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Platform specification

This specification is a 3rd party hardware format to be used in Arduino development software starting from the Arduino IDE 1.5.x series.
This specification allows a 3rd party vendor/maintainer to add support for new boards to the Arduino development software by providing a file to unzip into the hardware folder of Arduino's sketchbook folder.
It is also possible to add new 3rd party boards by providing just one configuration file.

Hardware Folders structure

The new hardware folders have a hierarchical structure organized in two levels:

  • the first level is the vendor/maintainer
  • the second level is the supported architecture

A vendor/maintainer can have multiple supported architectures. For example, below we have three hardware vendors called "arduino", "yyyyy" and "xxxxx":

hardware/arduino/avr/...     - Arduino - AVR Boards
hardware/arduino/sam/...     - Arduino - SAM (32bit ARM) Boards
hardware/yyyyy/avr/...       - Yyy - AVR
hardware/xxxxx/avr/...       - Xxx - AVR

The vendor "arduino" has two supported architectures (AVR and SAM), while "xxxxx" and "yyyyy" have only AVR.

If possible, follow existing architecture name conventions when creating hardware packages. The architecture folder name is used to determine library compatibility and also to permit referencing resources from another core of the same architecture so using a non-standard architecture name can only be harmful to your users. Architecture values are case sensitive (e.g. AVR != avr). Use the vendor folder name to differentiate your package, NOT the architecture name.

Architecture configurations

Each architecture must be configured through a set of configuration files:

  • platform.txt contains definitions for the CPU architecture used (compiler, build process parameters, tools used for upload, etc.)
  • boards.txt contains definitions for the boards (board name, parameters for building and uploading sketches, etc.)
  • programmers.txt contains definitions for external programmers (typically used to burn bootloaders or sketches on a blank CPU/board)

Configuration files format

A configuration file is a list of "key=value" properties. The value of a property can be expressed using the value of another property by putting its name inside brackets "{" "}". For example:

compiler.path=/tools/g++_arm_none_eabi/bin/
compiler.c.cmd=arm-none-eabi-gcc
[....]
recipe.c.o.pattern={compiler.path}{compiler.c.cmd}

In this example the property recipe.c.o.pattern will be set to /tools/g++_arm_none_eabi/bin/arm-none-eabi-gcc that is the composition of the two properties compiler.path and compiler.c.cmd.

Comments

Lines starting with # are treated as comments and will be ignored.

# Like in this example
# --------------------
# I'm a comment!

Automatic property override for specific OS

We can specify an OS-specific value for a property. For example the following file:

tools.bossac.cmd=bossac
tools.bossac.cmd.windows=bossac.exe

will set the property tools.bossac.cmd to the value bossac on Linux and Mac OS and bossac.exe on Windows. Suffixes supported are .linux, .windows and .macosx.

Global Predefined properties

The following automatically generated properties can be used globally in all configuration files:

  • {runtime.platform.path}: the absolute path of the board platform folder (i.e. the folder containing boards.txt)
  • {runtime.hardware.path}: the absolute path of the hardware folder (i.e. the folder containing the board platform folder)
  • {runtime.ide.path}: the absolute path of the Arduino IDE or Arduino CLI folder
  • {runtime.ide.version}: the version number of the Arduino IDE as a number (this uses two digits per version number component, and removes the points and leading zeroes, so Arduino IDE 1.8.3 becomes 01.08.03 which becomes runtime.ide.version=10803). When using Arduino development software other than the Arduino IDE, this is set to a meaningless version number.
  • {ide_version}: Compatibility alias for {runtime.ide.version}
  • {runtime.os}: the running OS ("linux", "windows", "macosx")

Compatibility note: Versions before Arduino IDE 1.6.0 only used one digit per version number component in {runtime.ide.version} (so 1.5.9 was 159, not 10509).

platform.txt

The platform.txt file contains information about a platform's specific aspects (compilers command line flags, paths, system libraries, etc.).

The following meta-data must be defined:

name=Arduino AVR Boards
version=1.5.3

The name will be shown in the Arduino IDE's Board menu or the Name field of arduino-cli core's output.
The version is currently unused, it is reserved for future use (probably together with the Boards Manager to handle dependencies on cores).

Build process

The platform.txt file is used to configure the build process. This is done through a list of recipes. Each recipe is a command line expression that explains how to call the compiler (or other tools) for every build step and which parameter should be passed.

