Has anybody got the save before power down working properly?

[jaikumar]

Hi all,
The application requires saving of 4 bytes to either internal or external eeprom before power off.

If we create a circuit such that to make pic work for a while before power down. We run in to other problems such as pic not resetting properly
due to capacitor not allowing the voltage to go down properly to reset the device.
Could any body give me some ideas for a proper circuit which would work.

Thanks & Regards,
Jai.

[Ttelmah]

Not really to do with circuitry. All to do with code.....

Multiple things. Don't try to hold the supply 'good' longer than you need. Trigger an interrupt to say 'supply failing' at a point you determine (say 4.5v). Then on a typical PIC, you can write to EEPROM, down to perhaps 3v. As soon as the interrupt fires, cut off all circuitry you can to save power. If your PIC supports changing clock speed to an internal clock, do this down to the slowest clock available. Perhaps 32KHz. Your consumption should then be under 50uA. Capacitor needed to give just 1v of drop over (say) 25mSec (four writes, plus a bit extra), is then only 1,.25uF!. Something like the typical 10uF, handles this easily....
Then once you have written the bytes, carry on monitoring the fail input. If the power recovers, turn the clock back up, and return from the routine. The commonest problem is a 'brownout' situation, where the supply drops enough to trigger the write, but then doesn't actually go off.
Use the chip's watchdog as well. The whole 'point' of this is to reset the device, when it remains alive, but has got somewhere it shouldn't.

Best Wishes

[bkamen]

[Ttelmah]

Yes,
Doen't someone like National, do a 'supply monitor' chip, that monitors the incoming supply, and generates both an interrupt, and a reset output?. Used one of these a while back for an app like this, and it helped simplify things a lot.
It depends masively on the nature of the supply involved, but if there is an incoming unregulated rail, this is well worth considering.

Best Wishes

[bkamen]