBlock, ms: u16) {
    // Set the timer to go off in `ms` ticks
    // 1 tick = 1 ms
    tim6.arr.write(|w| w.arr().bits(ms));

    // CEN: Enable the counter
    tim6.cr1.modify(|_, w| w.cen().set_bit());

    // Wait until the alarm goes off (until the update event occurs)
    while !tim6.sr.read().uif().bit_is_set() {}

    // Clear the update event flag
    tim6.sr.modify(|_, w| w.uif().clear_bit());
}

#[entry]
fn main() -> ! {
    let (mut leds, rcc, tim6) = aux9::init();

    // Power on the TIM6 timer
    rcc.apb1enr.modify(|_, w| w.tim6en().set_bit());

    // OPM Select one pulse mode
    // CEN Keep the counter disabled for now
    tim6.cr1.write(|w| w.opm().set_bit().cen().clear_bit());

    // Configure the prescaler to have the counter operate at 1 KHz
    // APB1_CLOCK = 8 MHz
    // PSC = 7999
    // 8 MHz / (7999 + 1) = 1 KHz
    // The counter (CNT) will increase on every millisecond
    tim6.psc.write(|w| w.psc().bits(7_999));

    let ms = 50;
    loop {
        for curr in 0..8 {
            let next = (curr + 1) % 8;

            leds[next].on();
            delay(tim6, ms);
            leds[curr].off();
            delay(tim6, ms);
        }
    }
}

This is what we'll be using. I hope your computer has one!

Nah, don't worry. This connector, the DE-9, went out of fashion on PCs quite some time ago; it got replaced by the Universal Serial Bus (USB). We won't be dealing with the DE-9 connector itself but with the communication protocol that this cable is/was usually used for.

So what's this serial communication? It's an asynchronous communication protocol where two devices exchange data serially, as in one bit at a time, using two data lines (plus a common ground). The protocol is asynchronous in the sense that neither of the shared lines carries a clock signal. Instead both parties must agree on how fast data will be sent along the wire before the communication occurs. This protocol allows duplex communication as data can be sent from A to B and from B to A simultaneously.

We'll be using this protocol to exchange data between the microcontroller and your computer. In contrast to the ITM protocol we have used before, with the serial communication protocol you can send data from your computer to the microcontroller.

The next practical question you probably want to ask is: How fast can we send data through this protocol?

This protocol works with frames. Each frame has one start bit, 5 to 9 bits of payload (data) and 1 to 2 stop bits. The speed of the protocol is known as baud rate and it's quoted in bits per second (bps). Common baud rates are: 9600, 19200, 38400, 57600 and 115200 bps.

To actually answer the question: With a common configuration of 1 start bit, 8 bits of data, 1 stop bit and a baud rate of 115200 bps one can, in theory, send 11,520 frames per second. Since each one frame carries a byte of data that results in a data rate of 11.52 KB/s. In practice, the data rate will probably be lower because of processing times on the slower side of the communication (the microcontroller).

Today's computers don't support the serial communication protocol. So you can't directly connect your computer to the microcontroller. But that's where the serial module comes in. This module will sit between the two and expose a serial interface to the microcontroller and an USB interface to your computer. The microcontroller will see your computer as another serial device and your computer will see the microcontroller as a virtual serial device.

Now, let's get familiar with the serial module and the serial communication tools that your OS offers. Pick a route:

• *nix
• Windows

With newer revisions, if you connect the discovery board to your computer you should see a new TTY device appear in /dev.

$ # Linux
$ dmesg | tail | grep -i tty
[13560.675310] cdc_acm 1-1.1:1.2: ttyACM0: USB ACM device

This is the USB <-> Serial device. On Linux, it's named tty* (usually ttyACM* or ttyUSB*).

If you don't see the device appear then you probably have an older revision of the board; check the next section, which contains instructions for older revisions. If you do have a newer revision skip the next section and move to the "minicom" section.

Connect the serial module to your computer and let's find out what name the OS assigned to it.

NOTE On macs, the USB device will named like this: /dev/cu.usbserial-*. You won't find it using dmesg, instead use ls -l /dev | grep cu.usb and adjust the following commands accordingly!

$ dmesg | grep -i tty
(..)
[  +0.000155] usb 3-2: FTDI USB Serial Device converter now attached to ttyUSB0

But what's this ttyUSB0 thing? It's a file of course! Everything is a file in *nix:

$ ls -l /dev/ttyUSB0
crw-rw-rw- 1 root uucp 188, 0 Oct 27 00:00 /dev/ttyUSB0

NOTE if the permissions above is crw-rw----, the udev rules have not been set correctly see udev rules

You can send out data by simply writing to this file:

$ echo 'Hello, world!' > /dev/ttyUSB0

You should see the TX (red) LED on the serial module blink, just once and very fast!

Dealing with serial devices using echo is far from ergonomic. So, we'll use the program minicom to interact with the serial device using the keyboard.

We must configure minicom before we use it. There are quite a few ways to do that but we'll use a .minirc.dfl file in the home directory. Create a file in ~/.minirc.dfl with the following contents:

$ cat ~/.minirc.dfl
pu baudrate 115200
pu bits 8
pu parity N
pu stopbits 1
pu rtscts No
pu xonxoff No

NOTE Make sure this file ends in a newline! Otherwise, minicom will fail to read it.

That file should be straightforward to read (except for the last two lines), but nonetheless let's go over it line by line:

• pu baudrate 115200. Sets baud rate to 115200 bps.
• pu bits 8. 8 bits per frame.
• pu parity N. No parity check.
• pu stopbits 1. 1 stop bit.
• pu rtscts No. No hardware control flow.
• pu xonxoff No. No software control flow.

Once that's in place, we can launch minicom.

$ # NOTE you may need to use a different device here
$ minicom -D /dev/ttyACM0 -b 115200

This tells minicom to open the serial device at /dev/ttyACM0 and set its baud rate to 115200. A text-based user interface (TUI) will pop out.

You can now send data using the keyboard! Go ahead and type something. Note that the TUI will not echo back what you type but, if you are using an external module, you may see some LED on the module blink with each keystroke.

minicom exposes commands via keyboard shortcuts. On Linux, the shortcuts start with Ctrl+A. On mac, the shortcuts start with the Meta key. Some useful commands below:

• Ctrl+A + Z. Minicom Command Summary
• Ctrl+A + C. Clear the screen
• Ctrl+A + X. Exit and reset
• Ctrl+A + Q. Quit with no reset

NOTE mac users: In the above commands, replace Ctrl+A with Meta.

Start by unplugging your discovery board.

Before plugging the discovery board or the serial module, run the following command on the terminal:

$ mode

It will print a list of devices that are connected to your computer. The ones that start with COM in their names are serial devices. This is the kind of device we'll be working with. Take note of all the COM ports mode outputs before plugging the serial module.

Now, plug the discovery board and run the mode command again. If you see a new COM port appear on the list then you have a newer revision of the discovery and that's the COM port assigned to the serial functionality on the discovery. You can skip the next paragraph.

If you didn't get a new COM port then you probably have an older revision of the discovery. Now plug the serial module; you should see new COM port appear; that's the COM port of the serial module.

Now launch putty. A GUI will pop out.

On the starter screen, which should have the "Session" category open, pick "Serial" as the "Connection type". On the "Serial line" field enter the COM device you got on the previous step, for example COM3.

Next, pick the "Connection/Serial" category from the menu on the left. On this new view, make sure that the serial port is configured as follows:

• "Speed (baud)": 115200
• "Data bits": 8
• "Stop bits": 1
• "Parity": None
• "Flow control": None

Finally, click the Open button. A console will show up now:

If you type on this console, the TX (red) LED on the Serial module should blink. Each key stroke should make the LED blink once. Note that the console won't echo back what you type so the screen will remain blank.

We've tested sending data. It's time to test receiving it. Except that there's no other device that can send us some data ... or is there?

Enter: loopbacks

You can send data to yourself! Not very useful in production but very useful for debugging.

Connect the TXO and the RXI pins of the serial module together using a male to male jumper wire as shown above.

Now enter some text into minicom/PuTTY and observe. What happens?

You should see three things:

• As before, the TX (red) LED blinks on each key press.
• But now the RX (green) LED blinks on each key press as well! This indicates that the serial module is receiving some data; the one it just sent.
• Finally, on the minicom/PuTTY console, you should see that what you type echoes back to the console.

If you have a newer revision of the board you can set up a loopback by shorting the PC4 and PC5 pins using a female to female jumper wire, like you did for the SWO pin.

