Sega Genesis gamepads comes in two flavor; 3 and 6 buttons. Method to read these controllers are different but retro-compatible. The firmware can accept and auto-detect both type.
Pin 7 “select” is use to “switch” button reading like a diffrent “page”.
6 button reading is well explained here.
Controller pinout
| Pin | FUNCTION | |||||
|---|---|---|---|---|---|---|
| 1 | UP/Z | |||||
| 2 | DOWN/Y | |||||
| 3 | LEFT/X | |||||
| 4 | RIGHT/MODE | |||||
| 5 | +5V | |||||
| 6 | BUTTON A/B | |||||
| 7 | SELECT | |||||
| 8 | GND | |||||
| 9 | START/BUTTON C |