I'm working with the Raspberry PI Pico to perform the basic task of reading data from a UART signal, modifying it, and writing it back out to a different UART address. However, I need to simultaneously be constantly monitoring an on-board sensor and sending the values it generates as well.
I found a good example at cortexm-threads but it performs some stack allocation like this:
let mut stack1 = [0xDEADBEEF; 512];
let mut stack2 = [0xDEADBEEF; 512];
How do I know (or find out) what memory addresses I can allocate the stacks to on the RP2040/Pico?
In the example, the 0xDEADBEEF will denote the initial per-cell value of stack1 and stack2 arrays, and can be set to anything. Since those arrays are function-local non-constants/non-statics, they will end up in the (main thread) stack.
Just make sure that the arrays are large enough for your use case, otherwise risking a stack overflow (How does a "stack overflow" occur and how do you prevent it?).
Regarding on where those variables end up being located in the device memory space: cortex-m will set the initial SP value to the largest possible memory address (0x20040000 on Pico - RP2040 SRAM is located between 0x20000000 and 0x20040000, sized 256 kB). See https://github.com/rust-embedded/cortex-m/blob/657af97d66b7157d6a6e5704d86dd59b398e7108/cortex-m-rt/link.x.in#L63. Thereby, the location of those variables will be close to the end of SRAM. See also https://docs.rust-embedded.org/embedonomicon/memory-layout.html
Regarding the multicore use case, see also https://github.com/rp-rs/rp-hal/blob/427344667e9f24f03d132fa08e2dfaa709bc805d/rp2040-hal/src/multicore.rs.
You could also achieve the similar functionality (but using only one core) with interrupt-driven approach, where you store each incoming UART-byte into a circular buffer, handle on-board sensor read on a (either DMA/timer) interrupt, and process the circular buffer (and possibly read sensor value) contents in the idle loop. For more information, see https://en.wikipedia.org/wiki/Circular_buffer and https://rtic.rs/,