rust-raspberrypi-OS-tutorials/.05_uart0/README.md

Tutorial 05 - UART0, PL011

Finally, we can set up UART0 thanks to the mailbox interface. This tutorial produces the same output as tutorial 04, but it prints the serial number on UART0.

uart.rs

In the init function, we use the mailbox to set a base clock for the UART:

mbox.buffer[0] = 9 * 4;
mbox.buffer[1] = mbox::REQUEST;
mbox.buffer[2] = mbox::tag::SETCLKRATE;
mbox.buffer[3] = 12;
mbox.buffer[4] = 8;
mbox.buffer[5] = mbox::clock::UART; // UART clock
mbox.buffer[6] = 4_000_000; // 4Mhz
mbox.buffer[7] = 0; // skip turbo setting
mbox.buffer[8] = mbox::tag::LAST;

// Insert a compiler fence that ensures that all stores to the
// mbox buffer are finished before the GPU is signaled (which
// is done by a store operation as well).
compiler_fence(Ordering::Release);

if mbox.call(mbox::channel::PROP).is_err() {
    return Err(UartError::MailboxError); // Abort if UART clocks couldn't be set
};

Afterwards, we can program the rate divisors:

self.IBRD.write(IBRD::IBRD.val(2)); // Results in 115200 baud
self.FBRD.write(FBRD::FBRD.val(0xB));

Baud rate calculation won't be covered in detail here. Please see this reference from ARM for details.

The API for using the UART is identical to the UART1 API.

main.rs

We query the board's serial number and display it on the serial console.