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