On Tue, 19 Oct 1999 11:18:44 -0400, you wrote:

>If you switch the ground instead of the +5 to
>the comparator, and you *also* have the ground
>for the divider resistors connected to this same
>"switched ground", then when the PIC output
>pin is LOW all works as desired, and when the
>PIC output pin is TRISTATED, then the comparator
>is switched off, and current through the divider
>resistors is reduced to almost nil.
except the divider has to come from the battery supply, not 5v, so you
can't just switch the Ov line - it will push current through the input
diode to 5v. This can actually work with some voltage regulators, as
current will flow in the divider _instead_ of the regulator, but it
can get messy...!

I used this to measure battery voltage using the PIc's ADC - you could
obviously adapt it for non a/d parts, either with a comparator, or a
voltage detector chip

+9V
|
+--------------------[5V reg]--->PIC supply
|       |
|      R3
 >|     |      C1
  |-----+-R4---||---< output pin
 /|
R1
|
+---------+----->a/d input
|         |
R2    [VOLT DET CHIP]--LOGIC O/P
|         |
0V       0V

R1=R2=10K
R3=R4=47K ( I used a digital transistor, so they are internal)
C1 = 10n

When the output pin goes low, the PNP transistor turns on for long
enough to measure the voltage. C1 acts as a level-shifter. This output
pin can often be shared with other functions. 
R1/R2 are low enough to avoid errors due to input leakage. 
You could even power a comparator like this, but watch out for
settling time. 


For brownout, use a reset chip on the processor's supply - smaller
cheaper & simpler than a micropower comparator - Motorola, uem, seiko
and many others do cheap ones (<50c) that draw about a microamp.
beware of using comparators for brownout protection - they can do odd
things at very low voltages, so they don't always hold their outputs
low all the way to 0V.