MicroLua / MicroLua

MicroLua - Lua for the RP2040 microcontroller

MicroLua allows programming the RP2040 microcontroller in Lua. It packages the latest Lua interpreter with bindings for the Pico SDK and a cooperative threading library.

MicroLua is licensed under the MIT license.

Mailing list: microlua@freelists.org

Why

Lua is a small, embeddable language. It is easy to learn and reasonably fast.
The RP2040 is a small, powerful microcontroller with a nice set of peripherals and an active developer community. Besides the official Pico and Pico W boards from Raspberry Pi, a variety of cheap modules in various shapes and configurations are readily available for purchase.

I had been wanting to play with Lua for a very long time, without having a concrete use case for it. Similarly, I wanted to explore the RP2040 but didn't have a concrete project for it. MicroLua served as an excuse to get started with both.

Features

Pristine, unpatched Lua interpreter: MicroLua runs the latest, unmodified Lua 5.4.x interpreter, imported as a git submodule. All customization is done through luaconf.h.
Per-core interpreter instances: MicroLua runs a separate Lua interpreter in each core. They don't share state except through C libraries.
Cooperative multithreading through Lua coroutines: MicroLua implements cooperative threads as coroutines. This enables multitasking without the need for locks. Blocking library calls (e.g. pico.time.sleep_us()) yield to other threads.
Thin bindings to C libraries: MicroLua exposes a growing subset of the functionality provided by the Pico SDK. The bindings are designed with a straightforward and consistent mapping to their underlying C implementation.
Support for Fennel: Fennel sources are transpiled to Lua.
Comprehensive suite of unit tests: They not only test the binding layer, but when possible also the underlying functionality of the Pico SDK.

Performance

Performance is adequate for applications that don't require very low latency, but it could be better. Event dispatch latency is currently in the hundreds of microseconds, which is fairly slow. This is mainly due to a naive threading implementation, and it can be improved.

So it probably isn't realistic to try bit-banging high-speed serial protocols or PWM in Lua, but that's what the PIO and PWM peripherals are for. Anything that requires precise timings should probably be implemented in C. But Lua is a great glue language for the non timing-critical logic, and very easy to interface to C code.

Roadmap

Add more bindings for the Pico SDK. PIO is high on the list, followed by USB, Wifi and Bluetooth. Eventually, most SDK libraries should have a binding.
Improve threading performance. A C implementation of the mlua.thread module, with a less naive timer list, could significantly improve event dispatch latency.
Tune garbage collection. Garbage collection parameters are currently left at their default value, which may not be ideal for a memory-constrained target.
Improve cross-core communication. Each core runs its own Lua interpreter, so they cannot communicate directly through shared Lua state. Currently, the only way for the cores to communicate is the SIO FIFOs, which is fairly limited. A form of memory-based cross-core channel would be useful.
Add multi-chip communication. As an extension of cross-core channels, cross-chip channels could enable fast communication between multiple RP2040 chips.

Building

Here's how to build and run the test suite on a Raspberry Pi Pico module.

# Configure the location of the Pico SDK. Adjust for your setup.
$ export PICO_SDK_PATH="${HOME}/pico-sdk"

# Clone the repository and initialize submodules.
$ git clone https://github.com/MicroLua/MicroLua.git
$ cd MicroLua
$ git submodule update --init

# Connect a Picoprobe to the target, on the UART and optionally on the debug
# port. Then view the Picoprobe's UART connection in a separate terminal.
# The "term" script uses socat.
$ tools/term /dev/ttyACM0

# Build the unit tests.
$ cmake -S . -B build -DPICO_BOARD=pico
$ make -j9 -C build/bin

# Start the target in BOOTSEL mode and flash it with picotool.
$ picotool load -v -x build/bin/mlua_tests.elf

# Alternatively, start the target in BOOTSEL mode and copy to its boot drive.
$ cp build/bin/mlua_tests.uf2 /mnt/RPI-RP2/

Examples

The MicroLua-examples repository contains example programs that demonstrate how to use the features of the RP2040 from Lua.

Here's the blink example in MicroLua, a translation of the blink example from the pico-examples repository.

_ENV = module(...)

local gpio = require 'hardware.gpio'
local pico = require 'pico'
local time = require 'pico.time'

function main()
    local LED_PIN = pico.DEFAULT_LED_PIN
    gpio.init(LED_PIN)
    gpio.set_dir(LED_PIN, gpio.OUT)
    while true do
        gpio.put(LED_PIN, 1)
        time.sleep_ms(250)
        gpio.put(LED_PIN, 0)
        time.sleep_ms(250)
    end
end

Documentation

Core functionality of MicroLua.
mlua.*: MicroLua libraries.
hardware.*: Bindings for the hardware_* libraries of the Pico SDK.
pico.*: Bindings for the pico_* libraries of the Pico SDK.

Contributing

While I'm happy to accept bug reports, feature requests and other suggestions on the mailing list, I am not actively looking for code contributions. In particular, please do not send pull requests on Github. MicroLua is developed in a private Mercurial repository, which is mirrored to Github.

FAQ

Why Lua? Why the RP2040?

Both Lua and the RP2040 evolve at a velocity that can be followed by a single developer in their spare time. Lua is developed by a small team: the language itself evolves very slowly and the interpreter has a couple of minor releases each year. Similarly, the RP2040 is developed by a small company, and a new chip may be released every few years.

Contrast this with e.g. Python: the language evolves rather quickly nowadays, and the interpreter gets contributions from dozens (hundreds? thousands?) of developers. Similarly, large semiconductor companies release new chips and variants every year. Developing for these targets is a game of catch-up that I don't want to play.

Does MicroLua support other microcontroller families?

Currently it doesn't, but it shouldn't be too difficult to add. The build system is already multi-platform, as it can create binaries for the RP2040 and for the build host. However, adding support for more platforms isn't a high priority.

MicroLua will likely support later RP devices if / when they get released.

How does MicroLua compare to other Lua projects for the Pico?

picolua is based on a patched Lua 5.4 interpreter, and aims to provide a full embedded development environment, including a shell and an editor. It exposes a limited subset of Pico-specific functionality.

MicroLua uses an unpatched Lua interpreter at the latest stable version, and aims to expose most of the functionality provided by the Pico SDK through a thin binding layer.

What's the relationship with MicroLua DS?

MicroLua DS was a development environment for building apps for the Nintendo DS in Lua. It was last released in January 2014. MicroLua has no relationship with MicroLua DS.

While the naming conflict is unfortunate, I felt that almost 10 years of inactivity was long enough that it was fair game to re-use the name.