Hi all,

I was happily on my way to using an MCP79412 RTCC chip to a PIC 18F =20
circuit, then remembered that there are 18F's with RTCC's built in.  =20
But the PIC has no separate battery backup option.  Main power =20
(automotive 12V) to the PIC circuit *will* get cut off regularly, and =20
sometimes for months, so I need to find some way to keep the clock =20
running during that time.

The MCP79412 would allow me to add an external battery, but if I can =20
add some type of power to the PIC and use it's RTCC instead (to =20
simplify and lower costs), that would be better.

I could add a battery management chip, but these seem to cost more =20
than the MCP79412 ($0.89ea in 100-pc qtys).

Alternatively I can add a (relatively) large capacitor to the PIC, =20
detect vehicle power and use a MOSFET to switch off power to the other =20
components on the board (other PICs, etc).  I'd probably raise the =20
main regulator output voltage to 5.4-ish volts, then use separate =20
diodes to the main PIC and the rest of the circuit.

And I could also do something similar as the cap, but with a CR2032 or =20
similar.

The simplicity of the MCP79412 is nice, as I don't have to mess with =20
the main PIC code for this.  The battery or capacitor option would be =20
nicer if it's just a dime or two in extra cost (not including the =20
battery, which I'd need for any of these solutions).

I'm wondering how others have tackled this.  Any ideas?

Cheers,
-Neil.



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