On Sun, 7 Jun 1998, David Duffy wrote:

> I came across a way to connect a keypad to micro with only one pin;
> It appears in "Electronic Design Online". This is the web version
> of the trade magazine - worth a read and best of all it's free !
>
> http://www.penton.com/ed/Pages/magpages/may0198/ifd/0501ifd.htm#5
>

It is a cool design. Unfortunately they also have a 555 timer (gag) in the
circuit (in addition to the microcontroller). I bet (or know?) that it's
possible to toss the 555 and still do the job with  a single I/O line. In
essence, the (simplified) circuit is:

          |
  PIC I/O +---/\/\/\-----+
          |      R       |
          |             === C
          |              |
                        ///

Where the value of R depends on which key is pressed and C is fixed (but
other wise arbitrary). There are a number of algorithms that can measure
'R' given 'C'. For example, BC tip #4 can be adapted to this. However I
think the easiest way is to discharge C and then charge it up with pulses
of current until it exceeds the PIC I/O input threshold. The number of
charge pulses required will be somewhat proportional to R.

Scott