Putting a PIC to sleep for a short while?

[evandude]

I need to be able to minimize power consumption while a PIC (12F683) sits in a delay loop (doing nothing else) for about 1mS or so. Do you think it would be better to put the PIC to sleep for this time period, or to switch it from the 8MHz internal oscillator, to the 31KHz internal low-power oscillator?
By the datasheet, it appears the difference between high- and low-speed internal oscillators is significant, about 800uA vs. 40uA at 5v. It also appears that since internal oscillators are shut down during sleep, I would require an external delay circuit to wake it back up again, which would use at least some amount of power and add to the circuitry. Just wondering if the power savings of sleep vs. low-speed intosc would be significant enough to be worth it.

Anyone know how soon the high-speed intosc is shut down after changing to the low-speed one? I couldn't find that at first glance through the datasheet. That would influence the decision.

My application is part of an RFID system where the PIC must transmit serial packets (less than a millisecond) with pauses (somewhere around 1mS) between each one. It's passive (powered by the reader) so minimizing power consumption between packets, rather than sitting in a delay loop doing nothing but consuming power, would be very beneficial.

 

[Pommie]

As the WDT uses the 31k int osc and can be set to be 1mS (just - 31k/32 prescaler) then sleep would seem to be the obvious answer. I'd not noticed it before, but with this chip the WDT has to be dissabled in the config and can then be enabled in software with SWDTEN.

Mike.