Tomu Hardware

A tiny ARM microprocessor which fits in your USB port.
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Tomu Hardware

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

Tomu Front Render Tomu Back Render


We're available on mailing lists and IRC:

About the PCB

Built out of a 3D printed part + 2 layer "thin" PCB. LCA2018 and Crowd Supply boards are 0.6mm.

Designs and documentation are available in:

  • pcb/: KiCad design files
  • docs/: PDF schematics, pin-out diagrams, and 3D renders

The board requirements are very lo-fi:

  • 6mil traces
  • 6mil clearance
  • 0.3mm drill / 0.6mm vias

The primary MCU is a Silicon Labs EFM32HG309 (datasheet, reference manual).

The important features are:

These make something which is almost hand solder-able fit into the tiny size.


All parts are available from Digikey, and are readily available from other suppliers.

Individual Parts

  • Resistors
    • 2 x 0402 - 15 Ohm - Digikey
    • 2 x 0402 - ~50 Ohm (Depends on LED choice)
  • Capacitors - Digikey Capacitors 0402, >1uF
    • 2 x Capacitor 1uF 0402
    • 1 x Capacitor 4.7uF 0402
  • 2 x LED 0402
  • 1 x EFM32HG309 or EMF32HG308


The PCB is thinner than a USB socket, and has exposed contacts on the reverse side, so it needs a case in order to make proper contact with the USB pins, and to not short on the shield of the USB port.

Early prototypes used a business card that had been folded over, however the crowd funding campaign made enough to cut steel and make a case.

Designs are available in the case/ directory. Cases for earlier board versions tend to require higher tolerance on 3D printers.


Why did you create this?

The aim was to prove that a device which fits in your USB port could be created by hobbyists in small quantities extremely cheaply.

Where does the name Tomu come from?

TOMU comes from the abbreviation "Tim's Open Micro USB".

What can I use this for?

What ever you want! If you do create something, please send me the info!


The contents of this repository excluding third_party directory is released under your choice of the following two licences:

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