The firmware for this synth is probably as basic as you can get, since most of the magic happens in the analog realm.
I used an Arduino Mega so I had plenty of IO available, since it needed to drive 16 oscillators and a couple of PWM signals. In the module version I'm using an Atmega32, which is turning out to become my go-to MCU for these kind of projects.

First thing the MCU needs to do is scan the keyboard matrix or process the MIDI input, both topics that have been covered a lot by other people so I'm not going to go into those here. Plenty of tutorials on the web already :)

The MCU sends out 16 pulse signals. These pulses are generated by very basic wavetable oscillators. The tables themselves are very short and consist only of booleans since the signal only needs to be high or low. The reason for all this is a little... lesson in trickery, if you will:

By having the pulse signal programmed in a table I can create all sorts of pulse-patterns to trigger the analog sawtooth oscillators with. This way I can very easily create different tones without making things complex. Take a look at the schematic below and see if you can figure what kind of effect multiple pulses of various widths would have.
There's nothing much special going on otherwise, look at some Arduino DDS synthesis tutorials if you want to know how wavetable oscillators are implemented.

The 2 low-pass filters and VCA's are both controlled through PWM, driving an LED + LDR vactrol. See schematics below.
Envelope-wise it's all very basic, just attack and decay, since I wanted to keep this synth straight forward and easy to use.