Garbled high bits on serial output
It seems this error is specific to the Rust ARM library I am using; quoting from https://github.com/klangner/sam3x/issues/9:


  It looks that at bound rate=9600 the timer sends data with the speed of ~888. And then computer can't understand the output.


Based on this, I started playing around with the baud rate I pass to the library when initializing it, and using 11000 instead of 9600 empirically seems to work (i.e. I am seeing a steady stream of 0x5A on my PC receiving at 9600).