The form perfectly follows the super retro function of this arcade-tastic chiptunes syntesizer from Portmanteau Devices, the Pianocade. A monophonic square-wave synth deserves to be played with arcade buttons!
The Pianocade’s built-in synthesizer is based on the sound hardware of the Nintendo NES and Game Boy (specifically, one of the square wave channels: it’s a monophonic square-wave synthesis engine with a 4-bit digital-to-analogue converter). Because it’s monophonic, it does what many early games did in lieu of chords: it cycles through the notes of the chord rapidly (arpeggiation).
It’s open source, and will come in both pre-built and kit form. Pre-orders are open at Pianocade.