A microSD-card-sized RISC-V board plugs directly into a USB port, providing a low-cost, fully open-source platform for bare-metal firmware experiments.
Icy Electronics has introduced Comu, a tiny development board built around WCH’s CH32V203 RISC-V microcontroller. The board is small enough to plug directly into a USB port without any additional cable or connector. Measuring just 13 x 9.4 millimetres (mm), it is slightly smaller than a microSD card and weighs only 0.4 grams. Despite its small size, the board includes two LEDs, four capacitive touch buttons and six general-purpose input/output (GPIO) pins exposed through test pads. The board is also designed as a fully open-source platform, making it possible to study and modify both the hardware and firmware.
At the centre of Comu is WCH’s CH32V203, a single-core, 32-bit RISC-V microcontroller unit (MCU) clocked at 144 megahertz (MHz), with 20KB of SRAM and 224KB of Flash, including a 64KB zero-wait region and 160KB of normal Flash. It also has a separate 24KB boot-memory region, of which 2KiB is used by Comu’s USB bootloader. The board draws all its power from the host USB port, while Icy Electronics rates run-mode consumption at about 49.3 microamps per MHz (μA/MHz), dropping to 19.4 μA/MHz in sleep mode.
Comu follows a series of tiny “omu” boards designed to plug directly into a USB port. The series began with the Arm Cortex-M0+-based Tomu in 2018 and was followed by the FPGA-equipped Fomu and Qomu, as well as the Somu security key. While the earlier boards used Arm cores or field-programmable gate arrays (FPGAs), Comu is built around a low-cost RISC-V MCU.At the time of writing, it is listed at $5.99 on Tindie and $5.88 on Lectronz, with the Tindie listing including a 3D-printed case.
According to the developer, a basic “blinky” LED programme on Comu can compile to as little as 40 bytes. The board comes with a custom 2KiB USB bootloader, leaving around 28KiB of the boot-memory region available for user code, in addition to the MCU’s main flash resources. The CH32V203 supports bare-metal C/C++ development through the open-source ch32fun project, along with Rust through ch32-hal, the Arduino IDE, and WCH’s MounRiver Studio toolchain. The complete hardware files, including KiCad schematics and PCB layouts, are available on GitHub. The PCB layout can also be inspected through KiCanvas.
With its compact USB form factor, RISC-V MCU and open-source hardware, Comu is aimed at developers looking to experiment with bare-metal firmware and USB-based embedded applications. The published hardware files also allow developers to inspect and modify the board design.
