On 7/1/05, Mario Mendes Jr. <mario@mmendes.com> 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).
> 

 Thre are many way of doing this, and I've guess you have already got
the answers:
1. use the ADC and a rezistive network divizor switched by buttons
2. use the comparator (internal or external) with the internal voltage
reference which has the required resolution
3. use an RC network with a fixed C and a keypad switched resistor
network and measure a time variation
4. use a paralel to serial converter and share the clock pin required
with other function

best regards,
Vasile

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist