Today I removed the small buttons board from the explorad breadboard to a new breadboard with a Teensy 2.0:

The yellow wire is the Serial1 Tx line (TTL, nothing fancy like RS232).

The code currently just debounces the buttons and sends a short string over Serial1 at 9600 8N1. Example:

b 0 0

means that button 0 (left) has been pressed. No further string is sent for that button until it is pressed again. The second '0' means that this was an "initial" press, not a software repeat (which I plan to implement), so

b 0 1

would mean that button 0 was pressed, and the software inserted a repetition.

There's still plenty of space for more hardware, like the LiPo charger, touch input hardware, and so on.