Matt,

Sounds like you may not be giving the A/D converter time to settle. Try
selecting the channel then delay for a few cycles before starting the A/D
conversion.

  Pete Casavant



PICLIST@MITVMA.MIT.EDU on 01/14/98 11:55:18 AM
Please respond to mjb@hazmat.com @ internet
To: PICLIST@MITVMA.MIT.EDU @ internet
cc:
Subject: 16C71 A/D converter crosstalk?


I'm having a problem reading the A/D converters on the 16C71.  I'm using
two, using VDD as Vref, and the problem occurs with the A/D conversion clock
= Fosc/32 or Frc (the conversion clocks I have tried).  The voltages are
read and sent out serially.

RA1 is hooked up to a LM34 Temp/Voltage converter, and RA2 is hooked up
to a simple voltage divider between VDD and GND, using 2.9K resistors

 (Vcc--^^^^--RA2--^^^^--GND)

Like this, both voltages are read accurately.  I wanted to see some changes,
so I shorted out the resistor between RA2 and GND, which should put GND at RA2.
It reads correctly, but doing this changes the reading off of RA1
(signifigantly).

Does the PIC have problems reading close (or at) the voltage rails?  From
my reading of the data-sheets, I think I'm in the legal areas (impedance wise).
If I short the upper resistor in the voltage divider, RA1's reading also changes
but to a different value.  RA1's reading will also change if I short either
resistor connected to RA2 with a 100 ohm resistor.

Can anybody tell me what I'm doing wrong? I can provide more details if they
would be useful.

-----------------------------
Matt Bennett                |
mjb@hazmat.com              |
http://www.hazmat.com/~mjb/ |