The Arduino development software, before starting the build, determines the list of files to compile. The list is composed of:

  • the user's Sketch
  • source code in the selected board's Core
  • source code in the Libraries used in the sketch

A temporary folder is created to store the build artifacts whose path is available through the global property {build.path}. A property {build.project_name} with the name of the project and a property {build.arch} with the name of the architecture is set as well.

  • {build.path}: The path to the temporary folder to store build artifacts
  • {build.project_name}: The project name
  • {build.arch}: The MCU architecture (avr, sam, etc...)

There are some other {build.xxx} properties available, that are explained in the boards.txt section of this guide.

Recipes to compile source code

We said that the Arduino development software determines a list of files to compile. Each file can be source code written in C (.c files), C++ (.cpp files) or Assembly (.S files). Every language is compiled using its respective recipe:

  • recipe.c.o.pattern: for C files
  • recipe.cpp.o.pattern: for CPP files
  • recipe.S.o.pattern: for Assembly files

The recipes can be built concatenating the following automatically generated properties (for each file compiled):

  • {includes}: the list of include paths in the format "-I/include/path -I/another/path...."
  • {source_file}: the path to the source file
  • {object_file}: the path to the output file

For example the following is used for AVR:

## Compiler global definitions
compiler.path={runtime.ide.path}/tools/avr/bin/
compiler.c.cmd=avr-gcc
compiler.c.flags=-c -g -Os -w -ffunction-sections -fdata-sections -MMD

[......]

## Compile c files
recipe.c.o.pattern="{compiler.path}{compiler.c.cmd}" {compiler.c.flags} -mmcu={build.mcu} -DF_CPU={build.f_cpu} -DARDUINO={runtime.ide.version} -DARDUINO_{build.board} -DARDUINO_ARCH_{build.arch} {build.extra_flags} {includes} "{source_file}" -o "{object_file}"

Note that some properties, like {build.mcu} for example, are taken from the boards.txt file which is documented later in this specification.

Recipes to build the core.a archive file

The core of the selected board is compiled as described in the previous paragraph, but the object files obtained from the compile are also archived into a static library named core.a using the recipe.ar.pattern.

The recipe can be built concatenating the following automatically generated properties:

  • {object_file}: the object file to include in the archive
  • {archive_file_path}: fully qualified archive file (ex. "/path/to/core.a"). This property was added in Arduino IDE 1.6.6/arduino builder 1.0.0-beta12 as a replacement for {build.path}/{archive_file}.
  • {archive_file}: the name of the resulting archive (ex. "core.a")

For example, Arduino provides the following for AVR:

compiler.ar.cmd=avr-ar
compiler.ar.flags=rcs

[......]

## Create archives
recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} "{archive_file_path}" "{object_file}"

Recipes for linking

All the artifacts produced by the previous steps (sketch object files, libraries object files and core.a archive) are linked together using the recipe.c.combine.pattern.

The recipe can be built concatenating the following automatically generated properties:

  • {object_files}: the list of object files to include in the archive ("file1.o file2.o ....")
  • {archive_file_path}: fully qualified archive file (ex. "/path/to/core.a"). This property was added in Arduino IDE 1.6.6/arduino builder 1.0.0-beta12 as a replacement for {build.path}/{archive_file}.
  • {archive_file}: the name of the core archive file (ex. "core.a")

For example the following is used for AVR:

compiler.c.elf.flags=-Os -Wl,--gc-sections
compiler.c.elf.cmd=avr-gcc

[......]

## Combine gc-sections, archives, and objects
recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} -mmcu={build.mcu} -o "{build.path}/{build.project_name}.elf" {object_files} "{archive_file_path}" "-L{build.path}" -lm

Recipes for extraction of executable files and other binary data

An arbitrary number of extra steps can be performed at the end of objects linking. These steps can be used to extract binary data used for upload and they are defined by a set of recipes with the following format:

recipe.objcopy.FILE_EXTENSION_1.pattern=[.....]
recipe.objcopy.FILE_EXTENSION_2.pattern=[.....]
[.....]

FILE_EXTENSION_x must be replaced with the extension of the extracted file, for example the AVR platform needs two files a .hex and a .eep, so we made two recipes like:

recipe.objcopy.eep.pattern=[.....]
recipe.objcopy.hex.pattern=[.....]