You should now be able to send data to yourself.

Now try to enter some text into minicom/PuTTY and observe.

NOTE: To rule out the possibility of the existing firmware doing weird things to the serial pins (PC4 and PC5) we recommend holding the reset button while you enter text into minicom/PuTTY.

If all is working you should see what you type echoed back to minicom/PuTTY console.

Now that you are familiar with sending and receiving data over serial port using minicom/PuTTY, let's make your microcontroller and your computer talk!

The microcontroller has a peripheral called USART, which stands for Universal Synchronous/Asynchronous Receiver/Transmitter. This peripheral can be configured to work with several communication protocols like the serial communication protocol.

Throughout this chapter, we'll use serial communication to exchange information between the microcontroller and your computer. But before we do that we have to wire up everything.

I mentioned before that this protocol involves two data lines: TX and RX. TX stands for transmitter and RX stands for receiver. Transmitter and receiver are relative terms though; which line is the transmitter and which line is the receiver depends from which side of the communication you are looking at the lines.

If you have a newer revision of the board and are using the on-board USB <-> Serial functionality then the auxiliary crate will set pin PC4 as the TX line and pin PC5 as the RX line.

Everything is already wired on the board so you don't need to wire anything yourself. You can move on to the next section.

If you are using an external USB <-> Serial module then you will need to enable the adapter feature of the aux11 crate dependency in Cargo.toml.

[dependencies.aux11]
path = "auxiliary"
# enable this if you are going to use an external serial adapter
features = ["adapter"] # <- uncomment this

We'll be using the pin PA9 as the microcontroller's TX line and PA10 as its RX line. In other words, the pin PA9 outputs data onto its wire whereas the pin PA10 listens for data on its wire.

We could have used a different pair of pins as the TX and RX pins. There's a table in page 44 of the Data Sheet that list all the other possible pins we could have used.

The serial module also has TX and RX pins. We'll have to cross these pins: that is connect the microcontroller's TX pin to the serial module's RX pin and the micro's RX pin to the serial module's TX pin. The wiring diagram below shows all the necessary connections.

These are the recommended steps to connect the microcontroller and the serial module:

• Close OpenOCD and itmdump
• Disconnect the USB cables from the F3 and the serial module.
• Connect one of F3 GND pins to the GND pin of the serial module using a female to male (F/M) wire. Preferably, a black one.
• Connect the PA9 pin on the back of the F3 to the RXI pin of the serial module using a F/M wire.
• Connect the PA10 pin on the back of the F3 to the TXO pin of the serial module using a F/M wire.
• Now connect the USB cable to the F3.
• Finally connect the USB cable to the Serial module.
• Re-launch OpenOCD and itmdump

Everything's wired up! Let's proceed to send data back and forth.

Our first task will be to send a single byte from the microcontroller to the computer over the serial connection.

This time, I'm going to provide you with an already initialized USART peripheral. You'll only have to work with the registers that are in charge of sending and receiving data.

Go into the 11-usart directory and let's run the starter code therein. Make sure that you have minicom/PuTTY open.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    // Send a single character
    usart1.tdr.write(|w| w.tdr().bits(u16::from(b'X')));

    loop {}
}

This program writes to the TDR register. This causes the USART peripheral to send one byte of information through the serial interface.

On the receiving end, your computer, you should see show the character X appear on minicom/PuTTY's terminal.

The next task will be to send a whole string from the microcontroller to your computer.

I want you to send the string "The quick brown fox jumps over the lazy dog." from the microcontroller to your computer.

It's your turn to write the program.

Execute your program inside the debugger, statement by statement. What do you see?

Then execute the program again but in one go using the continue command. What happens this time?

Finally, build the program in release mode and, again, run it one go. What happens this time?

If you wrote your program like this:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    // Send a string
    for byte in b"The quick brown fox jumps over the lazy dog.".iter() {
        usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
    }

    loop {}
}

You probably received something like this on your computer when you executed the program compiled in debug mode.

$ # minicom's terminal
(..)
The uic brwn oxjums oer helaz do.

And if you compiled in release mode, you probably only got something like this:

$ # minicom's terminal
(..)
T

What went wrong?

You see, sending bytes over the wire takes a relatively large amount of time. I already did the math so let me quote myself:

With a common configuration of 1 start bit, 8 bits of data, 1 stop bit and a baud rate of 115200 bps one can, in theory, send 11,520 frames per second. Since each one frame carries a byte of data that results in a data rate of 11.52 KB/s

Our pangram has a length of 45 bytes. That means it's going to take, at least, 3,900 microseconds (45 bytes / (11,520 bytes/s) = 3,906 us) to send the string. The processor is working at 8 MHz, where executing an instruction takes 125 nanoseconds, so it's likely going to be done with the for loop in less than 3,900 microseconds.

We can actually time how long it takes to execute the for loop. aux11::init() returns a MonoTimer (monotonic timer) value that exposes an Instant API that's similar to the one in std::time.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, mut itm) = aux11::init();

    let instant = mono_timer.now();
    // Send a string
    for byte in b"The quick brown fox jumps over the lazy dog.".iter() {
        usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
    }
    let elapsed = instant.elapsed(); // in ticks

    iprintln!(
        &mut itm.stim[0],
        "`for` loop took {} ticks ({} us)",
        elapsed,
        elapsed as f32 / mono_timer.frequency().0 as f32 * 1e6
    );

    loop {}
}

In debug mode, I get:

$ # itmdump terminal
(..)
`for` loop took 22415 ticks (2801.875 us)

This is less than 3,900 microseconds but it's not that far off and that's why only a few bytes of information are lost.

In conclusion, the processor is trying to send bytes at a faster rate than what the hardware can actually handle and this results in data loss. This condition is known as buffer overrun.

How do we avoid this? The status register (ISR) has a flag, TXE, that indicates if it's "safe" to write to the TDR register without incurring in data loss.

Let's use that to slowdown the processor.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, mut itm) = aux11::init();

    let instant = mono_timer.now();
    // Send a string
    for byte in b"The quick brown fox jumps over the lazy dog.".iter() {
        // wait until it's safe to write to TDR
        while usart1.isr.read().txe().bit_is_clear() {} // <- NEW!

        usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
    }
    let elapsed = instant.elapsed(); // in ticks

    iprintln!(
        &mut itm.stim[0],
        "`for` loop took {} ticks ({} us)",
        elapsed,
        elapsed as f32 / mono_timer.frequency().0 as f32 * 1e6
    );

    loop {}
}

This time, running the program in debug or release mode should result in a complete string on the receiving side.

$ # minicom/PuTTY's console
(..)
The quick brown fox jumps over the lazy dog.

The timing of the for loop should be closer to the theoretical 3,900 microseconds as well. The timing below is for the debug version.

$ # itmdump terminal
(..)
`for` loop took 30499 ticks (3812.375 us)

For the next exercise, we'll implement the uprint! family of macros. Your goal is to make this line of code work:

# #![allow(unused_variables)]
#fn main() {
    uprintln!(serial, "The answer is {}", 40 + 2);
#}

Which must send the string "The answer is 42" through the serial interface.

How do we go about that? It's informative to look into the std implementation of println!.

# #![allow(unused_variables)]
#fn main() {
// src/libstd/macros.rs
macro_rules! print {
    ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
}
#}

Looks simple so far. We need the built-in format_args! macro (it's implemented in the compiler so we can't see what it actually does). We'll have to use that macro in the exact same way. What does this _print function do?

# #![allow(unused_variables)]
#fn main() {
// src/libstd/io/stdio.rs
pub fn _print(args: fmt::Arguments) {
    let result = match LOCAL_STDOUT.state() {
        LocalKeyState::Uninitialized |
        LocalKeyState::Destroyed => stdout().write_fmt(args),
        LocalKeyState::Valid => {
            LOCAL_STDOUT.with(|s| {
                if s.borrow_state() == BorrowState::Unused {
                    if let Some(w) = s.borrow_mut().as_mut() {
                        return w.write_fmt(args);
                    }
                }
                stdout().write_fmt(args)
            })
        }
    };
    if let Err(e) = result {
        panic!("failed printing to stdout: {}", e);
    }
}
#}

That looks complicated but the only part we are interested in is: w.write_fmt(args) and stdout().write_fmt(args). What print! ultimately does is call the fmt::Write::write_fmt method with the output of format_args! as its argument.

Luckily we don't have to implement the fmt::Write::write_fmt method either because it's a default method. We only have to implement the fmt::Write::write_str method.

