nrf51 uart write blocks in qemu

[JoshMcguigan]

Hello! I am following along the rust discovery uart example by running the microbit v1 (nrf51) in qemu.

$ cargo build --features v1 --target thumbv6m-none-eabi

$ qemu-system-arm -machine microbit -display none -serial stdio -s -kernel target/thumbv6m-none-eabi/debug/uart

At first I thought the qemu emulation might be at fault, but I found this forum post where they seem to see a similar problem on real hardware with code that matches the uart write impl here:

nrf-hal/nrf-hal-common/src/uart.rs

Lines 134 to 141 in 939c017

	fn write(&mut self, byte: u8) -> nb::Result<(), Self::Error> {
	// Are we ready for sending out next byte?
	if self.0.events_txdrdy.read().bits() == 1 {
	// Reset ready for transmit event.
	self.0.events_txdrdy.reset();
	// Send byte.
	self.0.txd.write(|w| unsafe { w.bits(u32::from(byte)) });

This check for txdrdy hangs when trying to send the first byte. The reference manual says the txdrdy event is triggered only when a byte is actually sent. That aligns with the findings in the forum post that show very similar code blocking indefinitely.

Unfortunately I don't have real hardware to test this on at the moment.