For the controller, I decided to use the PSX controller (DualShock 1). I know that I could use something like a NES controller, which is probably easier to interface with (?), but I just thought that it might be used in my future projects, having more buttons is probably better. So, as I result, I got a cheap DualShock 1 clone off eBay.

I spent some time writing and testing the driver for it, and now it is working (partially). I am able to read all buttons out, including two analog joystick. However, I am unable to enter the escape mode and manually set it to work under analog mode rather than digital. The controller simply refuse to ACK my 0x44 Set Major Mode command. I guess this is not a big deal, the Game Boy does not even need the analog joystick, and I can always set the mode by pressing ANALOG key.

Here is a screenshot of the oscilloscope showing it working in the analog mode:

And here is a screenshot of it refuse to ACK my 0x44 command:

For more information about the driver itself, please refer to the code.