MicroPython external C modules

When developing modules for use with MicroPython you may find you run into limitations with the Python environment, often due to an inability to access certain hardware resources or Python speed limitations.

If your limitations can’t be resolved with suggestions in Maximising MicroPython Speed, writing some or all of your module in C is a viable option.

If your module is designed to access or work with commonly available hardware or libraries please consider implementing it inside the MicroPython source tree alongside similar modules and submitting it as a pull request. If however you’re targeting obscure or proprietary systems it may make more sense to keep this external to the main MicroPython repository.

This chapter describes how to compile such external modules into the MicroPython executable or firmware image.

Structure of an external C module

A MicroPython user C module is a directory with the following files:

  • *.c and/or *.h source code files for your module.

    These will typically include the low level functionality being implemented and the MicroPython binding functions to expose the functions and module(s).

    Currently the best reference for writing these functions/modules is to find similar modules within the MicroPython tree and use them as examples.

  • micropython.mk contains the Makefile fragment for this module.

    $(USERMOD_DIR) is available in micropython.mk as the path to your module directory. As it’s redefined for each c module, is should be expanded in your micropython.mk to a local make variable, eg EXAMPLE_MOD_DIR := $(USERMOD_DIR)

    Your micropython.mk must add your modules C files relative to your expanded copy of $(USERMOD_DIR) to SRC_USERMOD, eg SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c

    If you have custom CFLAGS settings or include folders to define, these should be added to CFLAGS_USERMOD.

    See below for full usage example.

Basic Example

This simple module named example provides a single function example.add_ints(a, b) which adds the two integer args together and returns the result.


├── example.c
└── micropython.mk


// Include required definitions first.
#include "py/obj.h"
#include "py/runtime.h"
#include "py/builtin.h"

// This is the function which will be called from Python as example.add_ints(a, b).
STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_t b_obj) {
    // Extract the ints from the micropython input objects
    int a = mp_obj_get_int(a_obj);
    int b = mp_obj_get_int(b_obj);

    // Calculate the addition and convert to MicroPython object.
    return mp_obj_new_int(a + b);
// Define a Python reference to the function above
STATIC MP_DEFINE_CONST_FUN_OBJ_2(example_add_ints_obj, example_add_ints);

// Define all properties of the example module.
// Table entries are key/value pairs of the attribute name (a string)
// and the MicroPython object reference.
// All identifiers and strings are written as MP_QSTR_xxx and will be
// optimized to word-sized integers by the build system (interned strings).
STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
    { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_example) },
    { MP_ROM_QSTR(MP_QSTR_add_ints), MP_ROM_PTR(&example_add_ints_obj) },
STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);

// Define module object.
const mp_obj_module_t example_user_cmodule = {
    .base = { &mp_type_module },
    .globals = (mp_obj_dict_t*)&example_module_globals,

// Register the module to make it available in Python



# Add all C files to SRC_USERMOD.

# We can add our module folder to include paths if needed
# This is not actually needed in this example.

Finally you will need to define MODULE_EXAMPLE_ENABLED to 1. This can be done by adding CFLAGS_EXTRA=-DMODULE_EXAMPLE_ENABLED=1 to the make command, or editing mpconfigport.h or mpconfigboard.h to add


Note that the exact method depends on the port as they have different structures. If not done correctly it will compile but importing will fail to find the module.

Compiling the cmodule into MicroPython

To build such a module, compile MicroPython (see getting started) with an extra make flag named USER_C_MODULES set to the directory containing all modules you want included (not to the module itself). For example:


├── modules/
│   └──example/
│       ├──example.c
│       └──micropython.mk
└── micropython/
   ... ├──stm32/

Building for stm32 port:

cd my_project/micropython/ports/stm32

Module usage in MicroPython

Once built into your copy of MicroPython, the module implemented in example.c above can now be accessed in Python just like any other builtin module, eg

import example
print(example.add_ints(1, 3))
# should display 4