Tuesday, January 26, 2021

CircuitPython: Programming Hardware in Python

CircuitPython

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circuitpython.org | Get CircuitPython | Documentation | Contributing | Branding | Differences from Micropython | Project Structure

CircuitPython is a beginner friendly, open source version of Python for tiny, inexpensive computers called microcontrollers. Microcontrollers are the brains of many electronics including a wide variety of development boards used to build hobby projects and prototypes. CircuitPython in electronics is one of the best ways to learn to code because it connects code to reality. Simply install CircuitPython on a supported board via drag and drop and then edit a code.py file on the CIRCUITPY drive. The code will automatically reload. No software installs are needed besides a text editor (we recommend Mu for beginners.)

CircuitPython features unified Python core APIs and a growing list of 150+ device libraries and drivers that work with it. These libraries also work on single board computers with regular Python via the Adafruit Blinka Library.

CircuitPython is based on MicroPython. See below for differences. CircuitPython development is sponsored by Adafruit and is available on their educational development boards. Please support both MicroPython and Adafruit.

Get CircuitPython

Official binaries for all supported boards are available through circuitpython.org/downloads. The site includes stable, unstable and continuous builds. Full release notes and assets are available through GitHub releases as well.

Documentation

Guides and videos are available through the Adafruit Learning System under the CircuitPython category. An API reference is also available on Read the Docs. A collection of awesome resources can be found at Awesome CircuitPython.

Specifically useful documentation when starting out:

Contributing

See CONTRIBUTING.md for full guidelines but please be aware that by contributing to this project you are agreeing to the Code of Conduct. Contributors who follow the Code of Conduct are welcome to submit pull requests and they will be promptly reviewed by project admins. Please join the Discord too.

Branding

While we are happy to see CircuitPython forked and modified, we'd appreciate it if forked releases not use the name "CircuitPython" or the Blinka logo. "CircuitPython" means something special to us and those who learn about it. As a result, we'd like to make sure products referring to it meet a common set of requirements.

If you'd like to use the term "CircuitPython" and Blinka for your product here is what we ask:

  • Your product is supported by the primary "adafruit/circuitpython" repo. This way we can update any custom code as we update the CircuitPython internals.
  • Your product is listed on circuitpython.org (source here). This is to ensure that a user of your product can always download the latest version of CircuitPython from the standard place.
  • Your product has a user accessible USB plug which appears as a CIRCUITPY drive when plugged in.

If you choose not to meet these requirements, then we ask you call your version of CircuitPython something else (for example, SuperDuperPython) and not use the Blinka logo. You can say it is "CircuitPython-compatible" if most CircuitPython drivers will work with it.

CircuitPython:

  • Supports native USB on all boards, allowing file editing without special tools.
  • Floats (aka decimals) are enabled for all builds.
  • Error messages are translated into 10+ languages.
  • Does not support concurrency within Python (including interrupts and threading). Some concurrency is achieved with native modules for tasks that require it such as audio file playback.

Behavior

  • The order that files are run and the state that is shared between them. CircuitPython's goal is to clarify the role of each file and make each file independent from each other.
  • boot.py (or settings.py) runs only once on start up before USB is initialized. This lays the ground work for configuring USB at startup rather than it being fixed. Since serial is not available, output is written to boot_out.txt.
  • code.py (or main.py) is run after every reload until it finishes or is interrupted. After it is done running, the vm and hardware is reinitialized. This means you cannot read state from code.py in the REPL anymore. CircuitPython's goal for this change includes reducing confusion about pins and memory being used.
  • After code.py the REPL can be entered by pressing any key. It no longer shares state with code.py so it is a fresh vm.
  • Autoreload state will be maintained across reload.
  • Adds a safe mode that does not run user code after a hard crash or brown out. The hope is that this will make it easier to fix code that causes nasty crashes by making it available through mass storage after the crash. A reset (the button) is needed after it's fixed to get back into normal mode.
  • RGB status LED indicating CircuitPython state, and errors through a sequence of colored flashes.
  • Re-runs code.py or other main file after file system writes over USB mass storage. (Disable with supervisor.disable_autoreload())
  • Entering the REPL after the main code is finished requires a key press which enters the REPL and disables autoreload.
  • Main is one of these: code.txt, code.py, main.py, main.txt
  • Boot is one of these: settings.txt, settings.py, boot.py, boot.txt

API

  • Unified hardware APIs. Documented on ReadTheDocs.
  • API docs are rST within the C files in shared-bindings.
  • No machine API.

Modules

  • No module aliasing. (uos and utime are not available as os and time respectively.) Instead os, time, and random are CPython compatible.
  • New storage module which manages file system mounts. (Functionality from uos in MicroPython.)
  • Modules with a CPython counterpart, such as time, os and random, are strict subsets of their CPython version. Therefore, code from CircuitPython is runnable on CPython but not necessarily the reverse.
  • tick count is available as time.monotonic()

Project Structure

Here is an overview of the top-level source code directories.

Core

The core code of MicroPython is shared amongst ports including CircuitPython:

  • docs High level user documentation in Sphinx reStructuredText format.
  • drivers External device drivers written in Python.
  • examples A few example Python scripts.
  • extmod Shared C code used in multiple ports' modules.
  • lib Shared core C code including externally developed libraries such as FATFS.
  • logo The CircuitPython logo.
  • mpy-cross A cross compiler that converts Python files to byte code prior to being run in MicroPython. Useful for reducing library size.
  • py Core Python implementation, including compiler, runtime, and core library.
  • shared-bindings Shared definition of Python modules, their docs and backing C APIs. Ports must implement the C API to support the corresponding module.
  • shared-module Shared implementation of Python modules that may be based on common-hal.
  • tests Test framework and test scripts.
  • tools Various tools, including the pyboard.py module.

Ports

Ports include the code unique to a microcontroller line and also variations based on the board.

Supported Support status
atmel-samd SAMD21 stable | SAMD51 stable
cxd56 stable
esp32s2 beta
litex alpha
mimxrt10xx alpha
nrf stable
stm F4 stable | others beta
unix alpha
  • stable Highly unlikely to have bugs or missing functionality.
  • beta Being actively improved but may be missing functionality and have bugs.
  • alpha Will have bugs and missing functionality.

The remaining port directories not listed above are in the repo to maintain compatibility with the MicroPython parent project.

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