Let's do that.

This is what the macro side of the equation looks like. What's left to be done by you is provide the implementation of the write_str method.

Above we saw that Write is in std::fmt. We don't have access to std but Write is also available in core::fmt.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use core::fmt::{self, Write};

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln, usart1};

macro_rules! uprint {
    ($serial:expr, $($arg:tt)*) => {
        $serial.write_fmt(format_args!($($arg)*)).ok()
    };
}

macro_rules! uprintln {
    ($serial:expr, $fmt:expr) => {
        uprint!($serial, concat!($fmt, "\n"))
    };
    ($serial:expr, $fmt:expr, $($arg:tt)*) => {
        uprint!($serial, concat!($fmt, "\n"), $($arg)*)
    };
}

struct SerialPort {
    usart1: &'static mut usart1::RegisterBlock,
}

impl fmt::Write for SerialPort {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        // TODO implement this
        // hint: this will look very similar to the previous program
        Ok(())
    }
}

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    let mut serial = SerialPort { usart1 };

    uprintln!(serial, "The answer is {}", 40 + 2);

    loop {}
}

So far we have sending data from the microcontroller to your computer. It's time to try the opposite: receiving data from your computer.

There's a RDR register that will be filled with the data that comes from the RX line. If we read that register, we'll retrieve the data that the other side of the channel sent. The question is: How do we know that we have received (new) data? The status register, ISR, has a bit for that purpose: RXNE. We can just busy wait on that flag.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    loop {
        // Wait until there's data available
        while usart1.isr.read().rxne().bit_is_clear() {}

        // Retrieve the data
        let _byte = usart1.rdr.read().rdr().bits() as u8;

        aux11::bkpt();
    }
}

Let's try this program! Let it run free using continue and then type a single character in minicom/PuTTY's console. What happens? What are the contents of the _byte variable?

(gdb) continue
Continuing.

Program received signal SIGTRAP, Trace/breakpoint trap.
0x8003d48 in __bkpt ()

(gdb) finish
Run till exit from #0  0x8003d48 in __bkpt ()
usart::main () at src/11-usart/src/main.rs:19
19              aux11::bkpt();

(gdb) p/c _byte
$1 = 97 'a'

Let's merge transmission and reception into a single program and write an echo server. An echo server sends back to the client the same text it sent. For this application, the microcontroller will be the server and you and your computer will be the client.

This should be straightforward to implement. (hint: do it byte by byte)

Alright, next let's make the server more interesting by having it respond to the client with the reverse of the text that they sent. The server will respond to the client every time they press the ENTER key. Each server response will be in a new line.

This time you'll need a buffer; you can use heapless::Vec. Here's the starter code:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};
use heapless::{consts, Vec};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    // A buffer with 32 bytes of capacity
    let mut buffer: Vec<u8, consts::U32> = Vec::new();

    loop {
        buffer.clear();

        // TODO Receive a user request. Each user request ends with ENTER
        // NOTE `buffer.push` returns a `Result`. Handle the error by responding
        // with an error message.

        // TODO Send back the reversed string
    }
}

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};
use heapless::{consts, Vec};

#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();

    // A buffer with 32 bytes of capacity
    let mut buffer: Vec<u8, consts::U32> = Vec::new();

    loop {
        buffer.clear();

        loop {
            while usart1.isr.read().rxne().bit_is_clear() {}
            let byte = usart1.rdr.read().rdr().bits() as u8;

            if buffer.push(byte).is_err() {
                // buffer full
                for byte in b"error: buffer full\n\r" {
                    while usart1.isr.read().txe().bit_is_clear() {}
                    usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
                }

                break;
            }

            // Carriage return
            if byte == 13 {
                // Respond
                for byte in buffer.iter().rev().chain(&[b'\n', b'\r']) {
                    while usart1.isr.read().txe().bit_is_clear() {}
                    usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
                }

                break;
            }
        }
    }
}

It's time to get rid of some wires. Serial communication can not only be emulated on top of the USB protocol; it can also be emulated on top of the Bluetooth protocol. This serial over Bluetooth protocol is known as RFCOMM.

Before we use the Bluetooth module with the microcontroller, let's first interact with it using minicom/PuTTY.

The first thing we'll need to do is: turn on the Bluetooth module. We'll have to share some of the F3 power to it using the following connection:

The recommend steps to wire this up are:

• Close OpenOCD and itmdump
• Disconnect the USB cables from the F3 and the serial module.
• Connect F3's GND pin to the Bluetooth's GND pin using a female to female (F/F) wire. Preferably, a black one.
• Connect F3's 5V pin to the Bluetooth's VCC pin using a F/F wire. Preferably, a red one.
• Then, connect the USB cable back to the F3.
• Re-launch OpenOCD and itmdump

Two LEDs, a blue one and a red one, on the Bluetooth module should start blinking right after you power on the F3 board.

Next thing to do is pair your computer and the Bluetooth module. AFAIK, Windows and mac users can simply use their OS default Bluetooth manager to do the pairing. The Bluetooth module default pin is 1234.

Linux users will have to follow (some of) these instructions.

If you have a graphical Bluetooth manager, you can use that to pair your computer to the Bluetooth module and skip most of these steps. You'll probably still have to this step though.

First, your computer's Bluetooth transceiver may be OFF. Check its status with hciconfig and turn it ON if necessary:

$ hciconfig
hci0:   Type: Primary  Bus: USB
        BD Address: 68:17:29:XX:XX:XX  ACL MTU: 310:10  SCO MTU: 64:8
        DOWN  <--
        RX bytes:580 acl:0 sco:0 events:31 errors:0
        TX bytes:368 acl:0 sco:0 commands:30 errors:0

$ sudo hciconfig hci0 up

$ hciconfig
hci0:   Type: Primary  Bus: USB
        BD Address: 68:17:29:XX:XX:XX  ACL MTU: 310:10  SCO MTU: 64:8
        UP RUNNING  <--
        RX bytes:1190 acl:0 sco:0 events:67 errors:0
        TX bytes:1072 acl:0 sco:0 commands:66 errors:0

Then you need to launch the BlueZ (Bluetooth) daemon:

• On systemd based Linux distributions, use:

$ sudo systemctl start bluetooth

• On Ubuntu (or upstart based Linux distributions), use:

$ sudo /etc/init.d/bluetooth start

You may also need to unblock your Bluetooth, depending on what rfkill list says:

$ rfkill list
9: hci0: Bluetooth
        Soft blocked: yes # <--
        Hard blocked: no

$ sudo rfkill unblock bluetooth

$ rfkill list
9: hci0: Bluetooth
        Soft blocked: no  # <--
        Hard blocked: no

$ hcitool scan
Scanning ...
        20:16:05:XX:XX:XX       Ferris
$ #                             ^^^^^^

$ bluetoothctl
[bluetooth]# scan on
[bluetooth]# agent on
[bluetooth]# pair 20:16:05:XX:XX:XX
Attempting to pair with 20:16:05:XX:XX:XX
[CHG] Device 20:16:05:XX:XX:XX Connected: yes
Request PIN code
[agent] Enter PIN code: 1234

We'll create a device file for our Bluetooth module in /dev. Then we'll be able to use it just like we used /dev/ttyUSB0.

$ sudo rfcomm bind 0 20:16:05:XX:XX:XX

Because we used 0 as an argument to bind, /dev/rfcomm0 will be the device file assigned to our Bluetooth module.

You can release (destroy) the device file at any time with the following command:

$ # Don't actually run this command right now!
$ sudo rfcomm release 0

After pairing your computer to the Bluetooth module, your OS should have created a device file / COM port for you. On Linux, it should be /dev/rfcomm*; on mac, it should be /dev/cu.*; and on Windows, it should be a new COM port.

We can now test the Bluetooth module with minicom/PuTTY. Because this module doesn't have LED indicators for the transmission and reception events like the serial module did, we'll test the module using a loopback connection:

Just connect the module's TXD pin to its RXD pin using a F/F wire.

Now, connect to the device using minicom/PuTTY:

$ minicom -D /dev/rfcomm0

Upon connecting, the blinking pattern of the Bluetooth module should change to: long pause then blink twice quickly.

Typing inside minicom/PuTTY terminal should echo back what you type.

The Bluetooth module and the F3 need to be configured to communicate at the same baud rate. The tutorial code initializes the UART1 serial device to a baud rate of 115200. The HC-05 Bluetooth module is configured at a baud rate of 9600 by default.

