Brandon,

I can think of two issues that you may want to consider.

1. What kind of glitches occur on the the 12V line during
    normal operation (powerup, drives spinning up/down, etc.)?
    You may want to make sure there is adequate filtering before the
    regulator to prevent spikes below the dropout voltage of the regulator.

2. What happens if the 12V goes away, but the 5V is still present (and maybe
only 4V)?
     It may make sense to come up with a  supply design that uses both the
12V and
     the 5V supplies with steering circuitry to give the PIC some power.
     Hint: find a pic that  can run at a voltage lower than 4.75V (I think
there are
     versions of the F877 that can).  Don't forget  you will need to use the
VRef+  input
     and a good voltage reference otherwise you would have to scale your
outputs between
     ground and an unknown voltage (Vcc).

Ken

----- Original Message -----
From: "Brandon Stewart" <brandonstewart@ATTBI.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, June 26, 2002 5:33 PM
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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