I've added an NES gamepad adapter to the project. This gives me a way to provide interactve input to games and get on with more of the build.

The implementation is based around a PIC micro which handles the I2C comms with the two gamepads.

I also needed a random number generator for the games so with a bit of space on the breadboard and a small addition to the design I added that too.

For more info ...

Hackaday project for the adapter ...

https://hackaday.io/project/181036-nes-controller-interface-using-pic16f18446

As usual I have a verilog simulation of the adapter....

https://github.com/Johnlon/spam-1/blob/master/verilog/gamepadAdapter/gamepadAdapter.v

The hardware schematic is ...

https://easyeda.com/editor#id=c41219d653c64b329350f3ddb47a70a8

The main routine of the microcontroller code is ... 

https://github.com/Johnlon/NESInterfaceAndPeripherals/blob/main/main.c