circuitpython/docs/develop/porting.rst

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.. _porting_to_a_board:
Porting MicroPython
===================
The MicroPython project contains several ports to different microcontroller families and
architectures. The project repository has a `ports <https://github.com/micropython/micropython/tree/master/ports>`_
directory containing a subdirectory for each supported port.
A port will typically contain definitions for multiple "boards", each of which is a specific piece of
hardware that that port can run on, e.g. a development kit or device.
The `minimal port <https://github.com/micropython/micropython/tree/master/ports/minimal>`_ is
available as a simplified reference implementation of a MicroPython port. It can run on both the
host system and an STM32F4xx MCU.
In general, starting a port requires:
- Setting up the toolchain (configuring Makefiles, etc).
- Implementing boot configuration and CPU initialization.
- Initialising basic drivers required for development and debugging (e.g. GPIO, UART).
- Performing the board-specific configurations.
- Implementing the port-specific modules.
Minimal MicroPython firmware
----------------------------
The best way to start porting MicroPython to a new board is by integrating a minimal
MicroPython interpreter. For this walkthrough, create a subdirectory for the new
port in the ``ports`` directory:
.. code-block:: bash
$ cd ports
$ mkdir example_port
The basic MicroPython firmware is implemented in the main port file, e.g ``main.c``:
.. code-block:: c
#include "py/compile.h"
#include "py/gc.h"
#include "py/mperrno.h"
#include "py/stackctrl.h"
#include "shared/runtime/gchelper.h"
#include "shared/runtime/pyexec.h"
// Allocate memory for the MicroPython GC heap.
static char heap[4096];
int main(int argc, char **argv) {
// Initialise the MicroPython runtime.
mp_stack_ctrl_init();
gc_init(heap, heap + sizeof(heap));
mp_init();
// Start a normal REPL; will exit when ctrl-D is entered on a blank line.
pyexec_friendly_repl();
// Deinitialise the runtime.
gc_sweep_all();
mp_deinit();
return 0;
}
// Handle uncaught exceptions (should never be reached in a correct C implementation).
void nlr_jump_fail(void *val) {
for (;;) {
}
}
// Do a garbage collection cycle.
void gc_collect(void) {
gc_collect_start();
gc_helper_collect_regs_and_stack();
gc_collect_end();
}
// There is no filesystem so stat'ing returns nothing.
mp_import_stat_t mp_import_stat(const char *path) {
return MP_IMPORT_STAT_NO_EXIST;
}
// There is no filesystem so opening a file raises an exception.
mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
mp_raise_OSError(MP_ENOENT);
}
We also need a Makefile at this point for the port:
.. code-block:: Makefile
# Include the core environment definitions; this will set $(TOP).
include ../../py/mkenv.mk
# Include py core make definitions.
include $(TOP)/py/py.mk
include $(TOP)/extmod/extmod.mk
# Set CFLAGS and libraries.
CFLAGS = -I. -I$(BUILD) -I$(TOP)
LIBS = -lm
# Define the required source files.
SRC_C = \
main.c \
mphalport.c \
shared/readline/readline.c \
shared/runtime/gchelper_generic.c \
shared/runtime/pyexec.c \
shared/runtime/stdout_helpers.c \
# Define the required object files.
OBJ = $(PY_CORE_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
# Define the top-level target, the main firmware.
all: $(BUILD)/firmware.elf
# Define how to build the firmware.
$(BUILD)/firmware.elf: $(OBJ)
$(ECHO) "LINK $@"
$(Q)$(CC) $(LDFLAGS) -o $@ $^ $(LIBS)
$(Q)$(SIZE) $@
# Include remaining core make rules.
include $(TOP)/py/mkrules.mk
Remember to use proper tabs to indent the Makefile.
MicroPython Configurations
--------------------------
After integrating the minimal code above, the next step is to create the MicroPython
configuration files for the port. The compile-time configurations are specified in
``mpconfigport.h`` and additional hardware-abstraction functions, such as time keeping,
in ``mphalport.h``.
The following is an example of an ``mpconfigport.h`` file:
.. code-block:: c
#include <stdint.h>
// Python internal features.
#define MICROPY_ENABLE_GC (1)
#define MICROPY_HELPER_REPL (1)
#define MICROPY_ERROR_REPORTING (MICROPY_ERROR_REPORTING_TERSE)
#define MICROPY_FLOAT_IMPL (MICROPY_FLOAT_IMPL_FLOAT)
// Enable u-modules to be imported with their standard name, like sys.
