The Tomu: An Arm Microcontroller That Fits in Your USB Port

Alasdair Allan
2 min readDec 15, 2017


There are a fair number of USB stick computers on the market, ranging from fairly cheap to rather expensive. It’s even possible to turn a Raspberry Pi Zero into one using a Zero Stem shim.

However, I was rather intrigued when I recently ran across the Tomu. The difference between the Tomu and most other USB stick computers I’ve come across? It’s so small it fits entirely inside your computer’s USB port.

The Tomu, top (left) and bottom (right) next to a U.S. penny. (📷: Tim Ansell)

The board is based around Silicon Labs “Happy Gecko” EFM32HG309 Arm Cortex-M0+ microcontroller running at 25 MHz, with 8 kb of RAM and 64 kb of flash onboard, and despite its small size, supports two capacitance touch buttons and two LEDs. The bill of materials of the board literally consists of just 12 parts, and that’s including the PCB.

The board was designed by Tim Ansell, who gave a lighting talk about the Tomu at 33c3 last year, and was inspired by the YubiKey 4 Nano.

The ultimate goal of the Tomu project is for it to be useable as a second factor authentication device. While the hardware is ready, the software is still a work in progress, although the necessary bootloader code is complete and working and can be flashed onto the board using a Raspberry Pi.

Tomu fits entirely inside a standard USB Type-A slot, except for a small area that pokes out to give you access to two buttons and two status LEDs. (📷: Tim Ansell)

The Tomu is now live on Crowd Supply, starting at $30. But you can should also be able to download the Gerbers and put one together yourself.

The hardware is fully documented, and the bootloader and other code are all available on GitHub. The project is entirely open source with the hardware under the CC BY-SA 4.0, and software under the Apache License Version 2.0.