Padu,

give us a code snippet of your port init. This sounds, that your input is=
n't
an input...

A simple test is, to connect a relative high resistor (let's say 100K) fr=
om
RA0 to GND, if RA0 is really an input the voltage should be 0V. Then conn=
ect
same R to +5V and RA0 must be +5V.

Peter


-----Urspr=FCngliche Nachricht-----
Von: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] Im Auftrag =
von
Padu
Gesendet: Donnerstag, 6. Oktober 2005 08:27
An: Microcontroller discussion list - Public.
Betreff: [PIC] 16F877A A/D Weirdness

Maybe I'm doing something wrong, but here's what I'm trying to do.

I'm trying to measure voltage of a battery that can go as high as 16V. I'=
m
using RA0 set as analog input for the A/D and I'm using Vdd as a referenc=
e.

In order to reduce the voltage to compatible levels, I've connected the
following voltage divider: from the + side of the battery, I have a 4.7K =
and
then a 2.0K in series going to the - side. Let's say that the battery has
12V across poles. I put my multimeter + probe in between the two resistor=
s
and the - probe on the - of the battery. It reads about 3.6V, perfect.

Now I connect my circuit's ground to the ground on the battery to be
measured, still 3.6V. The problem happens when I connect my RA0 pin to th=
e
divider (same spot where my probe is). Immediately the voltage drops to
2.65V.... why is that? 1V doesn't seem a lot but when I do the math to
output the real battery voltage, of course it doesn't match anymore.

What am I doing wrong?

I measured the voltage accross RA0 and ground, it is not zero. I figured
that perhaps it is because the pin is set as input and is floating?

Help is very much appreciated

Padu

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