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Porting boards

At some point you might need to port a new BOARD to RIOT, either because that specific development board is not yet supported or because you have a custom BOARD for your project.

If you want to port a BOARD to RIOT you have two choices: doing it inside of RIOTBASE or outside. In either case the file structure is basically the same and moving from one to another is easy.

This guide details the generic structure you need to add a new BOARD to RIOT, the different files as well as their functionality.

Note
We assume here that your CPU and CPU_MODEL is already supported in RIOT so no peripheral or cpu implementation is needed.

Porting flowchart

General structure

Like applications or modules, boards consist on a directory containing source files and makefiles. Usually a BOARD directory has the following structure

board-foo/
|----dist/
|----scripts
|----board.c
|----doc.txt
|----include/
|----periph_conf.h
|----board.h
|----gpio_params.h
|----Makefile
|----Makefile.dep
|----Makefile.features
|----Makefile.include

Source files

Header files in board-foo/include define physical mappings or configurations. e.g:

  • periph_conf.h: defines configurations and mappings for peripherals as well as clock configurations.
  • board.h: holds board specific definitions or mappings, for example LEDs, buttons. It might as well override default drivers parameters (e.g.: assigning specific pin connections to a LCD screen, radio, etc.). Some boards might also define optimized XTIMER_% values (e.g. XTIMER_BACKOFF).
  • gpio_params.h: if the board supports SAUL then its saul_gpio_params_t is defined here. (Analogously, a adc_params.h can contain a saul_adc_params_t, and pwm_params.h a saul_pwm_rgb_params_t and a saul_pwm_dimmer_params_t).
  • other: other specific headers needed by one BOARD
Note
Header files do not need to be defined in include/, but if defined somewhere else then they must be added to the include path. In Makefile.include: INCLUDES += -I<some>/<directory>/<path>

Board initialization functions are defined in board.c. This file can define a board_init() function that is called at startup. It is run before the scheduler is started, so it must not block (e.g. by performing I2C operations).

void board_init(void)
{
/* initialize GPIO or others... */
...
}
board_init
void board_init(void)
Initialize board specific hardware.

Makefiles

Makefile

A board's Makefile just needs to include Makefile.base in the RIOT repository and define the MODULE as board (see modules for more details)

MODULE = board
include $(RIOTBASE)/Makefile.base

Makefile.dep

Dependencies on other MODULES or FEATURES can be defined here. This might specify MODULES or dependencies that need to be pulled under specific configurations. e.g.: if your board has a sx1276 lora chip:

ifneq (,$(filter netdev_default,$(USEMODULE)))
USEMODULE += sx1276
endif
Note
Makefile.dep is processed only once so you have to take care of adding the dependency block for your board before its dependencies pull in their own dependencies.

Default configurations

As explained in Default Configurations, there are two pseudomodules that are used to indicate that certain drivers of devices present in the platform should be enabled. Each board (or CPU) has knowledge as to which drivers should be enabled in each case.

The previous code snippet shows how a board which has a Semtech SX1272 and SX1276 radios driver device, pulls in its driver when the default network interfaces are required.

When the pseudomodule saul_default is enabled, the board should pull in all the drivers of the devices it has which provide a [S]ensor [A]ctuator [U]ber [L]ayer interface. This is usually done as following:

ifneq (,$(filter saul_default,$(USEMODULE)))
USEMODULE += saul_gpio
USEMODULE += apds9960
USEMODULE += bmp280_i2c
USEMODULE += lis3mdl
USEMODULE += sht3x
endif

Makefile.features

This file defines all the features provided by the BOARD. These features might also need to be supported by the CPU. Here, define the CPU and CPU_MODEL (see build system basics for more details on these variables).

e.g.:

CPU = foo
CPU_MODEL = foobar
# Put defined MCU peripherals here (in alphabetical order)
FEATURES_PROVIDED += periph_i2c
FEATURES_PROVIDED += periph_spi
FEATURES_PROVIDED += periph_uart

Makefile.include

This file contains BSP or toolchain configurations for the BOARD. It should at least define the configuration needed for flashing (i.e. specify a default programmer) as well as the serial configuration (if one is available). The default serial port configuration is provided by makefiles/tools/serial.inc.mk and define the following values for the serial port (depends on the host OS):

PORT_LINUX ?= /dev/ttyACM0
PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))

So if the board is also using this, there's no need to redefine these variables in the board configuration.

For example a board that is using a custom serial port (via an USB to serial adapter) and that is flashed using openocd by default would have the following content in its Makefile.include:

# Define the default port depending on the host OS
PORT_LINUX ?= /dev/ttyUSB0
PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbserial*)))
# this board uses openocd
PROGRAMMER ?= openocd

Timer Configurations

When using the high level timer ztimer there is an overhead in calling the ztimer_sleep and ztimer_set functions. This offset can be compensated for. It can be measured by running tests/sys/ztimer_overhead on your board, i.e:

$ BOARD=my-new-board make -C tests/sys/ztimer_overhead flash term
main(): This is RIOT!
ZTIMER_USEC auto_adjust params:
ZTIMER_USEC->adjust_set = xx
ZTIMER_USEC->adjust_sleep = xx
ZTIMER_USEC auto_adjust params cleared
zitmer_overhead_set...
min=6 max=7 avg_diff=6
zitmer_overhead_sleep...
min=21 max=21 avg_diff=21
ZTIMER_USEC adjust params for my-new-board:
CONFIG_ZTIMER_USEC_ADJUST_SET 6
CONFIG_ZTIMER_USEC_ADJUST_SLEEP 21

The last two lines can be added as defines to the new board board.h:

#define CONFIG_ZTIMER_USEC_ADJUST_SET 6
#define CONFIG_ZTIMER_USEC_ADJUST_SLEEP 21

Alternatively, the pseudomodule ztimer_auto_adjust can be used in an application to enable automatic timer offset compensation at board startup. This however incurs overhead both in the text segment and at bootup time.

doc.txt

Although not explicitly needed, if upstreamed and as a general good practice, this file holds all BOARD documentation. This can include datasheet reference, documentation on how to flash, etc.

