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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 (and/or C++ if implemented for your port) 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. Both Make and CMake build tools are supported, and when writing an external module it’s a good idea to add the build files for both of these tools so the module can be used on all ports. But when compiling a particular port you will only need to use one method of building, either Make or CMake.

An alternative approach is to use Native machine code in .mpy files which allows writing custom C code that is placed in a .mpy file, which can be imported dynamically in to a running MicroPython system without the need to recompile the main firmware.

Structure of an external C module

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

  • *.c / *.cpp / *.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.

  • contains the Makefile fragment for this module.

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

    Your must add your modules source files to the SRC_USERMOD_C or SRC_USERMOD_LIB_C variables. The former will be processed for MP_QSTR_ and MP_REGISTER_MODULE definitions, the latter will not (e.g. helpers and library code that isn’t MicroPython-specific). These paths should include your expanded copy of $(USERMOD_DIR), e.g.:

    SRC_USERMOD_C += $(EXAMPLE_MOD_DIR)/modexample.c
    SRC_USERMOD_LIB_C += $(EXAMPLE_MOD_DIR)/utils/algorithm.c

    Similarly, use SRC_USERMOD_CXX and SRC_USERMOD_LIB_CXX for C++ source files.

    If you have custom compiler options (like -I to add directories to search for header files), these should be added to CFLAGS_USERMOD for C code and to CXXFLAGS_USERMOD for C++ code.

  • micropython.cmake contains the CMake configuration for this module.

    In micropython.cmake, you may use ${CMAKE_CURRENT_LIST_DIR} as the path to the current module.

    Your micropython.cmake should define an INTERFACE library and associate your source files, compile definitions and include directories with it. The library should then be linked to the usermod target.

    add_library(usermod_cexample INTERFACE)
    target_sources(usermod_cexample INTERFACE
    target_include_directories(usermod_cexample INTERFACE
    target_link_libraries(usermod INTERFACE usermod_cexample)

    See below for full usage example.

Basic example

The cexample module provides examples for a function and a class. The cexample.add_ints(a, b) function adds two integer args together and returns the result. The cexample.Timer() type creates timers that can be used to measure the elapsed time since the object is instantiated.

The module can be found in the MicroPython source tree in the examples directory and has a source file and a Makefile fragment with content as described above:

         ├── examplemodule.c
         └── micropython.cmake

Refer to the comments in these files for additional explanation. Next to the cexample module there’s also cppexample which works in the same way but shows one way of mixing C and C++ code in MicroPython.

Compiling the cmodule into MicroPython

To build such a module, compile MicroPython (see getting started), applying 2 modifications:

  1. Set the build-time flag USER_C_MODULES to point to the modules you want to include. For ports that use Make this variable should be a directory which is searched automatically for modules. For ports that use CMake this variable should be a file which includes the modules to build. See below for details.

  2. Enable the modules by setting the corresponding C preprocessor macro to 1. This is only needed if the modules you are building are not automatically enabled.

For building the example modules which come with MicroPython, set USER_C_MODULES to the examples/usercmodule directory for Make, or to examples/usercmodule/micropython.cmake for CMake.

For example, here’s how the to build the unix port with the example modules:

cd micropython/ports/unix
make USER_C_MODULES=../../examples/usercmodule

You may need to run make clean once at the start when including new user modules in the build. The build output will show the modules found:

Including User C Module from ../../examples/usercmodule/cexample
Including User C Module from ../../examples/usercmodule/cppexample

For a CMake-based port such as rp2, this will look a little different (note that CMake is actually invoked by make):

cd micropython/ports/rp2
make USER_C_MODULES=../../examples/usercmodule/micropython.cmake

Again, you may need to run make clean first for CMake to pick up the user modules. The CMake build output lists the modules by name:

Including User C Module(s) from ../../examples/usercmodule/micropython.cmake
Found User C Module(s): usermod_cexample, usermod_cppexample

The contents of the top-level micropython.cmake can be used to control which modules are enabled.

For your own projects it’s more convenient to keep custom code out of the main MicroPython source tree, so a typical project directory structure will look like this:

├── modules/
│   ├── example1/
│   │   ├── example1.c
│   │   ├──
│   │   └── micropython.cmake
│   ├── example2/
│   │   ├── example2.c
│   │   ├──
│   │   └── micropython.cmake
│   └── micropython.cmake
└── micropython/
   ... ├──stm32/

When building with Make set USER_C_MODULES to the my_project/modules directory. For example, building the stm32 port:

cd my_project/micropython/ports/stm32
make USER_C_MODULES=../../../modules

When building with CMake the top level micropython.cmake – found directly in the my_project/modules directory – should include all of the modules you want to have available:


Then build with:

cd my_project/micropython/ports/esp32
make USER_C_MODULES=../../../../modules/micropython.cmake

Note that the esp32 port needs the extra .. for relative paths due to the location of its main CMakeLists.txt file. You can also specify absolute paths to USER_C_MODULES.

All modules specified by the USER_C_MODULES variable (either found in this directory when using Make, or added via include when using CMake) will be compiled, but only those which are enabled will be available for importing. User modules are usually enabled by default (this is decided by the developer of the module), in which case there is nothing more to do than set USER_C_MODULES as described above.

If a module is not enabled by default then the corresponding C preprocessor macro must be enabled. This macro name can be found by searching for the MP_REGISTER_MODULE line in the module’s source code (it usually appears at the end of the main source file). This macro should be surrounded by a #if X / #endif pair, and the configuration option X must be set to 1 using CFLAGS_EXTRA to make the module available. If there is no #if X / #endif pair then the module is enabled by default.

For example, the examples/usercmodule/cexample module is enabled by default so has the following line in its source code:

MP_REGISTER_MODULE(MP_QSTR_cexample, example_user_cmodule);

Alternatively, to make this module disabled by default but selectable through a preprocessor configuration option, it would be:

MP_REGISTER_MODULE(MP_QSTR_cexample, example_user_cmodule);

In this case the module is enabled by adding CFLAGS_EXTRA=-DMODULE_CEXAMPLE_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.

Module usage in MicroPython

Once built into your copy of MicroPython, the module can now be accessed in Python just like any other builtin module, e.g.

import cexample
print(cexample.add_ints(1, 3))
# should display 4
from cexample import Timer
from time import sleep_ms

watch = Timer()
# should display approximately 1000