I would suggest hanging the whole battery checker off a PIC I/O pin,
assuming that the PIC Vcc is directly connected to the battery.

When the PIC wakes up, make the I/O pin high, which powers the
comparator/voltage reference or whatever.  It measures its own supply
voltage, which is pretty close to PIC Vcc, which is the battery.  Once the
circuit has made its determination, switch the PIC pin either low or
tri-state.

This way you don't need to take any particular precautions about the battery
comparator being low-power, it is not on long enough to make any differenct
to battery life.

If you're really clever, maybe you can figure out a way to get the results
back via the same I/O pin (maybe the battery checker leaves its answer in a
cap hooked to the same pin or something?)

> -----Original Message-----
> From: Brian Gregory [mailto:briang@CIX.COMPULINK.CO.UK]
> Sent: Wednesday, November 29, 2000 7:31 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: Low Battery Sensing
>
>
> I'm starting work on a project using a PIC powered by a battery.
>
> The idea is that the PIC will spend most of the time in sleep
> mode so the
> batteries will last a nice long time - at least a year would
> be our target.
>
> The PIC will probably be something like a PIC16C711.
>
> We need some way of providing an alarm when the battery is low. i.e. a
> voltage sensor that doesn't itself use much power.
>
> Are there any chips around to do this?
>
> Anyone got any ideas about how I could do this?
>
> Brian Gregory.
> briang@cix.co.uk
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics
>
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.