Hi Josh, For debouncing see
http://www.dattalo.com/technical/software/pic/vertcnt.html ... This has got
to be the best way to debounce switches I have ever seen - ingenious. Works
great. (Thanks Scott!) For 20 years I used to do the "take 3 samples and
compare - when they all match you are debounced". Scott's way is far more
elegant and faster. I now use a 2-bit vertical counter updated every 10mS to
debounce 8 inputs at a time. Works great.

Can you use a I/O port with internal pullups? Any way you slice it you will
need 11 I/O lines (or more external hardware). I would lean toward driving
one of 3 pins (in turn) and reading in 8 switches at a time. Others may
point out valid reasons for doing something different though.

-Mark

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
> Of Josh Koffman
> Sent: Saturday, April 09, 2005 12:03 PM
> To: Microcontroller discussion list - Public.
> Subject: [PIC] Matrix Keyboard Philosophy
>
>
> I'm going to be starting a project coming up using a preexisting
> keyboard assembly. It has a non symmetrical matrix though - it's not
> 4x4, it's 3x8 (if memory serves...it's not right in front of me at the
> moment). With a 4x4, it's easy to drive 4 of the pins and scan the
> other 4. But with a non symmetrical keyboard, would it be more
> efficient to drive the 3 pins or the 8 pins? My thoughts are that if I
> drove the 8 pins and watched the 3 pins, I'd need less pullup
> resistors (useful as board space is limited). I'll sacrifice code
> simplicity though, and it'd take 8 scans to read the whole keyboard.
> But I only have to watch 3 pins which might be easier.
>
> Any thoughts on the matter?
>
> Also, I'm looking to implement a debouncing routine so that eventually
> I will be able to trigger a command on the key down and the key up. My
> idea so far has been to read the keyboard, and shift the result for
> each key into a temp register, then look for patterns (ie 11110000 is
> a keypress, 00001111 is a key release...we discussed this a few months
> ago). This requires two steps though, reading the keyboard, then
> comparing all the temp registers to the patterns. Anyone have a better
> way?
>
> Thanks!
>
> Josh
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
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