I did something like this ages ago using diodes to multiplex lines.  Take a
look at http://www.mikerj.clara.net/keypad.html  The design is for a 3x4
keypad but is easily extendable to 4x4.  The code is shockingly bad (first
PIC assembler I ever wrote to be honest), but it has been used by a couple
of people.  Somewhere I have an updated version, and a HiTech C version.
I'll try to dig them out if there is any interest.

Mike

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