I got the blinky_basic example to compile. I figured out how to convert it from elf to bin format, then from bin to uf2 format, and get it flashed on my Circuit Playground Express board. I got the little red LED light blinking. So far so good.
The trickiest part for me was, after running
cargo build --example blinky_basic --release, figuring out how to turn the result into a UF2 file that could be flashed onto my Circuit Playground Express. Here are the notes on the final steps I had to figure out myself, in case it helps someone else: https://gw.handysoftware.com/wiki/cpif/circuit-playground#Programming_the_Circuit_Playground_Express_in_Rust
Now I'd like to write bytes from the Circuit Playground Express to the USB port to send messages and data to a connected PC. I'd like to implement something like
println! for simple debugging, and also eventually send data from the ADC to a connected PC. I searched the example code for other boards in the atsamd project and found some for reading from USB, which maybe could be adapted for the Circuit Playground Express, but I haven't tried them yet (pretty sure there will be a lot of trial and error due to differences between the boards.) But I didn't see any examples of writing to the USB.
Has anyone got a complete, working example of writing bytes from the Circuit Playground Express to the USB port? Or any advice or pointers?