.. _board_porting_guide:

Board Porting Guide
###################

To add Zephyr support for a new :term:`board`, you at least need a *board
directory* with various files in it. Files in the board directory inherit
support for at least one SoC and all of its features. Therefore, Zephyr must
support your :term:`SoC` as well.

Boards, SoCs, etc.
******************

Zephyr's hardware support hierarchy has these layers, from most to least
specific:

- Board: a particular CPU instance and its peripherals in a concrete hardware
  specification
- SoC: the exact system on a chip the board's CPU is part of
- SoC series: a smaller group of tightly related SoCs
- SoC family: a wider group of SoCs with similar characteristics
- CPU core: a particular CPU in an architecture
- Architecture: an instruction set architecture

You can visualize the hierarchy like this:

.. figure:: board/hierarchy.png
   :width: 500px
   :align: center
   :alt: Configuration Hierarchy

   Configuration Hierarchy

Here are some examples. Notice how the SoC series and family levels are
not always used.

.. list-table::
   :header-rows: 1

   * - Board
     - SoC
     - SoC series
     - SoC family
     - CPU core
     - Architecture
   * - :ref:`nrf52_pca10040 <nrf52_pca10040>`
     - nRF52832
     - nRF52
     - Nordic nRF5
     - Arm Cortex-M4
     - Arm
   * - :ref:`frdm_k64f <frdm_k64f>`
     - MK64F12
     - Kinetis K6x
     - NXP Kinetis
     - Arm Cortex-M4
     - Arm
   * - :ref:`stm32h474i_disco <stm32h747i_disco_board>`
     - STM32H747XI
     - STM32H7
     - STMicro STM32
     - Arm Cortex-M7
     - Arm
   * - :ref:`rv32m1_vega_ri5cy <rv32m1_vega>`
     - RV32M1
     - (Not used)
     - (Not used)
     - RI5CY
     - RISC-V

Make sure your SoC is supported
*******************************

Start by making sure your SoC is supported by Zephyr. If it is, it's time to
:ref:`create-your-board-directory`. If you don't know, try:

- checking :ref:`boards` for names that look relevant, and reading individual
  board documentation to find out for sure.
- asking your SoC vendor

If you need to add SoC, CPU core, or even architecture support, this is the
wrong page, but here is some general advice.

Architecture
============

See :ref:`architecture_porting_guide`.

CPU Core
========

CPU core support files go in ``core`` subdirectories under :zephyr_file:`arch`,
e.g. :zephyr_file:`arch/x86/core`.

See :ref:`gs_toolchain` for information about toolchains (compiler, linker,
etc.) supported by Zephyr. If you need to support a new toolchain,
:ref:`build_overview` is a good place to start learning about the build system.
Please reach out to the community if you are looking for advice or want to
collaborate on toolchain support.

SoC
===

Zephyr SoC support files are in architecture-specific subdirectories of
:zephyr_file:`soc`. They are generally grouped by SoC family.

When adding a new SoC family or series for a vendor that already has SoC
support within Zephyr, please try to extract common functionality into shared
files to avoid duplication. If there is no support for your vendor yet, you can
add it in a new directory ``zephyr/soc/<YOUR-ARCH>/<YOUR-SOC>``; please use
self-explanatory directory names.

.. _create-your-board-directory:

Create your board directory
***************************

Once you've found an existing board that uses your SoC, you can usually start
by copy/pasting its board directory and changing its contents for your
hardware.

You need to give your board a unique name. Run ``west boards`` for a list of
names that are already taken, and pick something new. Let's say your board is
called ``plank`` (please don't actually use that name).

Start by creating the board directory ``zephyr/boards/<ARCH>/plank``, where
``<ARCH>`` is your SoC's architecture subdirectory. (You don't have to put your
board directory in the zephyr repository, but it's the easiest way to get
started. See :ref:`custom_board_definition` for documentation on moving your
board directory to a separate repository once it's working.)

Your board directory should look like this:

.. code-block:: none

   boards/<ARCH>/plank
   ├── board.cmake
   ├── CMakeLists.txt
   ├── doc
   │   ├── plank.png
   │   └── index.rst
   ├── Kconfig.board
   ├── Kconfig.defconfig
   ├── plank_defconfig
   ├── plank.dts
   └── plank.yaml

Replace ``plank`` with your board's name, of course.

The mandatory files are:

#. :file:`plank.dts`: a hardware description in :ref:`devicetree` format. This
   declares your SoC, connectors, and any other hardware components such as
   LEDs, buttons, sensors, or communication peripherals (USB, BLE controller,
   etc).

#. :file:`Kconfig.board`, :file:`Kconfig.defconfig`, :file:`plank_defconfig`:
   software configuration in :ref:`kconfig` formats. This provides default
   settings for software features and peripheral drivers.