There are no specific properties set by the Arduino development software here. A full example for the AVR platform can be:

## Create eeprom
recipe.objcopy.eep.pattern="{compiler.path}{compiler.objcopy.cmd}" {compiler.objcopy.eep.flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.eep"

## Create hex
recipe.objcopy.hex.pattern="{compiler.path}{compiler.elf2hex.cmd}" {compiler.elf2hex.flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.hex"

Recipes to compute binary sketch size

At the end of the build the Arduino development software shows the final binary sketch size to the user. The size is calculated using the recipe recipe.size.pattern. The output of the command executed using the recipe is parsed through the regular expression set in the property recipe.size.regex. The regular expression must match the sketch size.

For AVR we have:

compiler.size.cmd=avr-size
[....]
## Compute size
recipe.size.pattern="{compiler.path}{compiler.size.cmd}" -A "{build.path}/{build.project_name}.hex"
recipe.size.regex=Total\s+([0-9]+).*

Recipes to export compiled binary

When you do a Sketch > Export compiled Binary in the Arduino IDE, the compiled binary is copied from the build folder to the sketch folder. Two binaries are copied; the standard binary, and a binary that has been merged with the bootloader file (identified by the .with_bootloader in the filename).

Two recipes affect how Export compiled Binary works:

  • recipe.output.tmp_file: Defines the binary's filename in the build folder.
  • recipe.output.save_file: Defines the filename to use when copying the binary file to the sketch folder.

As with other processes, there are pre and post build hooks for Export compiled Binary.

The recipe.hooks.savehex.presavehex.NUMBER.pattern and recipe.hooks.savehex.postsavehex.NUMBER.pattern hooks (but not recipe.output.tmp_file and recipe.output.save_file) can be built concatenating the following automatically generated properties:

{sketch_path}              - the absolute path of the sketch folder

Recipe to run the preprocessor

For detecting what libraries to include in the build, and for generating function prototypes, (just) the preprocessor is run. For this, the recipe.preproc.macros recipe exists. This recipe must run the preprocessor on a given source file, writing the preprocessed output to a given output file, and generate (only) preprocessor errors on standard output. This preprocessor run should happen with the same defines and other preprocessor-influencing-options as for normally compiling the source files.

The recipes can be built concatenating other automatically generated properties (for each file compiled):

  • {includes}: the list of include paths in the format "-I/include/path -I/another/path...."
  • {source_file}: the path to the source file
  • {preprocessed_file_path}: the path to the output file

For example the following is used for AVR:

preproc.macros.flags=-w -x c++ -E -CC
recipe.preproc.macros="{compiler.path}{compiler.cpp.cmd}" {compiler.cpp.flags} {preproc.macros.flags} -mmcu={build.mcu} -DF_CPU={build.f_cpu} -DARDUINO={runtime.ide.version} -DARDUINO_{build.board} -DARDUINO_ARCH_{build.arch} {compiler.cpp.extra_flags} {build.extra_flags} {includes} "{source_file}" -o "{preprocessed_file_path}"

Note that the {preprocessed_file_path} might point to (your operating system's equivalent) of /dev/null. In this case, also passing -MMD to gcc is problematic, as it will try to generate a dependency file called /dev/null.d, which will usually result in a permission error. Since platforms typically include {compiler.cpp.flags} here, which includes -MMD, the -MMD option is automatically filtered out of the recipe.preproc.macros recipe to prevent this error.

Note that older Arduino IDE versions used the recipe.preproc.includes recipe to determine includes, which is undocumented here. Since Arduino IDE 1.6.7 (arduino-builder 1.2.0) this was changed and recipe.preproc.includes is no longer used.

Pre and post build hooks (since Arduino IDE 1.6.5)

You can specify pre and post actions around each recipe. These are called "hooks". Here is the complete list of available hooks:

  • recipe.hooks.sketch.prebuild.NUMBER.pattern (called before sketch compilation)
  • recipe.hooks.sketch.postbuild.NUMBER.pattern (called after sketch compilation)
  • recipe.hooks.libraries.prebuild.NUMBER.pattern (called before libraries compilation)
  • recipe.hooks.libraries.postbuild.NUMBER.pattern (called after libraries compilation)
  • recipe.hooks.core.prebuild.NUMBER.pattern (called before core compilation)
  • recipe.hooks.core.postbuild.NUMBER.pattern (called after core compilation)
  • recipe.hooks.linking.prelink.NUMBER.pattern (called before linking)
  • recipe.hooks.linking.postlink.NUMBER.pattern (called after linking)
  • recipe.hooks.objcopy.preobjcopy.NUMBER.pattern (called before objcopy recipes execution)
  • recipe.hooks.objcopy.postobjcopy.NUMBER.pattern (called after objcopy recipes execution)
  • recipe.hooks.savehex.presavehex.NUMBER.pattern (called before savehex recipe execution)
  • recipe.hooks.savehex.postsavehex.NUMBER.pattern (called after savehex recipe execution)

