1-pin? Ok fine. I dare you... Let's see you do it with 0 pins! :)

That's why I put the "more external hardware" qualifier. Resistors are
external hardware. I'll have to think about the 1-pin solution. Seems like
it would be a challenge to be able to detect multiple keys pressed at the
same time. Gee Whiz, Thanks a lot Mike. Now I gotta think about this all
night :)

-Mark

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
> Of Mike Hord
> Sent: Monday, April 11, 2005 4:13 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Matrix Keyboard Philosophy
>
>
> > 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).
>
> Not entirely true.  It would be possible to do this 8x3 with one
> pin (tough)
> or 3 pins (easier), IF you pick the right pin(s); I guess this
> DOES fall under
> "more external hardware", although that hardware is just resistors.
>
> By using a pullup resistor on the columns and a pulldown on the rows (or
> vice-versa), you can create a voltage divider which will produce
> a different
> voltage depending on the button pressed.  Use a different resistance for
> each row.  Now you can poll each of three ADC pins and, if the value is
> in the right range, you know a valid button press has occured.
>
> Not necessarily the best possible way, from a CPU overhead/software
> simplicity point of view, but it's pretty easy on the pin count.
>
> Mike H.
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