One thing that is common on a Basic Stamp is to use the same arrangement
but instead of a voltage divider you make the varying resistance connect
to a capacitor. Then you can measure the time required to charge the
capacitor. Now you can just use a general-purpose I/O pin. You pick your
resistors so the values are far enough apart that you can't go wrong.
Still can't really press more than one (or could you? -- maybe you could
select values smart enough that any two would still be unique?).

I agree, I wouldn't tie up an A/D just for that, but the RC arrangement
is great -- I've used it myself.

Al Williams
AWC
* Floating point A/D
http://www.al-williams.com/awce/pak9.htm




> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Alan B. Pearce
> Sent: Monday, February 11, 2002 5:31 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: CHALLENGE: 4x4 keyboard and LCD with 1
> port = 8 bits
>
>
> >The "matrix voltage divider" method I posted yesterday may have been
> >cute, but it's awfully cheap 'n dirty: it doesn't resolve multiple
> >simultaneous keypresses; it requires a PIC with at least a
> 10-bit A/D
> >converter; the voltage levels it outputs for each keypress cannot be
> >known precisely at design time because of resistor
> tolerances; and the
> >voltage levels aren't distributed evenly over the A/D
> converter's input
> >range.  In sum, this voltage divider scheme makes extremely
> efficient
> >use of hardware but it's awfully inconvenient to use.
>
> With surface mount resistors being 1% tolerance, and E24
> ranges available, I suspect that with an hour or so of
> calculating it should be possible to make this arrangement
> reasonably foolproof.
>
> Another trick to do is to have the resistors set up as an
> R/2R network like is used inside many A/D converters, and
> then each switch represents one particular bit of the A/D
> inside the chip. Then for a 10 bit A/D converter one could
> probably get 9 switches reliably :)
>
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