Example: you want to execute 2 commands before sketch compilation and 1 after linking. You'll add to your platform.txt

recipe.hooks.sketch.prebuild.1.pattern=echo sketch compilation started at
recipe.hooks.sketch.prebuild.2.pattern=date

recipe.hooks.linking.postlink.1.pattern=echo linking is complete

Warning: hooks recipes are sorted before execution. If you need to write more than 10 recipes for a single hook, pad the number with a zero, for example:

recipe.hooks.sketch.prebuild.01.pattern=echo 1
recipe.hooks.sketch.prebuild.02.pattern=echo 2
...
recipe.hooks.sketch.prebuild.11.pattern=echo 11

Global platform.txt

Properties defined in a platform.txt created in the hardware subfolder of the Arduino IDE installation folder will be used for all platforms and will override local properties. This feature is currently only available when using the Arduino IDE.

platform.local.txt

Introduced in Arduino IDE 1.5.7. This file can be used to override properties defined in platform.txt or define new properties without modifying platform.txt (e.g. when platform.txt is tracked by a version control system). It should be placed in the architecture folder.

boards.txt

This file contains definitions and meta-data for the boards supported. Every board must be referred through its short name, the board ID. The settings for a board are defined through a set of properties with keys having the board ID as prefix.

For example the board ID chosen for the Arduino Uno board is "uno". An extract of the Uno board configuration in boards.txt looks like:

[......]
uno.name=Arduino Uno
uno.build.mcu=atmega328p
uno.build.f_cpu=16000000L
uno.build.board=AVR_UNO
uno.build.core=arduino
uno.build.variant=standard
[......]

Note that all the relevant keys start with the board ID uno.xxxxx.

The uno.name property contains the human-friendly name of the board. This is shown in the Board menu of the IDEs, the "Board Name" field of Arduino CLI's text output, or the "name" key of Arduino CLI's JSON output.

The uno.build.board property is used to set a compile-time variable ARDUINO_{build.board} to allow use of conditional code between #ifdefs. A build.board value is automatically generated if not defined. In this case the variable defined at compile time will be ARDUINO_AVR_UNO.

The other properties will override the corresponding global properties when the user selects the board. These properties will be globally available, in other configuration files too, without the board ID prefix:

uno.build.mcu           =>   build.mcu
uno.build.f_cpu         =>   build.f_cpu
uno.build.board         =>   build.board
uno.build.core          =>   build.core
uno.build.variant       =>   build.variant

This explains the presence of {build.mcu} or {build.board} in the platform.txt recipes: their value is overwritten respectively by {uno.build.mcu} and {uno.build.board} when the Uno board is selected! Moreover the following properties are automatically generated:

  • {build.core.path}: The path to the selected board's core folder (inside the core platform, for example hardware/arduino/avr/core/arduino)
  • {build.system.path}: The path to the core platform's system folder if available (for example hardware/arduino/sam/system)
  • {build.variant.path}: The path to the selected board variant folder (inside the variant platform, for example hardware/arduino/avr/variants/micro)

Cores

Cores are placed inside the cores subfolder. Many different cores can be provided within a single platform. For example the following could be a valid platform layout:

  • hardware/arduino/avr/cores/: Cores folder for "avr" architecture, package "arduino"
  • hardware/arduino/avr/cores/arduino: the Arduino Core
  • hardware/arduino/avr/cores/rtos: an hypothetical RTOS Core

The board's property build.core is used to find the core that must be compiled and linked when the board is selected. For example if a board needs the Arduino core the build.core variable should be set to:

uno.build.core=arduino

or if the RTOS core is needed, to:

uno.build.core=rtos

In any case the contents of the selected core folder are compiled and the core folder path is added to the include files search path.

Core Variants

Sometimes a board needs some tweaking on default core configuration (different pin mapping is a typical example). A core variant folder is an additional folder that is compiled together with the core and allows to easily add specific configurations.

