While I'm waiting on parts for the MIDI Interface, I spent the last couple of days working on a new hardware project (and avoiding working on a huge software project I'm also working on ).
The SNES connecter's 3D model is a little borked (wild guess: some normals are flipped), but you'll get the idea.
Current firmware maps the D-Pad to its four directions, L to up, R to down (handy for diagonal jump games) and all other buttons to fire. This should make any SNES (compatible) controller work like a normal Atari Joystick.
There is however an extension. Select/Y/B are connected through 3 resistors to POTA. The same goes for X/A/Start and POTB. The resistors are chosen in such a way that all eight combinations of the three buttons can be clearly determined. Note that this is not a resistor DAC, because that would result in eight different voltages, but Pokey measures current at +5V. The firmware only outputs 5V "into" the resistors if the button is pressed. Otherwise the pin is tri-state (i.e. not connected) and the connected resistor is ignored. That way, eight currents can be measured by Pokey and the values are sufficiently apart. Several factors influence the actual readings, e.g. temperature, resistors used and Pokey itself. Therefore if a game was to be written to utilize these buttons, it has to calibrate itself first.
BTW J1 is an Arduino Nano (Clone), which can easily be programmed over USB (connector is already mounted on the Nano's PCB). I have decided to just piggyback it on top of this board, because I can never match its functionality for €2,=, which is what it costs to buy one from China, incl. S&H.
All files will be released eventually and will be open source.