#define MICROPY_MODULE_WEAK_LINKS (1)
// Fine control over Python builtins, classes, modules, etc.
#define MICROPY_PY_ASYNC_AWAIT (0)
#define MICROPY_PY_BUILTINS_SET (0)
#define MICROPY_PY_ATTRTUPLE (0)
#define MICROPY_PY_COLLECTIONS (0)
#define MICROPY_PY_MATH (0)
#define MICROPY_PY_IO (0)
#define MICROPY_PY_STRUCT (0)
// Type definitions for the specific machine.
typedef intptr_t mp_int_t; // must be pointer size
typedef uintptr_t mp_uint_t; // must be pointer size
typedef long mp_off_t;
// We need to provide a declaration/definition of alloca().
#include <alloca.h>
// Define the port's name and hardware.
#define MICROPY_HW_BOARD_NAME "example-board"
#define MICROPY_HW_MCU_NAME "unknown-cpu"
#define MP_STATE_PORT MP_STATE_VM
#define MICROPY_PORT_ROOT_POINTERS \
const char *readline_hist[8];
This configuration file contains machine-specific configurations including aspects like if different
MicroPython features should be enabled e.g. ``#define MICROPY_ENABLE_GC (1)``. Making this Setting
``(0)`` disables the feature.
Other configurations include type definitions, root pointers, board name, microcontroller name
etc.
Similarly, an minimal example ``mphalport.h`` file looks like this:
.. code-block:: c
static inline void mp_hal_set_interrupt_char(char c) {}
Support for standard input/output
---------------------------------
MicroPython requires at least a way to output characters, and to have a REPL it also
requires a way to input characters. Functions for this can be implemented in the file
``mphalport.c``, for example:
.. code-block:: c
#include <unistd.h>
#include "py/mpconfig.h"
// Receive single character, blocking until one is available.
int mp_hal_stdin_rx_chr(void) {
unsigned char c = 0;
int r = read(STDIN_FILENO, &c, 1);
(void)r;
return c;
}
// Send the string of given length.
void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
int r = write(STDOUT_FILENO, str, len);
(void)r;
}
These input and output functions have to be modified depending on the
specific board API. This example uses the standard input/output stream.
Building and running
--------------------
At this stage the directory of the new port should contain::
ports/example_port/
├── main.c
├── Makefile
├── mpconfigport.h
├── mphalport.c
└── mphalport.h
The port can now be built by running ``make`` (or otherwise, depending on your system).
If you are using the default compiler settings in the Makefile given above then this
will create an executable called ``build/firmware.elf`` which can be executed directly.
To get a functional REPL you may need to first configure the terminal to raw mode:
.. code-block:: bash
$ stty raw opost -echo
$ ./build/firmware.elf
That should give a MicroPython REPL. You can then run commands like:
.. code-block:: bash
MicroPython v1.13 on 2021-01-01; example-board with unknown-cpu
>>> import sys
>>> sys.implementation
('micropython', (1, 13, 0))
>>>
Use Ctrl-D to exit, and then run ``reset`` to reset the terminal.
Adding a module to the port
---------------------------
To add a custom module like ``myport``, first add the module definition in a file
``modmyport.c``:
.. code-block:: c
#include "py/runtime.h"
STATIC mp_obj_t myport_info(void) {
mp_printf(&mp_plat_print, "info about my port\n");
return mp_const_none;
}
STATIC MP_DEFINE_CONST_FUN_OBJ_0(myport_info_obj, myport_info);
STATIC const mp_rom_map_elem_t myport_module_globals_table[] = {
{ MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_myport) },
{ MP_ROM_QSTR(MP_QSTR_info), MP_ROM_PTR(&myport_info_obj) },
};
STATIC MP_DEFINE_CONST_DICT(myport_module_globals, myport_module_globals_table);
const mp_obj_module_t myport_module = {
.base = { &mp_type_module },
.globals = (mp_obj_dict_t *)&myport_module_globals,
};
MP_REGISTER_MODULE(MP_QSTR_myport, myport_module);
You will also need to edit the Makefile to add ``modmyport.c`` to the ``SRC_C`` list, and
a new line adding the same file to ``SRC_QSTR`` (so qstrs are searched for in this new file),
like this:
.. code-block:: Makefile
SRC_C = \
main.c \
modmyport.c \
mphalport.c \
...
SRC_QSTR += modmyport.c
If all went correctly then, after rebuilding, you should be able to import the new module:
.. code-block:: bash
>>> import myport
>>> myport.info()
info about my port
>>>