Variants must be placed inside the variants folder in the current architecture. For example, Arduino AVR Boards uses:

  • hardware/arduino/avr/cores: Core folder for "avr" architecture, "arduino" package
  • hardware/arduino/avr/cores/arduino: The Arduino core
  • hardware/arduino/avr/variants/: Variant folder for "avr" architecture, "arduino" package
  • hardware/arduino/avr/variants/standard: ATmega328 based variants
  • hardware/arduino/avr/variants/leonardo: ATmega32U4 based variants

In this example, the Arduino Uno board needs the standard variant so the build.variant property is set to standard:

[.....]
uno.build.core=arduino
uno.build.variant=standard
[.....]

instead, the Arduino Leonardo board needs the leonardo variant:

[.....]
leonardo.build.core=arduino
leonardo.build.variant=leonardo
[.....]

In the example above, both Uno and Leonardo share the same core but use different variants.
In any case, the contents of the selected variant folder path is added to the include search path and its contents are compiled and linked with the sketch.

The parameter build.variant.path is automatically generated.

Tools

The Arduino development software uses external command line tools to upload the compiled sketch to the board or to burn bootloaders using external programmers. Currently avrdude is used for AVR based boards and bossac for SAM based boards, but there is no limit, any command line executable can be used. The command line parameters are specified using recipes in the same way used for platform build process.

Tools are configured inside the platform.txt file. Every Tool is identified by a short name, the Tool ID. A tool can be used for different purposes:

  • upload a sketch to the target board (using a bootloader preinstalled on the board)
  • program a sketch to the target board using an external programmer
  • erase the target board's flash memory using an external programmer
  • burn a bootloader into the target board using an external programmer

Each action has its own recipe and its configuration is done through a set of properties having key starting with tools prefix followed by the tool ID and the action:

[....]
tools.avrdude.upload.pattern=[......]
[....]
tools.avrdude.program.pattern=[......]
[....]
tools.avrdude.erase.pattern=[......]
[....]
tools.avrdude.bootloader.pattern=[......]
[.....]

A tool may have some actions not defined (it's not mandatory to define all four actions).
Let's look at how the upload action is defined for avrdude:

tools.avrdude.path={runtime.tools.avrdude.path}
tools.avrdude.cmd.path={path}/bin/avrdude
tools.avrdude.config.path={path}/etc/avrdude.conf

tools.avrdude.upload.pattern="{cmd.path}" "-C{config.path}" -p{build.mcu} -c{upload.protocol} -P{serial.port} -b{upload.speed} -D "-Uflash:w:{build.path}/{build.project_name}.hex:i"

A {runtime.tools.TOOL_NAME.path} and {runtime.tools.TOOL_NAME-TOOL_VERSION.path} property is generated for the tools of Arduino AVR Boards and any other platform installed via Boards Manager. {runtime.tools.TOOL_NAME.path} points to the latest version of the tool available.

The tool configuration properties are available globally without the prefix. For example, the tools.avrdude.cmd.path property can be used as {cmd.path} inside the recipe, and the same happens for all the other avrdude configuration variables.

Verbose parameter

It is possible for the user to enable verbosity from the Preferences panel of the IDEs or Arduino CLI's --verbose flag. This preference is transferred to the command line using the ACTION.verbose property (where ACTION is the action we are considering).
When the verbose mode is enabled the tools.TOOL_ID.ACTION.params.verbose property is copied into ACTION.verbose. When the verbose mode is disabled, the tools.TOOL_ID.ACTION.params.quiet property is copied into ACTION.verbose. Confused? Maybe an example will clear things:

tools.avrdude.upload.params.verbose=-v -v -v -v
tools.avrdude.upload.params.quiet=-q -q
tools.avrdude.upload.pattern="{cmd.path}" "-C{config.path}" {upload.verbose} -p{build.mcu} -c{upload.protocol} -P{serial.port} -b{upload.speed} -D "-Uflash:w:{build.path}/{build.project_name}.hex:i"

In this example if the user enables verbose mode, then {upload.params.verbose} is used in {upload.verbose}:

tools.avrdude.upload.params.verbose    =>    upload.verbose

If the user didn't enable verbose mode, then {upload.params.quiet} is used in {upload.verbose}:

tools.avrdude.upload.params.quiet      =>    upload.verbose

Sketch upload configuration

The Upload action is triggered when the user clicks on the "Upload" button on the IDE toolbar or uses arduino-cli upload. The upload.tool property determines the tool to be used for upload. A specific upload.tool property should be defined for every board in boards.txt:

[......]
uno.upload.tool=avrdude
[......]
leonardo.upload.tool=avrdude
[......]

