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Zephyr Embedded Controller (Zephyr EC)

[TOC]

Introduction

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:

  • Original Chrome EC: ECOS, cros-ec, and legacy EC
  • Zephyr-based EC: Zephyr EC

Getting Started

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.

The Zephyr Troubleshooting page lists few common errors and troubleshooting techniques used when working with Zephyr.

Source Code Organization

Zephyr EC images rely on multiple Chromium repositories to build Zephyr EC images.

The 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.

Zephyr EC Structure

Google merges upstream Zephyr changes into the corresponding Chromium repository daily.

Shared Code

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:

    • Battery charging
    • USB Power Delivery
    • AP chipset power sequencing
    • Motionsense (EC sensor support)
    • Keyboard handling
    • Verified boot support
  • 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:

    • Charge controllers
    • USB Power Delivery chips (TCPCs, PPCs, muxes, etc)
    • Temperature sensors
    • Motionsense sensors (accelerometers, gyrscopes, light sensors, etc)
  • 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/

Zephyr Subdirectory

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/program/ - 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.