Haven't tried it, but it should be easy enough to set the resistors like
this:
Suppose you have 15 keys. Set the voltages to values like
18h
28h
.
.
.
E8h
F8h

By ignoring the lower 4 bits, tolerance to drift should be quite
reasonable.
I'm not sure about the reliability of the AD conversion in AN512, but
maybe it's
not a problem to use just 3 bits of tolerance, allowing for 31 keys...
14h,24h,...,1Ch,2Ch,...,E4h,F4h,...,ECh,FCh

What do you think?
Anyone wants to sort out the resistors? 8)

Nuno Pedrosa.

Michael Rigby-Jones wrote:
>
> Neat idea.  However, by losing the LM317 and using the keypad and resitors
> with the single slope converter in AN512 (Microchip App Note), the method
> could be used on low end parts without an AD converter with just a small
> software overhead.  I suspect you wouldn't even need the reference resistor
> described in the app note, simplifying things even more.
>
> A nice compact routine that chooses the closest value in a lookup table
> would be nice, to allow for drift etc.  My asm is not great so I'll leave
> that to the rest of the gurus on here.

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