Also other upload parameters can be defined together, for example in the Arduino AVR Boards boards.txt we have:

[.....]
uno.name=Arduino Uno
uno.upload.tool=avrdude
uno.upload.protocol=arduino
uno.upload.maximum_size=32256
uno.upload.speed=115200
[.....]
leonardo.name=Arduino Leonardo
leonardo.upload.tool=avrdude
leonardo.upload.protocol=avr109
leonardo.upload.maximum_size=28672
leonardo.upload.speed=57600
leonardo.upload.use_1200bps_touch=true
leonardo.upload.wait_for_upload_port=true
[.....]

Most {upload.XXXX} variables are used later in the avrdude upload recipe in platform.txt:

[.....]
tools.avrdude.upload.pattern="{cmd.path}" "-C{config.path}" {upload.verbose} -p{build.mcu} -c{upload.protocol} -P{serial.port} -b{upload.speed} -D "-Uflash:w:{build.path}/{build.project_name}.hex:i"
[.....]

1200 bps bootloader reset

Most Arduino boards use a dedicated USB-to-serial chip, that takes care of restarting the main MCU (starting the bootloader) when the serial port is opened. However, boards that have a native USB connection (such as the Leonardo or Zero) will have to disconnect from USB when rebooting into the bootloader (after which the bootloader reconnects to USB and offers a new serial port for uploading). After the upload is complete, the bootloader disconnects from USB again, starts the sketch, which then reconnects to USB. Because of these reconnections, the standard restart-on-serial open will not work, since that would cause the serial port to disappear and be closed again. Instead, the sketch running on these boards interprets a bitrate of 1200 bps as a signal the bootloader should be started.

To let the Arduino development software perform these steps, two board parameters can be set:

  • use_1200bps_touch causes the selected serial port to be briefly opened at 1200 bps (8N1) before starting the upload.
  • wait_for_upload_port causes the upload procedure to wait for the serial port to (re)appear before and after the upload. This is only used when use_1200bps_touch is also set. When set, after doing the 1200 bps touch, the development software will wait for a new serial port to appear and use that as the port for uploads. Alternatively, if the original port does not disappear within a few seconds, the upload continues with the original port (which can be the case if the board was already put into bootloader manually, or the the disconnect and reconnect was missed). Additionally, after the upload is complete, the IDE again waits for a new port to appear (or the originally selected port to be present).

Note that the IDE implementation of this 1200 bps touch has some peculiarities, and the newer arduino-cli implementation also seems different (does not wait for the port after the reset, which is probably only needed in the IDE to prevent opening the wrong port on the serial monitor, and does not have a shorter timeout when the port never disappears).

Serial port

The port selected via the IDE or arduino-cli upload's --port option is available as a configuration property {serial.port}.

Upload using an external programmer

TODO... The platform.txt associated with the selected programmer will be used.

Burn Bootloader

TODO... The platform.txt associated with the selected board will be used.

Custom board options

It can sometimes be useful to provide user selectable configuration options for a specific board. For example, a board could be provided in two or more variants with different CPUs, or may have different crystal speed based on the board model, and so on...

When using Arduino CLI, the option can be selected via the FQBN.

In the Arduino IDE the options add extra menu items under the "Tools" menu.

In Arduino Web Editor, the options are displayed in the "Flavours" menu.

Let's see an example of how a custom option is implemented. The board used in the example is the Arduino Duemilanove. This board was produced in two models, one with an ATmega168 CPU and another with an ATmega328P.
We are going then to define a custom option, using the "cpu" MENU_ID, that allows the user to choose between the two different microcontrollers.

We must first define a set of menu.MENU_ID=Text properties. Text is what is displayed on the GUI for every custom menu we are going to create and must be declared at the beginning of the boards.txt file:

menu.cpu=Processor
[.....]

in this case, the menu name is "Processor".
Now let's add, always in the boards.txt file, the default configuration (common to all processors) for the duemilanove board:

menu.cpu=Processor
[.....]
duemilanove.name=Arduino Duemilanove
duemilanove.upload.tool=avrdude
duemilanove.upload.protocol=arduino
duemilanove.build.f_cpu=16000000L
duemilanove.build.board=AVR_DUEMILANOVE
duemilanove.build.core=arduino
duemilanove.build.variant=standard
[.....]