The optional files are:

- :file:`board.cmake`: used for :ref:`flash-and-debug-support`
- :file:`CMakeLists.txt`: if you need to add additional source files to
  your build.

  One common use for this file is to add a :file:`pinmux.c` file in your board
  directory to the build, which configures pin controllers at boot time. In
  that case, :file:`CMakeLists.txt` usually looks like this:

  .. code-block:: cmake

     if(CONFIG_PINMUX)
       zephyr_library()
       zephyr_library_sources(pinmux.c)
       zephyr_library_include_directories(${ZEPHYR_BASE}/drivers)
     endif()

- :file:`doc/index.rst`, :file:`doc/plank.png`: documentation for and a picture
  of your board. You only need this if you're :ref:`contributing-your-board` to
  Zephyr.
- :file:`plank.yaml`: a YAML file with miscellaneous metadata used by the
  :ref:`sanitycheck_script`.

.. _default_board_configuration:

Write your devicetree
*********************

The devicetree file :file:`boards/<ARCH>/plank/plank.dts` describes your board
hardware in the Devicetree Source (DTS) format (as usual, change ``plank`` to
your board's name). If you're new to devicetree, see :ref:`devicetree-intro`.

In general, :file:`plank.dts` should look like this:

.. code-block:: none

   /dts-v1/;
   #include <your_soc_vendor/your_soc.dtsi>

   / {
   	model = "A human readable name";
   	compatible = "yourcompany,plank";

   	chosen {
   		zephyr,console = &your_uart_console;
   		zephyr,sram = &your_memory_node;
   		/* other chosen settings  for your hardware */
   	};

   	/*
   	 * Your board-specific hardware: buttons, LEDs, sensors, etc.
   	 */

   	leds {
   		compatible = "gpio-leds";
   		led0: led_0 {
   			gpios = < /* GPIO your LED is hooked up to */ >;
   			label = "LED 0";
   		};
   		/* ... other LEDs ... */
   	};

   	buttons {
   		compatible = "gpio-keys";
   		/* ... your button definitions ... */
   	};

   	/* These aliases are provided for compatibility with samples */
   	aliases {
   		led0 = &led0; /* now you support the blinky sample! */
   		/* other aliases go here */
   	};
   };

   &some_peripheral_you_want_to_enable { /* like a GPIO or SPI controller */
   	status = "okay";
   };

   &another_peripheral_you_want {
   	status = "okay";
   };

If you're in a hurry, simple hardware can usually be supported by copy/paste
followed by trial and error. If you want to understand details, you will need
to read the rest of the devicetree documentation and the devicetree
specification.

Write Kconfig files
*******************

Zephyr uses the Kconfig language to configure software features. Your board
needs to provide some Kconfig settings before you can compile a Zephyr
application for it.

Setting Kconfig configuration values is documented in detail in
:ref:`setting_configuration_values`.

There are three mandatory Kconfig files in the board directory for a board
named ``plank``:

.. code-block:: none

   boards/<ARCH>/plank
   ├── Kconfig.board
   ├── Kconfig.defconfig
   └── plank_defconfig

:file:`Kconfig.board`
  Included by :zephyr_file:`boards/Kconfig` to include your board
  in the list of options.

  This should at least contain a definition for a ``BOARD_PLANK`` option,
  which looks something like this:

  .. code-block:: none

     config BOARD_PLANK
     	bool "Plank board"
     	depends on SOC_SERIES_YOUR_SOC_SERIES_HERE
     	select SOC_PART_NUMBER_ABCDEFGH

:file:`Kconfig.defconfig`
  Board-specific default values for Kconfig options.

  The entire file should be inside an ``if BOARD_PLANK`` / ``endif`` pair of
  lines, like this:

  .. code-block:: none

     if BOARD_PLANK

     # Always set CONFIG_BOARD here. This isn't meant to be customized,
     # but is set as a "default" due to Kconfig language restrictions.
     config BOARD
     	default "plank"

     # Other options you want enabled by default go next. Examples:

     config FOO
     	default y

     if NETWORKING
     config SOC_ETHERNET_DRIVER
     	default y
     endif # NETWORKING

     endif # BOARD_PLANK

:file:`plank_defconfig`
  A Kconfig fragment that is merged as-is into the final build directory
  :file:`.config` whenever an application is compiled for your board.

  You should at least select your board's SOC and do any mandatory settings for
  your system clock, console, etc. The results are architecture-specific, but
  typically look something like this:

  .. code-block:: none

     CONFIG_SOC_${VENDOR_XYZ3000}=y                      /* select your SoC */
     CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=120000000   /* set up your clock, etc */
     CONFIG_SERIAL=y

Build, test, and fix
********************

Now it's time to build and test the application(s) you want to run on your
board until you're satisfied.

