In a recent project, the PIC was powered from 12 volts DC via a 5 volt
regulator. I had a large capacitor on the o/p of the regulator, then used a
voltage divider from the 12 volt supply to a PIC pin such that the voltage
was just above the pins ON threshold voltage. When the 12 power dropped,
the PIC pin went low, and I saved the various values in eeprom.

At 03:40 PM 10/5/2003 -0700, you wrote:
>Hi,
>
>In a project at hand, I would like to save some data into the onboard
>EEPROM of a PIC16F870 when the power fails or is turned off. A large
>capacitor in the power supply ensures that when the main supply fails,
>there are still a few hundred milliseconds before the supply into PIC
>touches zero. How can one detect such a power failure and then
>immediately launch into the EEPROM write subroutine (while there is
>juice in the capacitor)? I don't really want to save the data regularly
>after every few seconds (analogous to Autosave in MS Word), but only
>when the power goes.
>
>Thanks,
>Mohit.
>
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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