The documentation must be under the proper doxygen group, you can compile the documentation by calling make doc and then open the generated html file on any browser.

/**
@defgroup boards_foo FooBoard
@ingroup boards
@brief Support for the foo board
@author FooName BarName <foo.bar@baz.com>
### User Interface
....
### Using UART
...
### Flashing the device
...
*/

Helper tools

To help you start porting a board, the RIOT build system provides the generate-board make target. It is a wrapper around the riotgen command line tool that is helpful when starting to port a board: all required files are generated with copyright headers, doxygen groups, etc, so you can concentrate on the port. The board source files are created in the boards/<board name> directory.

Usage:

From the RIOT base directory, run:

make generate-board

Then answer a few questions about the driver:

  • Board name: Enter a name for your board. It will be used as the name of the board directory under boards.
  • Board displayed name: Enter the name of the board, as displayed in the Doxygen documentation.
  • CPU name: Enter the name of the CPU embedded on the board.
  • CPU model name: Enter the precise model name of the CPU.
  • Features provided: CPU features provided (and configured) for this board.

Other global information (author name, email, organization) should be retrieved automatically from your git configuration.

Using Common code

To avoid code duplication, common code across boards has been grouped in boards/common. e.g. BOARDs based on the same cpu (boards/common/nrf52) or BOARDs having the same layout boards/common/nucleo64.

In the case of source files this means some functions like board_init can be already defined in the common code. Unless having specific configurations or initialization you might not need a board.c or board.h. Another common use case is common peripheral configurations, for example in the cfg_timer_tim5.h:

static const timer_conf_t timer_config[] = {
{
.dev = TIM5,
.max = 0xffffffff,
.rcc_mask = RCC_APB1ENR_TIM5EN,
.bus = APB1,
.irqn = TIM5_IRQn
}
};
#define TIMER_0_ISR isr_tim5
#define TIMER_NUMOF ARRAY_SIZE(timer_config)
APB1
@ APB1
Advanced Peripheral Bus 1
Definition periph_cpu.h:79
timer_conf_t
Timer device configuration.
Definition periph_cpu.h:264
timer_conf_t::dev
TC0_t * dev
Pointer to the used as Timer device.
Definition periph_cpu.h:265

If you want to use common makefiles, include them at the end of the specific Makefile, e.g. for a Makefile.features:

CPU = foo
CPU_MODEL = foobar
# Put defined MCU peripherals here (in alphabetical order)
FEATURES_PROVIDED += periph_i2c
FEATURES_PROVIDED += periph_spi
FEATURES_PROVIDED += periph_uart
include $(RIOTBOARD)/common/foo_common/Makefile.features

Boards outside of RIOTBASE

All BOARDs in RIOT reside in RIOTBOARD (RIOTBOARD being a make variable set to $(RIOTBOARD)/boards).

If one wants to use a BOARD outside of RIOTBOARD, the way to go is setting the EXTERNAL_BOARD_DIRS variable to the path to the directory containing your external boards, e.g.: EXTERNAL_BOARD_DIRS=/home/external-boards/ (this would commonly be done in your application Makefile or your environment). You can specify multiple directories separated by spaces.

/home/
|----RIOT/
|---- ...
|----external-boards/
|----board-foo/
|----dist/
|----scripts
|----board.c
|----doc.txt
|----include/
|----periph_conf.h
|----board.h
|----gpio_params.h
|----Makefile
|----Makefile.dep
|----Makefile.features
|----Makefile.include

If the external BOARD is very similar to a BOARD already present in RIOTBOARD, the external BOARD (board-foo) can inherit from that parent BOARD (e.g: foo-parent).

In this case some special considerations must be taken with the makefiles:

  • Makefile
    • MODULE cannot be board: foo-parent will already define MODULE = board, so use any other name, lets say MODULE = board-foo.
    • Include the location of the parent BOARD to inherit from (if there is one):
DIRS += $(RIOTBOARD)/foo-parent
  • Makefile.include
    • duplicate the include done by $(RIOTBASE)/Makefile.include to also include the parent board header. e.g: if inheriting from foo-parent `INCLUDES += $(addprefix -I,$(wildcard $(RIOTBOARD)/foo-parent/include))
  • Makefile.dep: board is added by default to USEMODULE but since board-foo is used for this BOARD, it must be explicitly included by adding USEMODULE += board-foo.
  • Then simply include in each Makefile.* the corresponding parent BOARD Makefile.*, just as it is done for common BOARD code (as explained in Using Common code). e.g: include $(RIOTBOARD)/foo-parent/Makefile.*include*

An example can be found in tests/build_system/external_board_native

Tools

Some scripts and tools available to ease BOARD porting and testing:

  • Run dist/tools/insufficient_memory/add_insufficient_memory_board.sh <board> if your board has little memory. This updates the Makefile.ci lists to exclude the BOARD from automated compile-tests of applications that do not fit on the BOARDs CPU.
  • Run dist/tools/compile_and_test_for_board/compile_and_test_for_board.py . <board> --with-test-only to run all automated tests on the new board.

Further reference