Pico 2 as debugprobe

Tuesday, 2025-06-03 Comments

How to use the Raspberry Pi Pico 2 as a hardware debugger.

On most popular microcontrollers used for educational purposes, there is already some hardware debugging support (also called a hardware debug probe) on the board itself, such as on the Micro:bit or the ESP32.

Having this debug probe allows you to debug the code running on the target Pico using GDB or other debugging tools.

A debug probe comes in the form of a small secondary chip that can be used to debug the main microcontroller on the board.

The Pico family of microcontrollers does not have this feature built-in. You have two options for debugging a Pico:

In this workshop, we will pursue the first option. If you get stuck, feel free to ask for a pre-made hardware debugger.

Turning a Pico 2 into a debugger Pico

The Raspberry Pi Foundation provides images for Picos that can be flashed to turn a Pico into a hardware debugger.

  1. Download the latest debugprobe_on_pico.uf2 flash image from the official debugprobereleases.
  2. Attach the Pico to your laptop while holding the white BOOTSEL button. A mass storage device will appear in your file manager. It will be called something like RP2350.
  3. Drop the downloaded .uf2 file onto the mass storage drive emulated by the Pico. Wait for a fraction of a second while the Pico unmounts and reboots as a fresh debugprobe.

Now you have successfully made a cheap hardware debugging probe.

Wire target to debugger

Let’s make some aliases:

Right now, there is no cabling between the debug probe and the target Pico. The cables should be connected such that D can detect T over the SWD debugging protocol.

Important: For this step, you need to have a JST-SH cable. You can find them on Kiwi, but they can be hard to find.

  1. Plug the white connector of the JST cable into the SWD socket of D.

  2. Place T and D in parallel with their USB ports facing upwards (to prevent confusion).

  3. Connect the male jumper pins. The three male header pins from T’s JST cable should be connected to D as follows:

    • T left (yellow) <-> D pin 5
    • T middle (black) <-> D pin 3
    • T right (orange) <-> D pin 4

    Instead of pin numbers, you can also use the pin names:

    • T SWCLK <-> D GP3
    • T SWDIO <-> D GP2
    • T GND <-> D GND

  4. Provide power to T using a single USB cable by forwarding power from D:

    • T GND pin 38 <-> D pin 38 (Connect ground)
    • T VSYS pin 39 <-> D pin 39 (Connect power supply)

Remark: You can also connect T to D for UART communication. However, I have not needed it so far.

Configure flashing from laptop

There is still one step remaining: we have to configure our laptop’s development environment to enable flashing (this applies to any microcontroller with an onboard or external debugger).

  1. Install cargo-embed, which is included in the probe-rs tool suite.

    cargo install probe-rs-tools

  2. Verify that cargo-embed is available in your shell’s PATH (cargo-[CMD] can be called with cargo [CMD]):

    cargo embed --version

  3. Add udev rules for probe-rs as described in the probe-rs documentation.

    sudo curl --output /etc/udev/rules.d/69-probe-rs.rules "https://probe.rs/files/69-probe-rs.rules"
sudo udevadm control --reload-rules
sudo udevadm trigger

Now you can flash changes in the source code directly to the target Pico (without re-plugging it or holding the BOOTSEL button). The debug probe Pico will function as an intermediary between your laptop and the target Pico.

cargo run

You should see two progress bars running to completion in your terminal. As soon as the flash process is finished:

While the Pico has a generous amount of flash memory, Embassy-produced binaries can sometimes be large. For microcontrollers with less memory, the Min-sized Rust guide provides tips for reducing binary size.