pymcu-compiler 0.1.0a1.post3

Python-to-MCU compiler driver

PyMCU

Python to bare-metal firmware — no runtime, no interpreter, no VM.
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[!IMPORTANT] Alpha — v0.1 — under active development. Core AVR compilation is stable and test-covered, but rough edges remain in error messages and tooling. Stability is expected for the supported feature set — if you hit a bug, please open an issue, it helps a lot.

Avoid pymcu.hal.* during the alpha — the native HAL API may change between releases. Use the MicroPython or CircuitPython compat API instead; those are stable and community-specified.

PyMCU compiles a statically-typed subset of Python into bare-metal AVR firmware — no runtime, no interpreter, no virtual machine. The same binary you would write in C.

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The pitch in one table

LED blink for ATmega328P @ 16 MHz — all variants do the same thing: configure PB5 as output, then loop LED on → wait 500 ms → LED off → wait 500 ms forever.

Source	Total flash	SRAM
C (avr-gcc -Os)	176 B	0 B
PyMCU (native HAL)	162 B	0 B
PyMCU (MicroPython API)	162 B	0 B
PyMCU (CircuitPython API)	164 B	0 B
Arduino (IDE defaults)	~1 024 B	9 B

PyMCU produces a smaller binary than C here. Why?

Pin("PB5", Pin.OUT) and delay_ms(500) are resolved entirely at compile time — the compiler sees through the Python objects and emits the same raw SBI/CBI port-toggle instructions a C programmer would write by hand. The delay loop PyMCU generates happens to use one fewer padding instruction than the loop avr-gcc -Os emits for this particular pattern. The interrupt vector table and startup stub are identical fixed overhead in both toolchains.

Native HAL and MicroPython API produce byte-for-byte identical firmware — both compile down to the same SBI/CBI toggle and the same delay loop. The API is a zero-cost abstraction. CircuitPython is 2 bytes larger because the Direction.OUTPUT setter clears the PORT register before setting DDR, as the CircuitPython spec requires.

These numbers are for a minimal blink. Real programs that use SRAM (global variables, buffers) will emit a small zeroing loop at startup, just like C does.

For complex drivers (custom protocols, timing-critical bit-bang): expect 2-3x flash vs hand-written C. PyMCU is not competing with C — the goal is to make microcontroller development approachable in Python you already know, without the overhead of Arduino. The output is still 100-1000x smaller than any embedded Python interpreter.

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Write code you already know

Pick the API that fits your background. Both compile to the same bare-metal firmware.

CircuitPython

# The exact same code that runs on a Pico under CircuitPython
import board
import digitalio
from time import sleep_ms

led = digitalio.DigitalInOut(board.LED)
led.direction = digitalio.Direction.OUTPUT

while True:
    led.value = True
    sleep_ms(500)
    led.value = False
    sleep_ms(500)

MicroPython

# The exact same code that runs on a Pico under MicroPython
from machine import Pin
from utime import sleep_ms

led = Pin(13, Pin.OUT)

while True:
    led.value(1)
    sleep_ms(500)
    led.value(0)
    sleep_ms(500)

pymcu build   # → dist/firmware.hex  (56 bytes flash, 0 bytes SRAM)
pymcu flash   # → avrdude upload to Arduino Uno

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First binary in under 5 minutes

1. Install

pipx install "pymcu-compiler[avr]"

Requires Python 3.11+ and pipx. The [avr] extra includes the AVR toolchain.

Package name: PyMCU is published as pymcu-compiler on PyPI while a PEP 541 request to reclaim the pymcu name is under review. Once approved, a pymcu metapackage will alias pymcu-compiler — installs and project configs will stay compatible.

