“Squeezing the full functionality of a computer out of just a 40 keys is a lot to ask for. Challenge accepted..
The people at rc2014.co.uk (read more here: https://rc2014.co.uk/history/) came up with this little Universal Micro Keyboard. It has the classic 8 x 5 matrix of the ZX81, ZX Spectrum, etc. and can be interfaced directly as a matrix keyboard.
With the addition of an ATMEGA328P it can be hooked up as plain old serial or as USB (via FTDI).”