The interface hardware is pretty simple. My end goal is to build it around an ItsyBitsy M4 Express board. Alas there aren't any yet, so I did the next best thing: I build it as a Metro M4 Express shield. Not the compact form factor I wanted, but all the same capabilities.

There's a microSD breakout hooked to the SPI lines, a 128x32 OLED on the I²C lines, and a rotary encoder (with push switch) using 3 digital inputs. 

See the related guides for more details on each of these:

• Metro M4
• microSD Breakout Tutorial
• OLED Displays
• Rotary Encoder 

Because I made the decision to use I²C between the interface circuit and the emulator circuit, any MCU with I²C capabilities would work.

One small wrinkle: the Metro M4 is a 3.3v system while the emulator is 5v. Adafruit to the rescue with the BSS138 based 4-channel I²C-safe Bi-directional Logic Level Converter. We just need to run the I²C lines through this between the two circuits.