metronome/rust/probe-flash.md

Flashing & debugging pm-kit with the Raspberry Pi Debug Probe

How to flash the Rust RP2350 firmware (rust/pm-kit) and watch its defmt logs live, from a Fedora Silverblue toolbox. This replaces the BOOTSEL/UF2 drag for development — over SWD you get one-command flash + reset + RTT log streaming, and panics print their message and file:line.

Wiring and chip name were verified against the official docs (links at the bottom); don't guess them.

0. Hardware wiring (once)

• Probe "D" port → Pico 2 DEBUG connector with the included 3-pin JST-SH cable (SC=SWCLK / SD=SWDIO / GND — connector is keyed). The "U" port is UART, not used here.
• Probe USB → computer. Pico 2 USB → power (the probe does SWD only, not power).
• Connect GND first when the target is separately powered.
• If probe-rs can't see the chip later, the probe's own firmware may predate the Pico 2 — update it: hold BOOTSEL on the probe, plug it in (mounts RPI-RP2), drag the latest debugprobe.uf2 from github.com/raspberrypi/debugprobe/releases.

1. Host udev rules (once, on the Silverblue HOST — not inside toolbox)

The host's udevd sets the device permissions; the rule's TAG+="uaccess" grants your UID an ACL, which is what maps correctly into the toolbox (group-based access shows as nobody:nobody).

sudo curl -L https://probe.rs/files/69-probe-rs.rules -o /etc/udev/rules.d/69-probe-rs.rules
sudo udevadm control --reload
sudo udevadm trigger
# then unplug/replug the probe

(/etc is writable on Silverblue. If your terminal opens inside a toolbox, run these on the host, e.g. flatpak-spawn --host sudo ….)

2. Install probe-rs in the toolbox (once)

toolbox enter                       # or: toolbox create probe && toolbox enter probe
sudo dnf install -y libusb1         # runtime lib the prebuilt binary links against
curl --proto '=https' --tlsv1.2 -LsSf \
  https://github.com/probe-rs/probe-rs/releases/latest/download/probe-rs-tools-installer.sh | sh
# follow the installer's PATH note (it lands in ~/.cargo/bin) or restart the shell
probe-rs --version
probe-rs list                       # should show the CMSIS-DAP Debug Probe

3. Flash + watch logs

curl -O https://metronome.varasys.io/pm-kit.elf
probe-rs run --chip RP235x pm-kit.elf

probe-rs run flashes the ELF, resets, and streams defmt over RTT. We ship the ELF (not the UF2) because defmt decodes its log strings from the ELF's symbol table. The firmware narrates: == pm-kit boot == → display init ok → parsed groove 0 → entering main loop → alive: … (~2×/sec). A panic/HardFault prints its location.

Building from source instead of the prebuilt ELF? .cargo/config.toml already sets runner = "probe-rs run --chip RP235x", so inside the pm-rust build image cargo run --release does build+flash+log in one — but that image needs probe-rs + USB passthrough too; the toolbox route above is simpler for just flashing what we ship.

Troubleshooting

• No probe was found → udev rules not applied (re-plug after step 1), or the device shows nobody:nobody in the toolbox. Quick unblock without re-plugging: sudo setfacl -m u:$USER:rw $(readlink -f /dev/serial/by-id/*Debug_Probe* 2>/dev/null) on the host, or just run sudo probe-rs run … inside the toolbox (it's privileged, so root can open the node).
• chip RP235x not found → your probe-rs is too old (RP2350 support landed Feb 2025); reinstall. Confirm the exact name with probe-rs chip list | grep -i rp23.
• Connects but can't halt/flash the core → update the probe firmware (step 0).

References

• Raspberry Pi Debug Probe docs — https://www.raspberrypi.com/documentation/microcontrollers/debug-probe.html
• probe-rs install — https://probe.rs/docs/getting-started/installation/
• probe-rs probe setup / udev — https://probe.rs/docs/getting-started/probe-setup/
• probe-rs RP235x support — https://github.com/probe-rs/probe-rs/issues/2787
• Toolbox USB ACL gotcha — https://discussion.fedoraproject.org/t/accessing-a-usb-device-in-toolbox-via-udev-rule/127904