The Bluetooth module supports an AT mode that allows you to examine and change its configuration and settings. To utilize the AT mode, connect the Bluetooth module to the F3 and FTDI as shown in the following diagram.

Recommended steps to enter AT mode:

• Disconnect the F3 and FTDI from your computer.
• Connect F3's GND pin to the Bluetooth's GND pin using a Female/Female (F/F) wire (preferably, a black one).
• Connect F3's 5V pin to the Bluetooth's VCC pin using a F/F wire (preferably, a red one).
• Connect the FTDI RXI pin to the Bluetooth's TXD pin using a Female/Male (F/M) wire.
• Connect the FTDI TXO pin to the Bluetooth's RXD pin using a Female/Male (F/M) wire.
• Now connect the FTDI to your computer via USB cable.
• Next connect the F3 to your computer via USB cable while simultaneously pressing and holding the button on the Bluetooth module (kinda tricky).
• Now, release the button and the Bluetooth module will enter AT mode. You can confirm this by observing that the red LED on the Bluetooth module is blinking in a slow pattern (approx 1-2 seconds on/off).

The AT mode always operates at a baud rate of 38400, so configure your terminal program for that baud rate and connect to the FTDI device.

When your serial connection is established, you may get a bunch of ERROR: (0) repeatedly being displayed. If this happens, just hit ENTER to stop the errors.

$ at
OK
OK
(etc...)

Answers OK repeatedly until you hit ENTER again.

$ at+name=ferris
OK

at+uart?
+UART:9600,0,0
OK
+UART:9600,0,0
OK
(etc ...)

$ at+uart=115200,0,0
OK

Now that we verify that the Bluetooth module works with minicom/PuTTY, let's connect it to the microcontroller:

Recommended steps to wire this up:

• Close OpenOCD and itmdump.
• Disconnect the F3 from your computer.
• Connect F3's GND pin to the module's GND pin using a female to female (F/F) wire (preferably, a black one).
• Connect F3's 5V pin to the module's VCC pin using a F/F wire (preferably, a red one).
• Connect the PA9 (TX) pin on the back of the F3 to the Bluetooth's RXD pin using a F/F wire.
• Connect the PA10 (RX) pin on the back of the F3 to the Bluetooth's TXD pin using a F/F wire.
• Now connect the F3 and your computer using an USB cable.
• Re-launch OpenOCD and itmdump.

And that's it! You should be able to run all the programs you wrote in section 11 without modification! Just make sure you open the right serial device / COM port.

NOTE If you are having trouble communicating with the bluetooth device, you may need to initialize USART1 with a lower baud rate. Lowering it from 115,200 bps to 9,600 bps might help, as described here

We just saw the serial communication protocol. It's a widely used protocol because it's very simple and this simplicity makes it easy to implement on top of other protocols like Bluetooth and USB.

However, it's simplicity is also a downside. More elaborated data exchanges, like reading a digital sensor, would require the sensor vendor to come up with another protocol on top of it.

(Un)Luckily for us, there are plenty of other communication protocols in the embedded space. Some of them are widely used in digital sensors.

The F3 board we are using has three motion sensors in it: an accelerometer, a magnetometer and gyroscope. The accelerometer and magnetometer are packaged in a single component and can be accessed via an I2C bus.

I2C stands for Inter-Integrated Circuit and is a synchronous serial communication protocol. It uses two lines to exchange data: a data line (SDA) and a clock line (SCL). Because a clock line is used to synchronize the communication, this is a synchronous protocol.

This protocol uses a master slave model where the master is the device that starts and drives the communication with a slave device. Several devices, both masters and slaves, can be connected to the same bus at the same time. A master device can communicate with a specific slave device by first broadcasting its address to the bus. This address can be 7 bits or 10 bits long. Once a master has started a communication with a slave, no other device can make use of the bus until the master stops the communication.

The clock line determines how fast data can be exchanged and it usually operates at a frequency of 100 KHz (standard mode) or 400 KHz (fast mode).

The I2C protocol is more elaborated than the serial communication protocol because it has to support communication between several devices. Let's see how it works using examples:

If the master wants to send data to the slave:

1. Master: Broadcast START
2. M: Broadcast slave address (7 bits) + the R/W (8th) bit set to WRITE
3. Slave: Responds ACK (ACKnowledgement)
4. M: Send one byte
5. S: Responds ACK
6. Repeat steps 4 and 5 zero or more times
7. M: Broadcast STOP OR (broadcast RESTART and go back to (2))

NOTE The slave address could have been 10 bits instead of 7 bits long. Nothing else would have changed.

If the master wants to read data from the slave:

1. M: Broadcast START
2. M: Broadcast slave address (7 bits) + the R/W (8th) bit set to READ
3. S: Responds with ACK
4. S: Send byte
5. M: Responds with ACK
6. Repeat steps 4 and 5 zero or more times
7. M: Broadcast STOP OR (broadcast RESTART and go back to (2))

NOTE The slave address could have been 10 bits instead of 7 bits long. Nothing else would have changed.

Two of the sensors in the F3, the magnetometer and the accelerometer, are packaged in a single component: the LSM303DLHC integrated circuit. These two sensors can be accessed via an I2C bus. Each sensor behaves like an I2C slave and has a different address.

Each sensor has its own memory where it stores the results of sensing its environment. Our interaction with these sensors will mainly involve reading their memory.

The memory of these sensors is modeled as byte addressable registers. These sensors can be configured too; that's done by writing to their registers. So, in a sense, these sensors are very similar to the peripherals inside the microcontroller. The difference is that their registers are not mapped into the microcontrollers' memory. Instead, their registers have to be accessed via the I2C bus.

The main source of information about the LSM303DLHC is its Data Sheet. Read through it to see how one can read the sensors' registers. That part is in:

Section 5.1.1 I2C Operation - Page 20 - LSM303DLHC Data Sheet

The other part of the documentation relevant to this book is the description of the registers. That part is in:

Section 7 Register description - Page 25 - LSM303DLHC Data Sheet

Let's put all that theory into practice!

Just like with the USART peripheral, I've taken care of initializing everything before you reach main so you'll only have to deal with the following registers:

• CR2. Control register 2.
• ISR. Interrupt and status register.
• TXDR. Transmit data register.
• RXDR. Receive data register.

These registers are documented in the following section of the Reference Manual:

Section 28.7 I2C registers - Page 868 - Reference Manual

We'll be using the I2C1 peripheral in conjunction with pins PB6 (SCL) and PB7 (SDA).

You won't have to wire anything this time because the sensor is on the board and it's already connected to the microcontroller. However, I would recommend that you disconnect the serial / Bluetooth module from the F3 to make it easier to manipulate. Later on, we'll be moving the board around quite a bit.

Your task is to write a program that reads the contents of the magnetometer's IRA_REG_M register. This register is read only and always contains the value 0b01001000.

The microcontroller will be taking the role of the I2C master and the magnetometer inside the LSM303DLHC will be the I2C slave.

Here's the starter code. You'll have to implement the TODOs.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux14::{entry, iprint, iprintln, prelude::*};

// Slave address
const MAGNETOMETER: u8 = 0b001_1110;

// Addresses of the magnetometer's registers
const OUT_X_H_M: u8 = 0x03;
const IRA_REG_M: u8 = 0x0A;

#[entry]
fn main() -> ! {
    let (i2c1, _delay, mut itm) = aux14::init();

    // Stage 1: Send the address of the register we want to read to the
    // magnetometer
    {
        // TODO Broadcast START

        // TODO Broadcast the MAGNETOMETER address with the R/W bit set to Write

        // TODO Send the address of the register that we want to read: IRA_REG_M
    }

    // Stage 2: Receive the contents of the register we asked for
    let byte = {
        // TODO Broadcast RESTART

        // TODO Broadcast the MAGNETOMETER address with the R/W bit set to Read

        // TODO Receive the contents of the register

        // TODO Broadcast STOP
        0
    };

    // Expected output: 0x0A - 0b01001000
    iprintln!(&mut itm.stim[0], "0x{:02X} - 0b{:08b}", IRA_REG_M, byte);

    loop {}
}

To give you some extra help, these are the exact bitfields you'll be working with:

• CR2: SADD1, RD_WRN, NBYTES, START, AUTOEND
• ISR: TXIS, RXNE, TC
• TXDR: TXDATA
• RXDR: RXDATA

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux14::{entry, iprint, iprintln, prelude::*};

// Slave address
const MAGNETOMETER: u8 = 0b001_1110;

