Interesting...

I am actually setting ADCON1 to xxxxx011 as I am using PORTE for I/O, but my
understanding that anything with an lsb of 1 sets RA3 to Vref. I can however
move that I/O somewhere else if setting ALL analog-enabled imputs to analog
somehow makes a difference. I will also check for an unintended
reset....intriguing.

I confess I am running an old MPLAB version (3.5 or something) and the
firmware in my Picstart is surely outdated as well. My next step is to
upgrade to 5.0 and freshen up my firmware too. You don't mention the device
you are using, is it in fact a '74A? Does it matter?



--Thanks everyone....Paul Howell

----------
>From: Barry King <barry@NRGSYSTEMS.COM>
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: Vref on PIC 16C74
>Date: Thu, Apr 6, 2000, 11:44 AM
>

> Thomas,
>
> The Selection of Vref is in ADCON1 as you said, make sure you are
> setting it to xxxxx001 to have RA3 as Vref.  Setting ADCON1 to all
> zeroes is the reset default, where Vcc is the ref.  Is it possible
> you're getting an unintended reset which trashes the ADCON1?
>
> The data sheet does not allow a Vref below 3V.  Its in the A/D
> converter section, DC characteristics.  (Parameter A20).  Vref range
> is allowed to be 3V to Vdd+0.3, (I assume the upper limit is where
> the over-voltage protection diodes on RA3 begin to turn on.)  That
> said, I am using mine with 2.55V reference (10 mV per count), and
> seems to be working fine.  I asked the FAE why the 3V min, and it
> comes down to the allowable offset in the comparator that is the
> heart of the A/D.  If Vref is smaller than the 3V spec, the
> comparator error budget is << 10 mV.  So a "good" one should work
> fine at 2.50V Vref.  I don't think that is your problem.
>
> -Barry.
> ------------
> Barry King, KA1NLH
> NRG Systems "Measuring the Wind's Energy"
> http://www.nrgsystems.com
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