The output pins were floating, and I used my dMM to get the readings from them
since I don't have a logic probe. And the Input Pin wasn't the pin that was
fluctuating, the output ones were, just for clarification. But I had a lot of
other devices on my board at the time, so what I did was I cleared out
everything but the 16f877 and then it seemed to work ok. I guess the problem
was noise (not too sure how it works)??? Anyway, thanks for all your help.

Jai

Quoting Olin Lathrop <olin_piclist@EMBEDINC.COM>:

> > Have a question about the A/D on the 16f877. I gave my first shot on
> coding a
> > simple converter that uses a divider from a pot as an input to one of
> the
> > pins. Now I'm pretty sure I have my code set right, because when I read
> the
> > results (I did about 5 test cases), they all seem pretty accurate,
> except with
> > a few problems.
> >
> > Some of the pins had constantly a constantly changing voltage that was
> > registering on my DMM. Most of the pins would be stable, either ~0 or
> 5.2V,
> > but some of them, each on different tests, would be fluctuating,
> sometimes
> > from 2.5-2.9V, and sometimes other values. Is this normal? Does the fact
> that
> > my program keeps going in a loop affect this (even though I'm reading
> the Go/!
> > Done flag to check when it's done)? And it seems to be different pins
> each
> > time, so there doesn't appear to be a definite pattern. On some
> readings, all
> > the pins are perfect. Any ideas?
>
> This depends on your external circuit.  How are the pins being driven?  If
> they are floating, then you could get the symptom you describe.  Microchip
> specifies the source impedence driving an A/D pin should not exceed
> 10Kohms.
>
> Note that as the A/D is switched between channels, you actually have a
> charge pump from one channel to the next.  A newly selected channel is
> connected to the same internal capacitor that the previous channel was
> connected to.  This actually creates an equivalent resistance from the
> previous channel voltage to the current channel pin.  This equivalent
> resistance is proportional to the period between selections of the same
> channel.  None of this should matter, however, if each channel's source
> impedence is 10Kohms or less and you are waiting the proper acquisition
> time.
>
>
> *****************************************************************
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> (978) 742-9014, http://www.embedinc.com
>
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