// Addresses of the magnetometer's registers
const OUT_X_H_M: u8 = 0x03;
const IRA_REG_M: u8 = 0x0A;

#[entry]
fn main() -> ! {
    let (i2c1, _delay, mut itm) = aux14::init();

    // Stage 1: Send the address of the register we want to read to the
    // magnetometer
    {
        // Broadcast START
        // Broadcast the MAGNETOMETER address with the R/W bit set to Write
        i2c1.cr2.write(|w| {
            w.start().set_bit();
            w.sadd1().bits(MAGNETOMETER);
            w.rd_wrn().clear_bit();
            w.nbytes().bits(1);
            w.autoend().clear_bit()
        });

        // Wait until we can send more data
        while i2c1.isr.read().txis().bit_is_clear() {}

        // Send the address of the register that we want to read: IRA_REG_M
        i2c1.txdr.write(|w| w.txdata().bits(IRA_REG_M));

        // Wait until the previous byte has been transmitted
        while i2c1.isr.read().tc().bit_is_clear() {}
    }

    // Stage 2: Receive the contents of the register we asked for
    let byte = {
        // Broadcast RESTART
        // Broadcast the MAGNETOMETER address with the R/W bit set to Read
        i2c1.cr2.modify(|_, w| {
            w.start().set_bit();
            w.nbytes().bits(1);
            w.rd_wrn().set_bit();
            w.autoend().set_bit()
        });

        // Wait until we have received the contents of the register
        while i2c1.isr.read().rxne().bit_is_clear() {}

        // Broadcast STOP (automatic because of `AUTOEND = 1`)

        i2c1.rxdr.read().rxdata().bits()
    };

    // Expected output: 0x0A - 0b01001000
    iprintln!(&mut itm.stim[0], "0x{:02X} - 0b{:08b}", IRA_REG_M, byte);

    loop {}
}

Reading the IRA_REG_M register was a good test of our understanding of the I2C protocol but that register contains uninteresting information.

This time, we'll read the registers of the magnetometer that actually expose the sensor readings. Six contiguous registers are involved and they start with OUT_X_H_M at address 0x03.

We'll modify our previous program to read these six registers. Only a few modifications are needed.

We'll need to change the address we request from the magnetometer from IRA_REG_M to OUT_X_H_M.

# #![allow(unused_variables)]
#fn main() {
    // Send the address of the register that we want to read: OUT_X_H_M
    i2c1.txdr.write(|w| w.txdata().bits(OUT_X_H_M));
#}

We'll have to request the slave for six bytes rather than just one.

# #![allow(unused_variables)]
#fn main() {
    // Broadcast RESTART
    // Broadcast the MAGNETOMETER address with the R/W bit set to Read
    i2c1.cr2.modify(|_, w| {
        w.start().set_bit();
        w.nbytes().bits(6);
        w.rd_wrn().set_bit();
        w.autoend().set_bit()
    });
#}

And fill a buffer rather than read just one byte:

# #![allow(unused_variables)]
#fn main() {
    let mut buffer = [0u8; 6];
    for byte in &mut buffer {
        // Wait until we have received the contents of the register
        while i2c1.isr.read().rxne().bit_is_clear() {}

        *byte = i2c1.rxdr.read().rxdata().bits();
    }

    // Broadcast STOP (automatic because of `AUTOEND = 1`)
#}

Putting it all together inside a loop alongside a delay to reduce the data throughput:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux14::{entry, iprint, iprintln, prelude::*};

// Slave address
const MAGNETOMETER: u8 = 0b001_1110;

// Addresses of the magnetometer's registers
const OUT_X_H_M: u8 = 0x03;
const IRA_REG_M: u8 = 0x0A;

#[entry]
fn main() -> ! {
    let (i2c1, mut delay, mut itm) = aux14::init();

    loop {
        // Broadcast START
        // Broadcast the MAGNETOMETER address with the R/W bit set to Write
        i2c1.cr2.write(|w| {
            w.start().set_bit();
            w.sadd1().bits(MAGNETOMETER);
            w.rd_wrn().clear_bit();
            w.nbytes().bits(1);
            w.autoend().clear_bit()
        });

        // Wait until we can send more data
        while i2c1.isr.read().txis().bit_is_clear() {}

        // Send the address of the register that we want to read: OUT_X_H_M
        i2c1.txdr.write(|w| w.txdata().bits(OUT_X_H_M));

        // Wait until the previous byte has been transmitted
        while i2c1.isr.read().tc().bit_is_clear() {}

        // Broadcast RESTART
        // Broadcast the MAGNETOMETER address with the R/W bit set to Read
        i2c1.cr2.modify(|_, w| {
            w.start().set_bit();
            w.nbytes().bits(6);
            w.rd_wrn().set_bit();
            w.autoend().set_bit()
        });

        let mut buffer = [0u8; 6];
        for byte in &mut buffer {
            // Wait until we have received something
            while i2c1.isr.read().rxne().bit_is_clear() {}

            *byte = i2c1.rxdr.read().rxdata().bits();
        }
        // Broadcast STOP (automatic because of `AUTOEND = 1`)

        iprintln!(&mut itm.stim[0], "{:?}", buffer);

        delay.delay_ms(1_000_u16);
    }
}

If you run this, you should printed in the itmdump's console a new array of six bytes every second. The values within the array should change if you move around the board.

$ # itmdump terminal
(..)
[0, 45, 255, 251, 0, 193]
[0, 44, 255, 249, 0, 193]
[0, 49, 255, 250, 0, 195]

But these bytes don't make much sense like that. Let's turn them into actual readings:

# #![allow(unused_variables)]
#fn main() {
        let x_h = u16::from(buffer[0]);
        let x_l = u16::from(buffer[1]);
        let z_h = u16::from(buffer[2]);
        let z_l = u16::from(buffer[3]);
        let y_h = u16::from(buffer[4]);
        let y_l = u16::from(buffer[5]);

        let x = ((x_h << 8) + x_l) as i16;
        let y = ((y_h << 8) + y_l) as i16;
        let z = ((z_h << 8) + z_l) as i16;

        iprintln!(&mut itm.stim[0], "{:?}", (x, y, z));
#}

Now it should look better:

$ # `itmdump terminal
(..)
(44, 196, -7)
(45, 195, -6)
(46, 196, -9)

This is the Earth's magnetic field decomposed alongside the XYZ axis of the magnetometer.

In the next section, we'll learn how to make sense of these numbers.

In this section, we'll implement a compass using the LEDs on the F3. Like proper compasses, our LED compass must point north somehow. It will do that by turning on one of its eight LEDs; the on LED should point towards north.

Magnetic fields have both a magnitude, measured in Gauss or Teslas, and a direction. The magnetometer on the F3 measures both the magnitude and the direction of an external magnetic field but it reports back the decomposition of said field along its axes.

See below, the magnetometer has three axes associated to it.

Only the X and Y axes are shown above. The Z axis is pointing "out" of your screen.

Let's get familiar with the readings of the magnetometer by running the following starter code:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*};

#[entry]
fn main() -> ! {
    let (_leds, mut lsm303dlhc, mut delay, mut itm) = aux15::init();

    loop {
        iprintln!(&mut itm.stim[0], "{:?}", lsm303dlhc.mag().unwrap());
        delay.delay_ms(1_000_u16);
    }
}

This lsm303dlhc module provides high level API over the LSM303DLHC. Under the hood it does the same I2C routine that you implemented in the last section but it reports the X, Y and Z values in a I16x3 struct instead of a tuple.

Locate where north is at your current location. Then rotate the board such that it's aligned "towards north": the North LED (LD3) should be pointing towards north.

Now run the starter code and observe the output. What X, Y and Z values do you see?

$ # itmdump terminal
(..)
I16x3 { x: 45, y: 194, z: -3 }
I16x3 { x: 46, y: 195, z: -8 }
I16x3 { x: 47, y: 197, z: -2 }

Now rotate the board 90 degrees while keeping it parallel to the ground. What X, Y and Z values do you see this time? Then rotate it 90 degrees again. What values do you see?

What's the simplest way in which we can implement the LED compass? Even if it's not perfect.

For starters, we'd only care about the X and Y components of the magnetic field because when you look at a compass you always hold it in horizontal position thus the compass is in the XY plane.

For example, what LED would you turn on in the following case. EMF stands for Earth's Magnetic Field and green arrow has the direction of the EMF (it points north).

What signs do the X and Y components of the magnetic field have in that scenario? Both are positive.

If we only looked at the signs of the X and Y components we could determine to which quadrant the magnetic field belongs to.

