On Wed, 2 Dec 1998, Rigby-Jones, Michael [PAI01:4837:EXCH] wrote:

> OK, I got bored last night and got to thinking if I could interface a 3x4
> matrix keypad using less than 7 lines without shift registers etc.  I
> suspect it must have been done, but I figured out a (possible) solution
> which uses only 4 lines and some diodes.
>
> I haven't got any hardware to try this out on at present, but I'd be
> interested in any comments.  I started writing the code in Hitech C but
> figured that I had to get to grips with PIC assembler some time so this is
> my first attempt, and is therefore probably full of standard mistakes and is
> most definately in need of optimisation.

I haven't seen your code yet but I have to say that AN512 allows one to
connect buttons through a SIPP R ladder with good results ;) 8 keys per
ladder is the usual limit, you can use 3 ladders with 4 pins to interface
24 keys to the PIC.

hope this helps,

Peter