> The datasheets I've read have said that you must TRIS analogs as inputs if
> you want to read the analog voltage. They say that if you TRIS the pin as
an
> output, the A/D can still happen, but that it will convert the pin voltage
> as driven by the digital output. I stand ready to be corrected.

There may be more than one A/D implementation.  I remember running into an
A/D gotcha on one of my first PIC projects, which might have been a 16C77 or
16C924.  I wasn't using analog in, so I ignored the A/D module.  What I
didn't realize was that the A/D wakes up enabled, and that it was taking
over the analog input pins.  I was trying to use some of them as digital
outputs, but I couldn't get them to do anything even though TRIS and the
port register were set correctly.  When I finally figured out to disable the
A/D module, everything worked the way I intended it.

I've also noticed that the A/D is supposed to wake up off now on newer PICs
like the 16F87x.  Perhaps these A/Ds also have slightly different operation.
I sorta remember that you can individually disable the A/D function on
select pins by using TRIS on the 17C series.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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