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Tomu, I’m

I’m Tomu, a tiny ARM microprocessor which fits in your USB port. I have two buttons and two LEDs!

I’m fully open source and am buildable by hobbyists! Designed for 2-factor authentication, usb experiments, or anything else you can think of.

I’m also available on Crowd Supply, fully assembled and tested.

Getting Started!

Got a Tomu? Great! For a hyper-fast quickstart, check out the Tomu Quickstart repo. All you need is make, a compiler, and dfu-util.

More information on the Quickstart repo, including links to precompiled samples, is available at the quickstart samples page.

If you’re building one from scratch, you’ll need to get a case and a bootloader. For more information, read on!


Because Tomu fits entirely inside your USB port, you need something to keep it from falling out. Early Tomu prototypes were held in place using a business card folded over and wedged in the USB port, but newer releases have support for 3D printed cases.

If you have a 3D printer, download the .stl file for the version of Tomu that you have. The version number is printed on the bottom side with the big USB connections. Depending on how thick your PCB is, you may need to adjust the FreeCAD file.


There are two types of bootloaders for Tomu: Serial and DFU. If you got your board from Crowd Supply, or updated it using the test jig at LCA 2018, or the LEDs blink red/green when you plug it in, then you have DFU. Otherwise, you have Serial.

Serial Bootloader (Legacy)

The serial bootloader is from SiLabs, and is known as AN0042.

To use the serial bootloader, Windows users will need to install, e.g. from M-Pression. Linux and macOS users do not need to use any special drivers. The device will show up as something like /dev/ttyUSB0.

  1. Enter the bootloader.
    • v0.2 boards require you to connect the C pin to Vcc.
    • v0.3 boards always enter the bootloader
  2. Connect a terminal.
    • TeraTerm on Windows is a good choice
    • screen works on Linux and macOS
  3. Interact with the bootloader by sending one-character commands:
    • i – show the bootloader version
    • u – upload a new program - send the binary using XMODEM
    • b – boot the current program

Note that the ‘d’ command will delete the bootloader, so use with caution!!!

Toboot (DFU Bootloader)

The name of the DFU bootloader is Toboot. DFU is a USB standard for updating firmware on a device. The canonical program to use is dfu-util, which is in most major Linux distributions. When Tomu is in Bootloader mode, it will flash red/green, and show up when you run dfu-util -l.

Toboot DFU mode

Version 1.0 of Toboot will auto-run a program if one exists. That means that once you load the Sample program, Toboot will always run the sample program. To get back into Toboot, you will need to connect both capacitive buttons when you apply power. Tweezers are useful here. A future version of Toboot will have a “developer mode” that will cause it to always boot into DFU mode.

To load a program, run dfu-util -D [program].bin. Toboot will flash the new program onto your Tomu and reboot. Don’t worry if you flash the wrong file, or flash an invalid program – Toboot will detect this and reboot into DFU mode if the file you flash fails to run.

Back Us!

Tomu is crowdfunding on Crowd Supply! We have met the following stretch goals, and their development is already underway:

DFU-compatible Bootloader

We would like a bootloader that gets out of the way and lets you run your normal application without needing to short out the C pin every time. We’d also like to not require drivers, or superuser access, or have to deal with other programs thinking Tomu is a GPS or modem (unless it’s behaving like one.) These are all shortcomings that the current bootloader suffers from.

Enough people have backed Tomu that we have developed an initial DFU bootloader that meets all these goals. Although the initial version is ready, it has some wrinkles that we intend to iron out before shipment.

Injection-Molded Plastic Case

Tomu is tiny, and fits entirely inside your USB port. USB ports have metal shields around them, so Tomu requires a case both to fit snugly inside the port and to protect the components from shorting out against the shield.

The current solution is to 3D print your own case. Since we’ve met this stretch goal, we’ve redesigned the board and have begun working on a steel tool to injection-mold a plastic case.

U2F / FIDO Firmware

GNU Chopstx has been ported to Tomu, complete with U2F support. That means you can use Tomu like any U2F token to add a second authentication factor when you log in. Chrome supports U2F natively, and Firefox supports it via a flag (and will support it fully in a few months)


The U2F firmware source is located on Github at gl-sergei/u2f-token.


To build the U2F firmware, ensure you have an ARM compiler installed (e.g. sudo apt install gcc-arm-none-eabi openssl) as well as Python pip (e.g. pip install --user --upgrade asn1crypto), then run:

git clone
cd u2f-token
git submodule update --init
cd src

Loading onto Tomu

The build system produces an output file build/u2f.bin. Upload this file to Tomu.

Updating udev rules

On Linux-based systems you will need to update your entry in /etc/udev/rules.d/10-tomu.rules (or create it if you haven’t already) to reflect the new product and vendor IDs so that you’ll have permissions to the device.

ACTION=="add|change", KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="cdab", TAG+="uaccess"

Generating private keys on a host machine

During a firmware update, your U2F private key will be erased from the Tomu. This means you won’t be able to use it to login into websites anymore.

This can be alleviated by generating your private key on a host machine and pushing them on your Tomu.


Help us!

These are currently the top items which need to be done related to the Tomu. If you help out, @mithro will probably send you a Tomu device!

Creating (or porting) a FOSS USB stack to EFM32HG

To make it easy for people to develop new applications for the Tomu, we need a good USB stack which is compatible with the EFM32HG.

There are a couple of possible options:

QEmu Emulation of the EFM32HG

We would like to have good emulation of the features in the EFM32HG309 processor so that people can write software for the board without having to have the hardware.

The biggest part of this is the emulation of the USB stack.


Tomu Hardware

Built out of a 3d printed part + 2 layer “thin” PCB (0.4mm, 0.6mm or 0.8mm thick).

Boards have:

All Tomu boards have:




The important thing to note is that you need a PCB that is 0.8mm thickness or less. The default thickness is normally 1.0mm and 1.0mm is too thick.

These gerbers should be possible with the following manufacturers:


The Tomu hardware is under your choice of:

Software for Tomu is under various licenses, please consult the license included with the code.

Open Source Hardware Certification AU0000001