This is lower in cost than using a dedicated keypad encoder, such as the ones I mentioned earlier in the thread, it's more flexible, and it doesn't rely on supply of rare components.

It's also going to do EXACTLY what you want, namely to isolate whatever processes you have running from the task of scanning the keypad. What's more, it will consume a minimal number of pins, since you can interface to the slave processor serially, in nybbles, or however you like. You don't have to use async signalling. You can use i2c or whatever suits you and you can have an interrupt if and only if there's a key pressed, and otherwise ignore the thing.

RE