Starting roughly in July of 2021, Chromebooks switched from the original Google Chrome EC to an application based on the Zephyr Project RTOS.
The following terms are used to to describe these two implementations:
The Zephyr build documentation details the usage of Google's meta build tool zmake, used to configure and build Zephyr EC binaries. Start here if you only need to build a Zephyr EC binary for an existing board.
The project configuration documentation details the steps required to create a new Chromebook program (reference board) and for creating a new project variant.
The Zephyr New Board Checklist links to the documentation needed to configure individual EC features.
Zephyr EC images rely on multiple Chromium repositories to build Zephyr EC images.
third_party/zephyr/main - Google's local mirror of the main Zephyr RTOS source located on Github.third_party/zephyr/cmsis - Google's local mirror of the CMSIS module source from Githubplatform/ec - local repository containing code shared by the legacy EC and the Zephyr ECThe current Zephyr EC builds only the CMSIS module from the upstream Zephyr source, but may use other modules in the future.
The diagram below shows the relationship of the upstream Zephyr repositories and the Chromium repositories.
Google merges upstream Zephyr changes into the corresponding Chromium repository daily.
The two EC implementations currently share a significant amount code found in the following directories (all paths relative to the Chrome EC base directory platform/ec.)
common/ - Upper-level code shared across boards and the EC implementations. This includes the following features:
driver/ - Low-level drivers for on-board peripherals controlled by the EC. This does not include any drivers for modules integrated directly into the EC chipset (such as GPIO controllers, I2C, controllers, keyboard controller). On-board peripheral drivers include:
include/ - Header files for the common/ and driver/ code
The following legacy EC directories are not used by the Zephyr EC application:
baseboard/board/chip/core/Only the Zephyr EC application uses the following directory under the platform/ec repository:
zephyr/The following provides an overview of the sub-directories found under zephyr/.
zephyr/app/ - The Zephyr EC application entry point. The Zephyr kernel calls ec_app_main() routine after initializing all kernel services and chip-level drivers,zephyr/boards/ - Contains the EC chip-level support. This directory and the organization is required by the Zephyr build system. This should not be confused with the legacy EC boards/ directory, as it does not contain any Chromebook specific board code.zephyr/cmake/ - Configuration files for the CMake build system.zephyr/drivers/ - Drivers conforming to the Zephyr device model to implement Chrome EC specific features. Google plans to eventually move these drivers upstream.zephyr/dts/ - Devicetree description files for Google's Zephyr drivers that are not yet available upstream.zephyr/emul/ - Emulator source code that has not yet been moved upstream.zephyr/include/ - Header files for files found in the zephyr/ sub-directory.zephyr/linker/ - Linker directive files used to construct the Zephyr EC binaries.zephyr/projects/ - Program-specific configurations for each program supported by Zephyr.zephyr/shim/ - Source code that adapts the legacy EC APIs to the equivalent Zephyr OS API.zephyr/subsys/ - Staging area for subsystem code that will be moved upstream.zephyr/tests/ - Host based emulation tests.zephyr/zmake/ - Source code for the zmake/ meta tool.