In the previous example, the magnetic field was in the first quadrant (x and y were positive) and it made sense to turn on the SouthEast LED. Similarly, we could turn a different LED if the magnetic field was in a different quadrant.

Let's try that logic. Here's the starter code:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, Direction, I16x3};

#[entry]
fn main() -> ! {
    let (mut leds, mut lsm303dlhc, mut delay, _itm) = aux15::init();

    loop {
        let I16x3 { x, y, .. } = lsm303dlhc.mag().unwrap();

        // Look at the signs of the X and Y components to determine in which
        // quadrant the magnetic field is
        let dir = match (x > 0, y > 0) {
            // Quadrant ???
            (true, true) => Direction::Southeast,
            // Quadrant ???
            (false, true) => panic!("TODO"),
            // Quadrant ???
            (false, false) => panic!("TODO"),
            // Quadrant ???
            (true, false) => panic!("TODO"),
        };

        leds.iter_mut().for_each(|led| led.off());
        leds[dir].on();

        delay.delay_ms(1_000_u16);
    }
}

There's a Direction enum in the led module that has 8 variants named after the cardinal points: North, East, Southwest, etc. Each of these variants represent one of the 8 LEDs in the compass. The Leds value can be indexed using the Direction enum; the result of indexing is the LED that points in that Direction.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, Direction, I16x3};

#[entry]
fn main() -> ! {
    let (mut leds, mut lsm303dlhc, mut delay, _itm) = aux15::init();

    loop {
        let I16x3 { x, y, .. } = lsm303dlhc.mag().unwrap();

        // Look at the signs of the X and Y components to determine in which
        // quadrant the magnetic field is
        let dir = match (x > 0, y > 0) {
            // Quadrant I
            (true, true) => Direction::Southeast,
            // Quadrant II
            (false, true) => Direction::Northeast,
            // Quadrant III
            (false, false) => Direction::Northwest,
            // Quadrant IV
            (true, false) => Direction::Southwest,
        };

        leds.iter_mut().for_each(|led| led.off());
        leds[dir].on();

        delay.delay_ms(1_000_u16);
    }
}

This time, we'll use math to get the precise angle that the magnetic field forms with the X and Y axes of the magnetometer.

We'll use the atan2 function. This function returns an angle in the -PI to PI range. The graphic below shows how this angle is measured:

Although not explicitly shown in this graph the X axis points to the right and the Y axis points up.

Here's the starter code. theta, in radians, has already been computed. You need to pick which LED to turn on based on the value of theta.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

// You'll find this useful ;-)
use core::f32::consts::PI;

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, Direction, I16x3};
// this trait provides the `atan2` method
use m::Float;

#[entry]
fn main() -> ! {
    let (mut leds, mut lsm303dlhc, mut delay, _itm) = aux15::init();

    loop {
        let I16x3 { x, y, .. } = lsm303dlhc.mag().unwrap();

        let _theta = (y as f32).atan2(x as f32); // in radians

        // FIXME pick a direction to point to based on `theta`
        let dir = Direction::Southeast;

        leds.iter_mut().for_each(|led| led.off());
        leds[dir].on();

        delay.delay_ms(100_u8);
    }
}

Suggestions/tips:

• A whole circle rotation equals 360 degrees.
• PI radians is equivalent to 180 degrees.
• If theta was zero, what LED would you turn on?
• If theta was, instead, very close to zero, what LED would you turn on?
• If theta kept increasing, at what value would you turn on a different LED?

#![deny(unsafe_code)]
#![no_main]
#![no_std]

// You'll find this useful ;-)
use core::f32::consts::PI;

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, Direction, I16x3};
use m::Float;

#[entry]
fn main() -> ! {
    let (mut leds, mut lsm303dlhc, mut delay, _itm) = aux15::init();

    loop {
        let I16x3 { x, y, .. } = lsm303dlhc.mag().unwrap();

        let theta = (y as f32).atan2(x as f32); // in radians

        let dir = if theta < -7. * PI / 8. {
            Direction::North
        } else if theta < -5. * PI / 8. {
            Direction::Northwest
        } else if theta < -3. * PI / 8. {
            Direction::West
        } else if theta < -PI / 8. {
            Direction::Southwest
        } else if theta < PI / 8. {
            Direction::South
        } else if theta < 3. * PI / 8. {
            Direction::Southeast
        } else if theta < 5. * PI / 8. {
            Direction::East
        } else if theta < 7. * PI / 8. {
            Direction::Northeast
        } else {
            Direction::North
        };

        leds.iter_mut().for_each(|led| led.off());
        leds[dir].on();

        delay.delay_ms(100_u8);
    }
}

We have been working with the direction of the magnetic field but what's its real magnitude? The number that the magnetic_field function reports are unit-less. How can we convert those values to Gauss?

The documentation will answer that question.

Section 2.1 Sensor characteristics - Page 10 - LSM303DLHC Data Sheet

The table in that page shows a magnetic gain setting that has different values according to the values of the GN bits. By default, those GN bits are set to 001. That means that magnetic gain of the X and Y axes is 1100 LSB / Gauss and the magnetic gain of the Z axis is 980 LSB / Gauss. LSB stands for Least Significant Bits and the 1100 LSB / Gauss number indicates that a reading of 1100 is equivalent to 1 Gauss, a reading of 2200 is equivalent to 2 Gauss and so on.

So, what we need to do is divide the X, Y and Z values that the sensor outputs by its corresponding gain. Then, we'll have the X, Y and Z components of the magnetic field in Gauss.

With some extra math we can retrieve the magnitude of the magnetic field from its X, Y and Z components:

# #![allow(unused_variables)]
#fn main() {
let magnitude = (x * x + y * y + z * z).sqrt();
#}

Putting all this together in a program:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, I16x3};
use m::Float;

#[entry]
fn main() -> ! {
    const XY_GAIN: f32 = 1100.; // LSB / G
    const Z_GAIN: f32 = 980.; // LSB / G

    let (_leds, mut lsm303dlhc, mut delay, mut itm) = aux15::init();

    loop {
        let I16x3 { x, y, z } = lsm303dlhc.mag().unwrap();

        let x = f32::from(x) / XY_GAIN;
        let y = f32::from(y) / XY_GAIN;
        let z = f32::from(z) / Z_GAIN;

        let mag = (x * x + y * y + z * z).sqrt();

        iprintln!(&mut itm.stim[0], "{} mG", mag * 1_000.);

        delay.delay_ms(500_u16);
    }
}

This program will report the magnitude (strength) of the magnetic field in milligauss (mG). The magnitude of the Earth's magnetic field is in the range of 250 mG to 650 mG (the magnitude varies depending on your geographical location) so you should see a value in that range or close to that range -- I see a magnitude of around 210 mG.

Some questions:

Without moving the board, what value do you see? Do you always see the same value?

If you rotate the board, does the magnitude change? Should it change?

If we rotate the board, the direction of the Earth's magnetic field with respect to the magnetometer should change but its magnitude should not! Yet, the magnetometer indicates that the magnitude of the magnetic field changes as the board rotates.

Why's that the case? Turns out the magnetometer needs to be calibrated to return the correct answer.

The calibration involves quite a bit of math (matrices) so we won't cover it here but this Application Note describes the procedure if you are interested. Instead, what we'll do in this section is visualize how off we are.

Let's try this experiment: Let's record the readings of the magnetometer while we slowly rotate the board in different directions. We'll use the iprintln macro to format the readings as Tab Separated Values (TSV).

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux15::{entry, iprint, iprintln, prelude::*, I16x3};

#[entry]
fn main() -> ! {
    let (_leds, mut lsm303dlhc, mut delay, mut itm) = aux15::init();

    loop {
        let I16x3 { x, y, z } = lsm303dlhc.mag().unwrap();

        iprintln!(&mut itm.stim[0], "{}\t{}\t{}", x, y, z);

        delay.delay_ms(100_u8);
    }
}

You should get an output in the console that looks like this:

$ # itmdump console
-76     213     -54
-76     213     -54
-76     213     -54
-76     213     -54
-73     213     -55

You can pipe that to a file using:

$ # Careful! Exit any running other `itmdump` instance that may be running
$ itmdump -F -f itm.txt > emf.txt

Rotate the board in many different direction while you log data for a several seconds.

Then import that TSV file into a spreadsheet program (or use the Python script shown below) and plot the first two columns as a scatter plot.

