It’s really just an ATTiny85 – a favorite of the crowd that ... Four of the six IO pins on the ‘Tiny85 are broken out on a pin header; two are used to communicate with the BTLE module.

[mitxela]’s polyphonic synth for the ATtiny 2313 does just that ... and audio output provided eight digital pins summed with a bunch of resistors. Yes, this is only a square wave synth, and ...