Let's start with the Debug Header, the little 3 pin interface lets you peek into the Pons of the Raspberry Pi brain. You can find this header on the Raspberry Pi 5, the Raspberry Pi Pico (H, and WH), and now happily the Adafruit RP23XX boards thus far. By connecting a Pico Probe to this interface, you can get a lot of debug information back on your big main computer. ARM refers to this type of interfacing / debugging as Semihosting.

With the 3-Pin JST cable, cable plugged into the D header on the Pico Probe and the other end of that cable onto the Debug Header on your Feather (Shown above); With the Pico Probe connected to your computer, and you can connect the Feather to any USB power source, or back to your computer as well. Once you've done that, you're ready to issue the cargo run command, so long as you have the required software installed below.

Back to the code we cloned all of the way at the top of this article. With all of the software above installed, and your computer connected to the debug probe, the probe connected to the feather, and the feather connected to power or your computer you now have a system that is ready to go. We really can run that cargo run command and have Rust code on our feather. But what are all of these files and what do they do?

.cargo/config.toml and cargo run

The .cargo/config.toml file configures cargo run to use probe-rs to flash to the boards ROM. You will need a Raspberry Pi Debug Probe in order to use this, but it makes development MUCH easier, faster, and more fun! You connect the Debug Probe's D (for debug, D for defmt 😉) side to the board's Debug Port. Once done connect the Adafruit board, and Debug Probe to your computer. You can flash at will with cargo run and see any debug messages in your computer's terminal thanks to defmt.

memory.x

If you've never seen a memory.x file before, and have no clue what it is; I don't blame you for being curious. It's an odd file, filled with things that aren't Rust or C, or anything else that fits the norm. This file actually tells the linker where to put sections of the binary. It makes sure everything is in order so that when the microcontroller jumps to flash memory, the expected data is there ready for it. It also tells the the linker how much RAM the target board or chip has.

build.rs

This build script copies the memory.x file from the crate root into a directory where the linker can always find it at build time. For many projects this is optional, as the linker always searches the project root directory -- wherever Cargo.toml is. However, if you are using a workspace or have a more complicated build setup, this build script becomes required. Additionally, by requesting that Cargo re-run the build script whenever memory.x is changed, updating memory.x ensures a rebuild of the application with the new memory settings.