AVR-Chat

> Backup Power: 110 VAC power is normally available but I'd like to 
provide backup
> power to maintain the RTC clock function mentioned above.  What's 
the
> easiest/cheapest way to do this?  One thought was to just purchase 
the smallest
> battery UPS I can find.  Another thought was a 12v battery 
supplying the
> processor, etc. with a 'float' charger attached to it.  Tradeoffs?

Most RTC's can be powered from two sources.  one is about 1/2 volt 
higher (or less).  the lower voltge one is often a button battery.  
when the high voltage, if you call 5 volts high, is lost, the button 
battery kicks in at 3. something volts and then kicks out when the 
power is restored.

The button will have a realtivly short life of 5-10 years in moderate 
use, 2-3 in continuous use and about 25 years in reserve.

Not too bad for a 50 cent battery and a $1.50 for a holder.

I use the 12.5mm solder leg verson for my RTC's, or the 22mm for 
larger projects like data loggers that are only powered briefly and 
are unplugged 99% of the time.

For the sunny climate, a solar charger will be of great use to re-
charge your battery packs.

also, read the data sheet for the RTC.  Some have the ability to 
trickle charge the back-up battery.

Dave