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Embedded Swift on the Raspberry Pi Pico

A proof of concept for executing Embedded Swift code on the Raspberry Pi Pico. The main program is written in C (built with the official Pico C SDK) and calls into a statically linked Swift library.

Prerequisites and Installation

Hardware

• A Raspberry Pi Pico
• Optional but recommended: a Raspberry Pi Debug Probe for more convenient flashing (see below). This also enables debugging.

Software

• Host OS: macOS 13.x or 14.x

  Tested on macOS 13.6.2. It’ll probably work on Linux with minimal modifications to tell CMake how to find the Swift toolchain, but I haven’t tested this.

• A recent nightly Swift toolchain from swift.org. Tested with the Xcode toolchain from December 7, 2023.

• A clone of the Raspberry Pi Pico C/C++ SDK:

  cd ..
  git clone https://github.com/raspberrypi/pico-sdk.git
  cd pico-sdk/
  git submodule update --init
  cd ..

  This project expects to find the SDK in a sibling directory named pico-sdk. You can change this below if your SDK is in a different place.

• The GCC toolchain for ARM embedded platforms (the Pico SDK builds with GCC by default):

  brew install --cask gcc-arm-embedded

• CMake and Ninja:

  brew install cmake ninja

  The Pico SDK uses CMake as its build system and we’re piggybacking on that. And CMake’s Swift support only works with Ninja. The Swift library is also built with CMake. The unfortunate consequence is that we can’t easily use a SwiftPM package for the Swift library as we’d have to tell CMake how to build the package.

Configuration

Open the file CMakeLists.txt in the root folder. Edit these two lines to match your setup:

…
set(PICO_SDK_PATH "${CMAKE_CURRENT_LIST_DIR}/../pico-sdk")
…
set(Swift_Toolchain "org.swift.59202312071a")
…

Building

cd pico-embedded-swift
mkdir build
cd build
cmake -G Ninja ..
ninja

This produces the executable in the build directory in several formats, e.g. SwiftPico.elf, ``SwiftPico.uf2, SwiftPico.bin`.

Running on the Pico

You have two options to copy the executable to the Pico:

1. Via the USB Mass Storage interface: Connect the Pico to your computer while holding down the BOOTSEL button. When you release the button, the Pico will appear as a removable drive. Now copy SwiftPico.uf2 to the Pico “drive”. The Pico will automatically reboot and run the program (you can ignore macOS’s “disk not ejected properly” message).

2. Via the debug probe. The debug probe is connected to your PC and talks to the Pico via its debug port. This allows you to reflash the Pico without having to disconnect it.

   I use probe.rs for this, which is a tool from the Rust community, but it works in this context too. Provided you have Rust installed, you can install probe-rs with cargo install probe-rs-debugger --features cli.

   probe-rs run --chip RP2040 SwiftPico.elf