#!/usr/bin/python

import csv
import math
import matplotlib.pyplot as plt
import numpy as np
import seaborn as sns
import sys

# apply plot style
sns.set()

x = []
y = []

with open(sys.argv[1], 'r') as f:
    rows = csv.reader(f, delimiter='\t')

    for row in rows:
        # discard rows that are missing data
        if len(row) != 3 or not row[0] or not row[1]:
            continue

        x.append(int(row[0]))
        y.append(int(row[1]))

r = math.ceil(max(max(np.abs(x)), max(np.abs(y))) / 100) * 100

plt.plot(x, y, '.')
plt.xlim(-r, r)
plt.ylim(-r, r)
plt.gca().set_aspect(1)
plt.tight_layout()

plt.savefig('emf.svg')
plt.close

If you rotated the board on a flat horizontal surface, the Z component of the magnetic field should have remained relatively constant and this plot should have been a circumference (not a ellipse) centered at the origin. If you rotated the board in random directions, which was the case of plot above, then you should have gotten a circle made of a bunch of points centered at the origin. Deviations from the circle shape indicate that the magnetometer needs to be calibrated.

Take home message: Don't just trust the reading of a sensor. Verify it's outputting sensible values. If it's not, then calibrate it.

In this section we'll be playing with the accelerometer that's in the board.

What are we building this time? A punch-o-meter! We'll be measuring the power of your jabs. Well, actually the maximum acceleration that you can reach because acceleration is what accelerometers measure. Strength and acceleration are proportional though so it's a good approximation.

The accelerometer is also built inside the LSM303DLHC package. And just like the magnetometer, it can also be accessed using the I2C bus. It also has the same coordinate system as the magnetometer. Here's the coordinate system again:

Just like in the previous unit, we'll be using a high level API to directly get the sensor readings in a nicely packaged struct.

What's the first thing we'll do?

Perform a sanity check!

The starter code prints the X, Y and Z components of the acceleration measured by the accelerometer. The values have already been "scaled" and have units of gs. Where 1 g is equal to the acceleration of the gravity, about 9.8 meters per second squared.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux16::{entry, iprint, iprintln, prelude::*, I16x3, Sensitivity};

#[entry]
fn main() -> ! {
    let (mut lsm303dlhc, mut delay, _mono_timer, mut itm) = aux16::init();

    // extend sensing range to `[-12g, +12g]`
    lsm303dlhc.set_accel_sensitivity(Sensitivity::G12).unwrap();
    loop {
        const SENSITIVITY: f32 = 12. / (1 << 14) as f32;

        let I16x3 { x, y, z } = lsm303dlhc.accel().unwrap();

        let x = f32::from(x) * SENSITIVITY;
        let y = f32::from(y) * SENSITIVITY;
        let z = f32::from(z) * SENSITIVITY;

        iprintln!(&mut itm.stim[0], "{:?}", (x, y, z));

        delay.delay_ms(1_000_u16);
    }
}

The output of this program with the board sitting still will be something like:

$ # itmdump console
(..)
(0.0, 0.0, 1.078125)
(0.0, 0.0, 1.078125)
(0.0, 0.0, 1.171875)
(0.0, 0.0, 1.03125)
(0.0, 0.0, 1.078125)

Which is weird because the board is not moving yet its acceleration is non-zero. What's going on? This must be related to the gravity, right? Because the acceleration of gravity is 1 g. But the gravity pulls objects downwards so the acceleration along the Z axis should be negative not positive ...

Did the program get the Z axis backwards? Nope, you can test rotating the board to align the gravity to the X or Y axis but the acceleration measured by the accelerometer is always pointing up.

What happens here is that the accelerometer is measuring the proper acceleration of the board not the acceleration you are observing. This proper acceleration is the acceleration of the board as seen from a observer that's in free fall. An observer that's in free fall is moving toward the center of the the Earth with an acceleration of 1g; from its point of view the board is actually moving upwards (away from the center of the Earth) with an acceleration of 1g. And that's why the proper acceleration is pointing up. This also means that if the board was in free fall, the accelerometer would report a proper acceleration of zero. Please, don't try that at home.

Yes, physics is hard. Let's move on.

To keep things simple, we'll measure the acceleration only in the X axis while the board remains horizontal. That way we won't have to deal with subtracting that fictitious 1g we observed before which would be hard because that 1g could have X Y Z components depending on how the board is oriented.

Here's what the punch-o-meter must do:

• By default, the app is not "observing" the acceleration of the board.
• When a significant X acceleration is detected (i.e. the acceleration goes above some threshold), the app should start a new measurement.
• During that measurement interval, the app should keep track of the maximum acceleration observed
• After the measurement interval ends, the app must report the maximum acceleration observed. You can report the value using the iprintln macro.

Give it a try and let me know how hard you can punch ;-).

#![deny(unsafe_code)]
#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux16::{entry, iprint, iprintln, prelude::*, I16x3, Sensitivity};
use m::Float;

#[entry]
fn main() -> ! {
    const SENSITIVITY: f32 = 12. / (1 << 14) as f32;
    const THRESHOLD: f32 = 0.5;

    let (mut lsm303dlhc, mut delay, mono_timer, mut itm) = aux16::init();

    lsm303dlhc.set_accel_sensitivity(Sensitivity::G12).unwrap();

    let measurement_time = mono_timer.frequency().0; // 1 second in ticks
    let mut instant = None;
    let mut max_g = 0.;
    loop {
        let g_x = f32::from(lsm303dlhc.accel().unwrap().x).abs() * SENSITIVITY;

        match instant {
            None => {
                // If acceleration goes above a threshold, we start measuring
                if g_x > THRESHOLD {
                    iprintln!(&mut itm.stim[0], "START!");

                    max_g = g_x;
                    instant = Some(mono_timer.now());
                }
            }
            // Still measuring
            Some(ref instant) if instant.elapsed() < measurement_time => {
                if g_x > max_g {
                    max_g = g_x;
                }
            }
            _ => {
                // Report max value
                iprintln!(&mut itm.stim[0], "Max acceleration: {}g", max_g);

                // Measurement done
                instant = None;

                // Reset
                max_g = 0.;
            }
        }

        delay.delay_ms(50_u8);
    }
}

We have barely scratched the surface! There's lots of stuff left for you to explore.

NOTE: If you're reading this, and you'd like to help add examples or exercises to the Discovery book for any of the items below, or any other relevant embedded topics, we'd love to have your help!

Please open an issue if you would like to help, but need assistance or mentoring for how to contribute this to the book, or open a Pull Request adding the information!

These topics discuss strategies for writing embedded software. Although many problems can be solved in different ways, these sections talk about some strategies, and when they make sense (or don't make sense) to use.

All our programs executed a single task. How could we achieve multitasking in a system with no OS, and thus no threads. There are two main approaches to multitasking: preemptive multitasking and cooperative multitasking.

In preemptive multitasking a task that's currently being executed can, at any point in time, be preempted (interrupted) by another task. On preemption, the first task will be suspended and the processor will instead execute the second task. At some point the first task will be resumed. Microcontrollers provide hardware support for preemption in the form of interrupts.

In cooperative multitasking a task that's being executed will run until it reaches a suspension point. When the processor reaches that suspension point it will stop executing the current task and instead go and execute a different task. At some point the first task will be resumed. The main difference between these two approaches to multitasking is that in cooperative multitasking yields execution control at known suspension points instead of being forcefully preempted at any point of its execution.

All our programs have been continuously polling peripherals to see if there's anything that needs to be done. However, some times there's nothing to be done! At those times, the microcontroller should "sleep".

When the processor sleeps, it stops executing instructions and this saves power. It's almost always a good idea to save power so your microcontroller should be sleeping as much as possible. But, how does it know when it has to wake up to perform some action? "Interrupts" are one of the events that wake up the microcontroller but there are others and the wfi and wfe are the instructions that make the processor "sleep".

Microcontrollers (like our STM32F3) have many different capabilities. However, many share similar capabilities that can be used to solve all sorts of different problems.

These topics discuss some of those capabilities, and how they can be used effectively in embedded development.

This peripheral is a kind of asynchronous memcpy. So far our programs have been pumping data, byte by byte, into peripherals like UART and I2C. This DMA peripheral can be used to perform bulk transfers of data. Either from RAM to RAM, from a peripheral, like a UART, to RAM or from RAM to a peripheral. You can schedule a DMA transfer, like read 256 bytes from USART1 into this buffer, leave it running in the background and then poll some register to see if it has completed so you can do other stuff while the transfer is ongoing.