Now let's define the possible values of the "cpu" option:

[.....]
duemilanove.menu.cpu.atmega328=ATmega328P
[.....]
duemilanove.menu.cpu.atmega168=ATmega168
[.....]

We have defined two values: "atmega328" and "atmega168".
Note that the property keys must follow the format BOARD_ID.menu.MENU_ID.OPTION_ID=Text, where Text is what is displayed under the "Processor" menu in the IDE's GUI.
Finally, the specific configuration for each option value:

[.....]
## Arduino Duemilanove w/ ATmega328P
duemilanove.menu.cpu.atmega328=ATmega328P
duemilanove.menu.cpu.atmega328.upload.maximum_size=30720
duemilanove.menu.cpu.atmega328.upload.speed=57600
duemilanove.menu.cpu.atmega328.build.mcu=atmega328p

## Arduino Duemilanove w/ ATmega168
duemilanove.menu.cpu.atmega168=ATmega168
duemilanove.menu.cpu.atmega168.upload.maximum_size=14336
duemilanove.menu.cpu.atmega168.upload.speed=19200
duemilanove.menu.cpu.atmega168.build.mcu=atmega168
[.....]

Note that when the user selects an option value, all the "sub properties" of that value are copied in the global configuration. For example when the user selects "ATmega168" from the "Processor" menu, or uses the FQBN arduino:avr:duemilanove:cpu=atmega168 with Arduino CLI, the configuration under atmega168 is made available globally:

duemilanove.menu.cpu.atmega168.upload.maximum_size     =>   upload.maximum_size
duemilanove.menu.cpu.atmega168.upload.speed            =>   upload.speed
duemilanove.menu.cpu.atmega168.build.mcu               =>   build.mcu

There is no limit to the number of custom menus that can be defined.

TODO: add an example with more than one submenu

Referencing another core, variant or tool

Inside the boards.txt we can define a board that uses a core provided by another vendor/mantainer using the syntax VENDOR_ID:CORE_ID. For example, if we want to define a board that uses the "arduino" core from the "arduino" vendor we should write:

[....]
myboard.name=My Wonderful Arduino Compatible board
myboard.build.core=arduino:arduino
[....]

Note that we don't need to specify any architecture since the same architecture of "myboard" is used, so we just say "arduino:arduino" instead of "arduino:avr:arduino".

The platform.txt settings are inherited from the referenced core platform, thus there is no need to provide a platform.txt unless there are some specific properties that need to be overridden.

The libraries from the referenced platform are used, thus there is no need to provide those libraries. If libraries are provided the list of available libraries are the sum of the 2 libraries where the referencing platform has priority over the referenced platform.

In the same way we can use variants and tools defined on another platform:

[....]
myboard.build.variant=arduino:standard
myboard.upload.tool=arduino:avrdude
myboard.bootloader.tool=arduino:avrdude
[....]

Using this syntax allows us to reduce the minimum set of files needed to define a new "hardware" to just the boards.txt file.

Note that referencing a variant in another platform does not inherit any properties from that platform's platform.txt (like referencing a core does).

Platform Terminology

Because boards can reference cores, variants and tools in different platforms, this means that a single build or upload can use data from up to four different platforms. To keep this clear, the following terminology is used:

  • The "board platform" is the platform that defines the currently selected board (e.g. the platform that contains the board.txt the board is defined in.
  • The "core platform" is the the platform that contains the core to be used.
  • The "variant platform" is the platform that contains the variant to be used.
  • The "tool platform" is the platform that contains the tool used for the current operation.

In the most common case, without any references, all of these will refer to the same platform.

Note that the above terminology is not in widespread use, but was invented for clarity within this document. In the actual arduino-cli code, the "board platform" is called targetPlatform, the "core platform" is called actualPlatform, the others are pretty much nameless.

boards.local.txt

Introduced in Arduino IDE 1.6.6. This file can be used to override properties defined in boards.txt or define new properties without modifying boards.txt.

keywords.txt

As of Arduino IDE 1.6.6, per-platform keywords can be defined by adding a keywords.txt file to the platform's architecture folder. These keywords are only highlighted in the Arduino IDE when one of the boards of that platform are selected. This file follows the same format as the keywords.txt used in libraries. Each keyword must be separated from the keyword identifier by a tab.