Hi Andy / Peter,

How about using a 74x165 and 74x595 to interface your key matrix (pullup or pulldown resistors are required here).

This will give you a 4 or 3 wire serial interface and is more efficient than a MCU dedicated for just this task.

I suggest to scan it in a timer interrupt because you'll need software based key debounce and that works better in a fixed time interval.

Cheers, Roger