I have managed to get a consistent 10 bit conversion using AN512, but there
is obviously a trade off with conversion time against resolution.  The other
issue is detecting multiple keypresses which may be tricky, but may not be
important in a lot of applications.

Damn, you've got me all interested now.  Hmmm...where did I put MPLAB?

Regards


Mike Rigby-Jones
mrjones@nortelnetworks.com

P.S. I've been getting 2 bounces for every posting irrespective of where my
reply goes, top, bottom or both.

> 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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