RebbleOS is an open-source reimplementation of the firmware for the devices formerly manufactured by Pebble Technologies, Inc. The firmware is based on FreeRTOS, and aims to be binary-compatible with applications that were written for the original Pebble OS, as well as radio-compatible with smartphone applications that are designed to work with Pebble.

RebbleOS needs your help! This section discusses what you need to know to get started working on the project.

RebbleOS currently can be built for snowy (Pebble Time and Pebble Time Steel) and tintin (Pebble and Pebble Steel). To build RebbleOS, follow these steps:

• Obtain a checkout of the RebbleOS source code.
• Create a localconfig.mk if your cross-compiler is in an unusual location. For instance, if you have the SDK installed in /home/me, add the following line to your localconfig.mk: PEBBLE_TOOLCHAIN_PATH=/home/me/Pebble/SDK/pebble-sdk-4.5-linux64/arm-cs-tools/bin. For more information on localconfig.mk variables, consult the Makefile.
• Build the firmware: make
• If you wish to run the firmware in qemu, copy the resources necessary into Resources/. Take a look at Utilities/mk_resources.sh for more information on that.
• To run the firmware in qemu, try make snowy_qemu.

If you wish to build firmware to run on your device, you may also wish to consider a script like buildfw.sh. Running RebbleOS on hardware is currently out of scope for this document.

(This section is, admittedly, somewhat aspirational. Do not be surprised if code within RebbleOS does not necessarily conform to this structure yet!)

RebbleOS is composed of four major components: the hardware abstraction layer, the core operating system, the PebbleOS compatibility layer, and system applications. We break down these components as follows:

• Hardware abstraction layer. This subsystem provides a unified interface for the rest of the system, and abstracts away platform-specific elements. The HAL lives in the directory hw/; symbols that the HAL exports to the rest of the system are prefixed with hw_. The main entity that the HAL works on is a "platform"; for an example, take a look at hw/platform/snowy/config.mk. A platform depends on various chip components, and potentially other driver components; it exports a platform.h that includes all defines that the rest of the system may need. The HAL is, in theory, independent of the rest of the OS; it does not call into the rest of the system other than through debugging mechanisms and through callbacks that it is provided.

• Core OS. This subsystem provides basic services that any smartwatch OS, even if not implementing a Pebble-like API, might need. HAL accesses are marshalled through concurrency protection; higher-level power management takes place; and, flash wear leveling and filesystem management happens in the core OS. The core OS lives in rcore/, and symbols exported from the core OS are prefixed with rcore_. It calls on FreeRTOS, which lives in FreeRTOS/.

• Pebble compatibility layer. The core OS provides basic isolation between threads and framebuffer management primitives, but the Pebble compatibility layer provides higher level operations, like Pebble-style layers, input management and routing, and UI services. The Pebble compatibility layer lives in rwatch/, and symbols exported from it are prefixed with rwatch_. (Functions that are exactly analogous to Pebble APIs may be named with their exact name.)

• System applications. We'll, uh, get there when we have some. Yeah.

RebbleOS is an open-source project licensed (primarily) under a BSD-style license. For more information, please see the LICENSE and AUTHORS files. Additionally, contributors and members of the RebbleOS community are expected to abide by our code of conduct; for more information on that, please see the CODE-OF-CONDUCT.md file. Reuse of this project is not only permitted, but encouraged! If you do something cool with RebbleOS, please get in touch with us. The easiest way to do so is through the Rebble Discord server, channel #firmware. We look forward to meeting you!