The maximum recommended source impedance for a PIC ADC input is 10K which
means approx.  1mA drain during an ADC conversion.
So put the other end of the divider into a processor I/O pin.
Wake the micro up periodically, switch the I/O pin from highZ  to  'low',
take the measurment and switch back to highZ.
Go b ack to sleep if nothing has to be done.

A rather better scheme is to use a dedicated external supervisory circuit,
such as the MAX6457 Voltage Monitor (SOT23 package) which runs at 3,5uA
The output can hardware interrupt the processor out of sleep should the
supply voltage reach a critical  threshold,


Perry Curling-Hope

Research and Development
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