RP2040 volume control

A simple volume control firmware for the RP2040 - developed on the Pico

I opted to write this in a relatively simple interrupt-driven flow, using the GPIO BANK0 interrupt handler to check the pins on a clickable rotary encoder and dispatch USB-HID reports accordingly. The USB_IRQ is just used to poll the USB device.

For more complex use-cases it's probably to use RTIC, and bind one of the timers for a watchdog.

Usage

Deployment

The project is already configured with elf2uf2-rs as a runner, which means you can just plug in your Pico in update mode and run using

cargo run

Debugging

No UART output is available, the logs are printed out using defmt-rtt, which you can see using probe-run, but if you want to debug too, you need to set up openocd, either on your own probe or on a Raspberry Pi (which is what I opted to do). This allowed me to build, deploy and debug from my M1 Mac.

Follow section 5.1 in Getting started with Pico to build openocd.

Hook up the SWD pins as following:

Raspberry Pi host	Pico target
GND (pin 20)	SWD GND
GPIO24 (pin 18)	SWDIO
GPIO25 (pin 22)	SWDCLK

Run openocd on the Raspberry Pi host, binding to an external interface for the remote debugging:

openocd -f interface/raspberrypi-swd.cfg -f target/rp2040.cfg -c "bindto 0.0.0.0"

Now connect using gdb, setting the target binary and the remote (extended) debugging stub, setting RPI_IP to your Pi's address

gdb-multiarch -ex "target extended-remote $RPI_IP:3333" target/thumbv6m-none-eabi/debug/rp2040-volume-knob

Since I'm on an M1 mac with no gdb-multiarch, I use an Ubuntu VM with multipass, with my home folder mounted inside the VM.

You are now debugging the target, and can use monitor commands such as monitor reset init to restart the target, or load with our without a path to load a new .elf file. To get defmt logs out - you need to extract the location of the RTT control block using the following command:

$ rust-nm -S target/thumbv6m-none-eabi/debug/rp2040-volume-knob | grep RTT # requires cargo-binutils
2003fae8 00000030 D _SEGGER_RTT

In the above case, the control block is as 0x2003fae8 and is 0x30 bytes in size.

Using this, we can ask (via gdb) the openocd monitor to set up a rtt source on a tcp address, using the following gdb commands:

monitor rtt server start 8765 0
monitor rtt setup 0x2003fae8 0x30 "SEGGER RTT"
monitor rtt start

At which points - the RTT server will be listening on the RPi (not localhost), and you can stream its output into

nc 192.168.1.148 8765 | defmt-print -e target/thumbv6m-none-eabi/debug/rp2040-volume-knob