What is the minimum current draw you can get on your PIC 18F* in 'RTC only'
mode?
You may want to use a CR2050 (same as CR2032 but thicker, higher capacity).

If the current is low enough to last for how long you need from battery,
 power the RTC PIC with a diode off the power bus, and a diode
from your battery. No management chip needed.
Just be sure to have a bypass cap on the RTC PIC since the diode
makes the supply soft.
 That PIC should be happy running at 4.4 volts
(diode drop) if you're not pushing the clock speed to max.
BUT you'll want to turn off the BOD since it wastes 50ua (On the
16F parts anyaway) so use an external reset device with better specs.

Anyone needing or knowing of a position for a PIC
hardware/software/firmware designer
in the Edmonton area, feel free to contact me privately.

Robert


On Wed, Oct 2, 2013 at 8:58 PM, Neil <picdude3@narwani.org> wrote:

> Hi all,
>
> I was happily on my way to using an MCP79412 RTCC chip to a PIC 18F
> circuit, then remembered that there are 18F's with RTCC's built in.
> But the PIC has no separate battery backup option.  Main power
> (automotive 12V) to the PIC circuit *will* get cut off regularly, and
> sometimes for months, so I need to find some way to keep the clock
> running during that time.
>
> The MCP79412 would allow me to add an external battery, but if I can
> add some type of power to the PIC and use it's RTCC instead (to
> simplify and lower costs), that would be better.
>
> I could add a battery management chip, but these seem to cost more
> than the MCP79412 ($0.89ea in 100-pc qtys).
>
> Alternatively I can add a (relatively) large capacitor to the PIC,
> detect vehicle power and use a MOSFET to switch off power to the other
> components on the board (other PICs, etc).  I'd probably raise the
> main regulator output voltage to 5.4-ish volts, then use separate
> 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
> similar.
>
> The simplicity of the MCP79412 is nice, as I don't have to mess with
> the main PIC code for this.  The battery or capacitor option would be
> nicer if it's just a dime or two in extra cost (not including the
> 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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>
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