UNA Watch modular GPS smartwatch ships with open-source STM32 firmware and hardware designs

Pimoroni has begun selling the UNA Watch, a modular GPS smartwatch that originated from a Kickstarter campaign in April 2025. The device centers on an STMicroelectronics STM32U595 Arm Cortex-M33 microcontroller with 2.5MB of RAM and 4GB of eMMC storage, paired with a 1.2-inch 240×240 memory-in-pixel LCD. Positioning capabilities span six satellite systems including GPS, GLONASS, Galileo, BeiDou, NavIC, and QZSS, while fitness tracking relies on an optical heart-rate sensor, accelerometer, gyroscope, magnetometer, barometric altimeter, and temperature sensor.

The open-source software stack runs FreeRTOS and exposes a public C++ SDK for building native applications compiled into Arm Cortex-M ELF binaries. UNA publishes kernel firmware source code, hardware designs under Creative Commons Attribution 4.0, and the SDK under the MIT license. Developers can build applications with CMake and ARM GCC, test them in a desktop simulator for Windows or Linux, and deploy as .uapp files over USB. The modular hardware allows battery and display replacement with a Torx T3 screwdriver.

Open-source architecture and developer platform

• STM32U595 platform: Arm Cortex-M33 core with 2.5MB RAM and 4GB eMMC storage, running FreeRTOS with native C++ application support through position-independent ELF binaries and a shared C library to minimize memory footprint.
• Multi-constellation GNSS: Integrated receiver supports GPS, GLONASS, Galileo, BeiDou, NavIC, and QZSS for global positioning coverage without cellular dependency.
• Sensor array: Optical heart-rate monitor, accelerometer, gyroscope, magnetometer, barometric altimeter, and temperature sensor enable comprehensive fitness and environmental tracking.
• SDK and build toolchain: Public C++ SDK with CMake-based builds using STMicroelectronics ARM GCC toolchain, producing .uapp packages; desktop simulator available for Windows via TouchGFX/Visual Studio and x86-64 Linux via GCC and SDL2.
• Open hardware and firmware: Housing and PCB designs published under CC-BY-4.0, SDK under MIT license, and kernel firmware source fully available for custom builds and installation.

Why it matters

The UNA Watch represents a rare commercially produced wearable where both hardware designs and firmware are fully open, giving developers and privacy-conscious users complete control over a device category typically dominated by proprietary stacks. For the embedded Linux and open hardware communities, it provides a reference platform for understanding low-power GNSS integration, FreeRTOS application architecture, and modular industrial design that could inform future open-source wearable projects. The desktop Linux simulator and standard GCC toolchain also lower barriers for developers already working in open-source environments.