Alex Davis made Squareinator – a SN76489 Monosynth.

After being handed an old package of the SN76489 sound chip, Alex wondered if could be used as a synthesizer.  After a bit of research he was inspired to make the Squareinator.

Squareinator drives a SN76489 vintage soundchip as a monophonic
synthesizer, using all three squarewaves plus pseudo-random noise to
deliver a fat synth sound. A multi-mode 12 db/oct VCF adds additional
character and is playable at full oscillation. All synth functions are
exposed as MIDI CCs. The ATMEGA32U4 in the Teensy 2.0 allows us to use
the high-speed timer PLL to provide the 2 mHz clock required by the
SN76489 with no extra parts.

You can find the code for the project as well as a schematic on GitHub.