Depends on what exactly you are trying to monitor -- fluctations
only, or a power-out problem condition.  For the latter, this
obviously won't work.

Otherwise, I did something similar for an automotive voltmeter,
but set up the voltage divider so that 25.5V translated to 5V
at the analog input.  This way, I was able to avoid a bunch of
math, as 25.5V ==> 5V ==> 255 on the upper 8 bits of the A/D-
converted value.  Only other thing was to insert the decimal
point in the right place.

Be careful about the max voltage you put on any of the analog
inputs.  IIRC, max voltage at any pin is 0.3V above VDD, or
5.3V in this case.  RA4 has a different spec which I don't
remember offhand.  You may want to protect it with a zener
or something.

Cheers,
-Neil.




-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Brandon Stewart
Sent: Wednesday, June 26, 2002 4:33 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Monitoring a voltage with a 16F877? the same voltage
source??


I am building a project that fits into an empty computer bay and is powered
from the computer's power supply.  This is done off of a 12 volt line and
through a 7805 -> which provides the 5V for the PIC to run on.  Now, I plan
to set up voltage dividers between the computer's 5 volt line and ground,
and the 12 volt line and ground.  This way I can use the A/D inputs of the
PIC to monitor the computer's power supply voltage.  This is done by
choosing resistor values that provide monitoring voltages at 2.5 volts for
each of the two lines being monitored.  This is done to allow for the
greatest fluctuation in the power supply voltage, whereby I can still
monitor it without 'clipping'.

==>  My question is, should I worry about sensing a voltage that is
providing the power to my PIC to start with?  Remember, I am monitoring a 5
& a 12 volt line, and this 12 volt line is the same one that powers my PIC
through a 7805.



Thanks,
Brandon

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