For example:

.. code-block:: console

   west build -b plank samples/hello_world
   west flash

For ``west flash`` to work, see :ref:`flash-and-debug-support` below. You can
also just flash :file:`build/zephyr/zephyr.elf`, :file:`zephyr.hex`, or
:file:`zephyr.bin` with any other tools you prefer.

.. _porting-general-recommendations:

General recommendations
***********************

For consistency and to make it easier for users to build generic applications
that are not board specific for your board, please follow these guidelines
while porting.

- Unless explicitly recommended otherwise by this section, leave peripherals
  and their drivers disabled by default.

- Configure and enable a system clock, along with a tick source.

- Provide pin and driver configuration that matches the board's valuable
  components such as sensors, buttons or LEDs, and communication interfaces
  such as USB, Ethernet connector, or Bluetooth/Wi-Fi chip.

- If your board uses a well-known connector standard (like Arduino, Mikrobus,
  Grove, or 96Boards connectors), add connector nodes to your DTS and configure
  pin muxes accordingly.

- Configure components that enable the use of these pins, such as
  configuring an SPI instance to use the usual Arduino SPI pins.

- If available, configure and enable a serial output for the console
  using the ``zephyr,console`` chosen node in the devicetree.

- If your board supports networking, configure a default interface.

- Enable all GPIO ports connected to peripherals or expansion connectors.

- If available, enable pinmux and interrupt controller drivers.

- It is recommended to enable the MPU by default, if there is support for it
  in hardware. For boards with limited memory resources it is acceptable to
  disable it.

.. _flash-and-debug-support:

Flash and debug support
***********************

Zephyr supports :ref:`west-build-flash-debug` via west extension commands.

To add ``west flash`` and ``west debug`` support for your board, you need to
create a :file:`board.cmake` file in your board directory. This file's job is
to configure a "runner" for your board. (There's nothing special you need to
do to get ``west build`` support for your board.)

"Runners" are Zephyr-specific Python classes that wrap :ref:`flash and debug
host tools <debug-host-tools>` and integrate with west and the zephyr build
system to support ``west flash`` and related commands. Each runner supports
flashing, debugging, or both. You need to configure the arguments to these
Python scripts in your :file:`board.cmake` to support those commands like this
example :file:`board.cmake`:

.. code-block:: cmake

   board_runner_args(nrfjprog "--nrf-family=NRF52")
   board_runner_args(jlink "--device=nrf52" "--speed=4000")
   board_runner_args(pyocd "--target=nrf52" "--frequency=4000000")

   include(${ZEPHYR_BASE}/boards/common/nrfjprog.board.cmake)
   include(${ZEPHYR_BASE}/boards/common/jlink.board.cmake)
   include(${ZEPHYR_BASE}/boards/common/pyocd.board.cmake)

This example configures the ``nrfjprog``, ``jlink``, and ``pyocd`` runners.

.. warning::

   Runners usually have names which match the tools they wrap, so the ``jlink``
   runner wraps Segger's J-Link tools, and so on. But the runner command line
   options like ``--speed`` etc. are specific to the Python scripts.

For more details:

- Run ``west flash --context`` to see a list of available runners which support
  flashing, and ``west flash --context -r <RUNNER>`` to view the specific options
  available for an individual runner.
- Run ``west debug --context`` and ``west debug --context <RUNNER>`` to get
  the same output for runners which support debugging.
- Run ``west flash --help`` and ``west debug --help`` for top-level options
  for flashing and debugging.
- See :ref:`west-runner` for Python APIs.
- Look for :file:`board.cmake` files for other boards similar to your own for
  more examples.

To see what a ``west flash`` or ``west debug`` command is doing exactly, run it
in verbose mode:

.. code-block:: sh

   west --verbose flash
   west --verbose debug

Verbose mode prints any host tool commands the runner uses.

The order of the ``include()`` calls in your :file:`board.cmake` matters. The
first ``include`` sets the default runner if it's not already set. For example,
including ``nrfjprog.board.cmake`` first means that ``nrjfprog`` is the default
flash runner for this board. Since ``nrfjprog`` does not support debugging,
``jlink`` is the default debug runner.

.. _contributing-your-board:

Contributing your board
***********************

If you want to contribute your board to Zephyr, first -- thanks!

There are some extra things you'll need to do:

#. Make sure you've followed all the :ref:`porting-general-recommendations`.
   They are requirements for boards included with Zephyr.

#. Add documentation for your board using the template file
   :zephyr_file:`doc/templates/board.tmpl`. See :ref:`zephyr_doc` for
   information on how to build your documentation before submitting
   your pull request.

#. Prepare a pull request adding your board which follows the
   :ref:`contribute_guidelines`.
