The cartridge is slowly, but surely, taking shape. I'm simultaneously designing a PCB which I will have etched once the cartridge seems to work fine. Right now, I'm working with something that looks like this:
I tried clocking the MCU at 20MHz, but it wouldn't cooperate properly. Maybe I could shop around for one fabrication model which does, but what's the point. If it says 16MHz, I'll respect that. Instead, I have decided push my code to the extreme: I'm using a 12MHz crystal. "What, are you mad?" you say, in which the answer is "probably". I don't want this thing to barely work at 16MHz. It needs to be rock solid at that speed, so I'm clocking it down instead.
Oh, and the emulation code is disabled at the moment, it's only printing out debug info now. Bluetooth communication works fine, too - here's a picture of the output, where it is only connected to the C64:
*Sniff*... right back atcha, C64, right back atcha big time.