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'