2. Create a project

pymcu new blink
cd blink

3. Choose your API and write the program

CircuitPython style — add pymcu-circuitpython to dependencies:

# pyproject.toml
[project]
dependencies = ["pymcu-compiler", "pymcu-circuitpython"]

[tool.pymcu]
board     = "arduino_uno"
frequency = 16000000

# src/main.py
import board
import digitalio
from time import sleep_ms

led = digitalio.DigitalInOut(board.LED)
led.direction = digitalio.Direction.OUTPUT

while True:
    led.value = True
    sleep_ms(500)
    led.value = False
    sleep_ms(500)

MicroPython style — add pymcu-micropython to dependencies:

# pyproject.toml
[project]
dependencies = ["pymcu-compiler", "pymcu-micropython"]

[tool.pymcu]
board     = "arduino_uno"
frequency = 16000000

# src/main.py
from machine import Pin
from utime import sleep_ms

led = Pin(13, Pin.OUT)

while True:
    led.value(1)
    sleep_ms(500)
    led.value(0)
    sleep_ms(500)

4. Build and flash

pymcu build
# Compiling src/main.py...
# → dist/firmware.hex

pymcu flash --port /dev/cu.usbmodem*
# avrdude: flash verified

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Choosing an API

Package	API surface	Install
pymcu-circuitpython	digitalio, analogio, busio, pwmio, time, board, neopixel	pip install pymcu-circuitpython
pymcu-micropython	machine (Pin/UART/ADC/PWM/SPI/I2C/Timer/WDT), utime	pip install pymcu-micropython
pymcu.hal.*	Direct register-level HAL — lowest overhead	pymcu-stdlib (installed automatically with pymcu-compiler)

Start with MicroPython or CircuitPython — they are stable, community-specified, and backed by real hardware compatibility guarantees. The pymcu.hal.* native HAL is functional but its API may change between alpha releases — avoid it unless you need direct register access not yet covered by the compat layers.

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Supported targets

Architecture	Chips
AVR (ATmega)	ATmega48/88/168/328P, ATmega2560, ATmega32U4
AVR (ATtiny)	ATtiny25/45/85, ATtiny24/44/84, ATtiny13/13A, ATtiny2313/4313

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HAL coverage (ATmega328P / Arduino Uno)

Module	Features
pymcu.hal.gpio	Pin — high / low / toggle / irq / pulse_in
pymcu.hal.uart	UART — write / read / println / RX interrupt
pymcu.hal.adc	AnalogPin — poll + interrupt; internal temperature
pymcu.hal.timer	Timer(n, prescaler) — CTC mode; millis() / micros()
pymcu.hal.pwm	PWM — multi-channel; set_duty / set_freq
pymcu.hal.spi	SPI + SoftSPI
pymcu.hal.i2c	I2C + SoftI2C
pymcu.hal.eeprom	EEPROM — write(addr, val) / read(addr)
pymcu.hal.watchdog	Watchdog — enable / disable / feed
pymcu.hal.power	sleep_idle / power_down / standby

Drivers: DHT11, DS18B20, LM35, HD44780 LCD, SSD1306 OLED, MAX7219, BMP280, WS2812 NeoPixel.

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What Python features are supported

PyMCU accepts Python syntax but enforces a strict compile-time type system.

Supported:

• Integer types: uint8, int8, uint16, int16, uint32, int32, float
• Fixed arrays buf: uint8[16] and heap-bounded lists x: list[uint8] = list()
• for, while, if, match / case, with, class, @inline, lambda
• @interrupt ISR handlers, asm("...") inline assembly
• try / except / raise / finally (AVR only; raise and except in same function)
• CircuitPython and MicroPython compat packages

Not supported:

• dict / set (hash tables require heap)
• Runtime f"value={x}" with non-constant expressions (compile-time constants only)
• async / await — use @interrupt + polling loop instead
• Closures capturing mutable variables — use explicit parameters
• *args / **kwargs

The compiler rejects unsupported features with a clear error at compile time. See the Language Limitations page for the full list.

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CLI reference

Command	Description
pymcu new <name>	Scaffold a new project
pymcu build	Compile src/ → dist/firmware.hex
pymcu flash	Upload via avrdude
pymcu clean	Remove build artifacts

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Sustainability

Post-alpha development will be slower and community-driven. If PyMCU saves you time, consider sponsoring the project — the goal is $200-300/month to cover the AI tooling costs that made this first release possible and keep active development going.

Sponsor on GitHub

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License

All components are licensed under the MIT License. Your compiled firmware output is entirely yours — no runtime license, no attribution required.

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Contributing

See CONTRIBUTING.md and LANGUAGE_ROADMAP.md.

Credits

Special thanks to Richard Wardlow, creator of the original pyMCU project (2012). See CREDITS.md for the full acknowledgement.