On Sun, May 08, 2005 at 03:11:51PM +0100, Dave Turner wrote:
> Oh, i didn't know the RC could be so innacurate. 

RC is never used for timing sensitive applications. At the nominal 1%
a RTC will drift 36 seconds an hour.

1% represents 10000 PPM. Most crystals are in the range of 20 PPM.

And each of these will drift differently based on temperature.

My conclusion is that an external source is the best bet. Jinx has posted
about the $2 USD battery powered clock modules. Also one can use the WWVB
60 KHz clock signal broadcast from Colorodo. Finally the easiest and most 
stable tick source is the 60 Hz AC signal out of the power outlet. A
transformer and an optosiolator will give you 216000 ticks per hour like
clockwork.

When I get back to my sunrise/sunset controller I plan to use the power 
outlet signal to keep accurate time. The 32Khz crystal will be my backup
during power outages as this project has a gel-cell battery backup.

BAJ
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