In order to interact with the real world, it is often necessary for the microcontroller to respond immediately when some kind of event occurs.

Microcontrollers have the ability to be interrupted, meaning when a certain event occurs, it will stop whatever it is doing at the moment, to instead respond to that event. This can be very useful when we want to stop a motor when a button is pressed, or measure a sensor when a timer finishes counting down.

Although these interrupts can be very useful, they can also be a bit difficult to work with properly. We want to make sure that we respond to events quickly, but also allow other work to continue as well.

In Rust, we model interrupts similar to the concept of threading on desktop Rust programs. This means we also must think about the Rust concepts of Send and Sync when sharing data between our main application, and code that executes as part of handling an interrupt event.

In a nutshell, PWM is turning on something and then turning it off periodically while keeping some proportion ("duty cycle") between the "on time" and the "off time". When used on a LED with a sufficiently high frequency, this can be used to dim the LED. A low duty cycle, say 10% on time and 90% off time, will make the LED very dim wheres a high duty cycle, say 90% on time and 10% off time, will make the LED much brighter (almost as if it were fully powered).

In general, PWM can be used to control how much power is given to some electric device. With proper (power) electronics between a microcontroller and an electrical motor, PWM can be used to control how much power is given to the motor thus it can be used to control its torque and speed. Then you can add an angular position sensor and you got yourself a closed loop controller that can control the position of the motor at different loads.

We have used the microcontroller pins as digital outputs, to drive LEDs. But these pins can also be configured as digital inputs. As digital inputs, these pins can read the binary state of switches (on/off) or buttons (pressed/not pressed).

(spoilers reading the binary state of switches / buttons is not as straightforward as it sounds ;-)

There are a lots of digital sensors out there. You can use a protocol like I2C and SPI to read them. But analog sensors also exist! These sensors just output a voltage level that's proportional to the magnitude they are sensing.

The ADC peripheral can be use to convert that "analog" voltage level, say 1.25 Volts,into a "digital" number, say in the [0, 65535] range, that the processor can use in its calculations.

As you might expect a DAC is exactly the opposite of ADC. You can write some digital value into a register to produce a voltage in the [0, 3.3V] range (assuming a 3.3V power supply) on some "analog" pin. When this analog pin is connected to some appropriate electronics and the register is written to at some constant, fast rate (frequency) with the right values you can produce sounds or even music!

This peripheral can be used to track time in "human format". Seconds, minutes, hours, days, months and years. This peripheral handles the translation from "ticks" to these human friendly units of time. It even handles leap years and Daylight Save Time for you!

SPI, I2S, SMBUS, CAN, IrDA, Ethernet, USB, Bluetooth, etc.

Different applications use different communication protocols. User facing applications usually have an USB connector because USB is an ubiquitous protocol in PCs and smartphones. Whereas inside cars you'll find plenty of CAN "buses". Some digital sensors use SPI, others use I2C and others, SMBUS.

These topics cover items that are not specific to our device, or the hardware on it. Instead, they discuss useful techniques that could be used on embedded systems.

As part of our Punch-o-meter exercise, we used the Accelerometer to measure changes in acceleration in three dimensions. Our board also features a sensor called a Gyroscope, which allows us to measure changes in "spin" in three dimensions.

This can be very useful when trying to build certain systems, such as a robot that wants to avoid tipping over. Additionally, the data from a sensor like a gyroscope can also be combined with data from accelerometer using a technique called Sensor Fusion (see below for more information).

While some motors are used primarily just to spin in one direction or the other, for example driving a remote control car forwards or backwards, it is sometimes useful to measure more precisely how a motor rotates.

Our microcontroller can be used to drive Servo or Stepper motors, which allow for more precise control of how many turns are being made by the motor, or can even position the motor in one specific place, for example if we wanted to move the arms of a clock to a particular direction.

The STM32F3DISCOVERY contains three motion sensors: an accelerometer, a gyroscope and a magnetometer. On their own these measure: (proper) acceleration, angular speed and (the Earth's) magnetic field. But these magnitudes can be "fused" into something more useful: a "robust" measurement of the orientation of the board. Where robust means with less measurement error than a single sensor would be capable of.

This idea of deriving more reliable data from different sources is known as sensor fusion.

So where to next? There are several options:

• You could check out the examples in the f3 board support crate. All those examples work for the STM32F3DISCOVERY board you have.

• You could try out this motion sensors demo. Details about the implementation and source code are available in this blog post.

• You could check out Real Time for The Masses. A very efficient preemptive multitasking framework that supports task prioritization and dead lock free execution.

• You could try running Rust on a different development board. The easiest way to get started is to use the cortex-m-quickstart Cargo project template.

• You could check out this blog post which describes how Rust type system can prevent bugs in I/O configuration.

• You could check out my blog for miscellaneous topics about embedded development with Rust.

• You could check out the embedded-hal project which aims to build abstractions (traits) for all the embedded I/O functionality commonly found on microcontrollers.

• You could join the Weekly driver initiative and help us write generic drivers on top of the embedded-hal traits and that work for all sorts of platforms (ARM Cortex-M, AVR, MSP430, RISCV, etc.)

Upon trying to establish a new connection with the device you get an error that looks like this:

$ openocd -f (..)
(..)
Error: open failed
in procedure 'init'
in procedure 'ocd_bouncer'

• All: The device is not (properly) connected. Check the USB connection using lsusb or the Device Manager.
• Linux: You may not have enough permission to open the device. Try again with sudo. If that works, you can use these instructions to make OpenOCD work without root privilege.
• Windows: You are probably missing the ST-LINK USB driver. Installation instructions here.

Upon trying to establish a new connection with the device you get an error that looks like this:

$ openocd -f (..)
(..)
Error: jtag status contains invalid mode value - communication failure
Polling target stm32f3x.cpu failed, trying to reexamine
Examination failed, GDB will be halted. Polling again in 100ms
Info : Previous state query failed, trying to reconnect
Error: jtag status contains invalid mode value - communication failure
Polling target stm32f3x.cpu failed, trying to reexamine
Examination failed, GDB will be halted. Polling again in 300ms
Info : Previous state query failed, trying to reconnect

The microcontroller may have get stuck in some tight infinite loop or it may be continuously raising an exception, e.g. the exception handler is raising an exception.

• Close OpenOCD, if running
• Press and hold the reset (black) button
• Launch the OpenOCD command
• Now, release the reset button

A running OpenOCD session suddenly errors with:

# openocd -f (..)
Error: jtag status contains invalid mode value - communication failure
Polling target stm32f3x.cpu failed, trying to reexamine
Examination failed, GDB will be halted. Polling again in 100ms
Info : Previous state query failed, trying to reconnect
Error: jtag status contains invalid mode value - communication failure
Polling target stm32f3x.cpu failed, trying to reexamine
Examination failed, GDB will be halted. Polling again in 300ms
Info : Previous state query failed, trying to reconnect

The USB connection was lost.

• Close OpenOCD
• Disconnect and re-connect the USB cable.
• Re-launch OpenOCD

While flashing the device, you get:

$ arm-none-eabi-gdb $file
Start address 0x8000194, load size 31588
Transfer rate: 22 KB/sec, 5264 bytes/write.
Ignoring packet error, continuing...
Ignoring packet error, continuing...

Closed itmdump while a program that "printed" to the ITM was running. The current GDB session will appear to work normally, just without ITM output but the next GDB session will error with the message that was shown in the previous section.

Or, itmdump was called after the monitor tpiu was issued thus making itmdump delete the file / named-pipe that OpenOCD was writing to.

• Close/kill GDB, OpenOCD and itmdump
• Remove the file / named-pipe that itmdump was using (for example, itm.txt).
• Launch OpenOCD
• Then, launch itmdump
• Then, launch the GDB session that executes the monitor tpiu command.

   Compiling volatile-register v0.1.2
   Compiling rlibc v1.0.0
   Compiling r0 v0.1.0
error[E0463]: can't find crate for `core`

error: aborting due to previous error

error[E0463]: can't find crate for `core`

error: aborting due to previous error

error[E0463]: can't find crate for `core`

error: aborting due to previous error

Build failed, waiting for other jobs to finish...
Build failed, waiting for other jobs to finish...
error: Could not compile `r0`.

To learn more, run the command again with --verbose.

You are using a toolchain older than nightly-2018-04-08 and forgot to call rustup target add thumbv7em-none-eabihf.

Update your nightly and install the thumbv7em-none-eabihf target.

$ rustup update nightly

$ rustup target add thumbv7em-none-eabihf