On July 1, 2005 12:34 pm, Mario Mendes Jr. wrote:
> Hi guys,
>
> I'm working on a little project and have just one i/o pin left in the
> pict.  Two things about it, first, is that I feel that really must
> find something for that pin to do as not to go to waste =) and,
> second, I would really like to include a keyboard/keypad in this
> project.  I once saw a "16-button analog keypad" circuit which used 1
> pic i/o pin configured as analog input, (obviously) 16 momentary
> switches and some resistors.  The theory behind it was that for each
> key pressed, a different voltage would be present to the analog input
> and the pic could tell what key was from that voltage.
>
> The only problem is that I tried all sorts of google search phrases
> and can't find anything other than the usual 16 button keypad
> interfaces and PC/AT keyboard interfaces.
>
> Anyone here has a link or details about how this is implemented?  I
> would be specially helpfull if you had a diagram WITH the resitor
> values in it =) but I can try to come up with those values on my own
> if you have a schematic (or a good explanation of how everthing is
> connected).

If you search the archives of the piclist for:
"[PIC] Matrix Keyboard Philosophy"
It was recently discussed (approx April 9 2005) and has a few ideas.
Personally, I somewhat like the idea similar to reading a PC joystick or 
a 555-type timer, by discharging a capacitor, then timing the amount of 
time needed to achieve a crossover from 0 to 1 bit (see AppNote 512):
http://ww1.microchip.com/downloads/en/AppNotes/00512e.pdf

There are also other valid ideas there as well.
As a side note, reading 4 bits via an analog method is easy, 8 is 
tougher, and 16 may be somewhat difficult due to noise on the power 
lines or other induced noises.


> Thanks in advance.
>
>
> -Mario
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