Re: gEDA-user: Using 60Hz mains frequency for timing?

[Dan McMahill <dan@xxxxxxxxxxxx>]

Randall Nortman wrote:
> Just a quick non-gEDA design question -- I have the choice between
> using the zero crossings of the 60Hz mains voltage or my MCU clock
> (generated from an 18.432MHz quartz crystal producing a 48MHz CPU
> clock via PLL built into the MCU) for low-resolution timing.  The
> crystal is not designed as a watch crystal, so its tolerance is
> probably pretty poor, and furthermore this board will see wide
> temperature swings, which I think has an affect on the crystal
> frequency as well.  I have no idea how precise the 60Hz line frequency
> from the power utility is, but it at least is probably not
> temperature-dependant.  Either one is easy to use -- I just want to be
> as accurate as possible.
> 
> Anybody have suggestions?  TIA,
> 

Without even trying, you are probably better than +/- 200 ppm over 
temperature with your crystal.  Without knowing what you're trying to 
do, I'd guess that you'll have a harder time getting the 60 Hz signal 
into a usable form than the crystal.

For some info on 60 Hz mains stability, see:
http://www.leapsecond.com/pages/mains/


You mentioned "low resolution" timing so I'll bet the crystal is just 
fine.  That also would make your circuit work in cases where you may not 
have a 60 Hz mains.  For example some other countries use 50 Hz and if 
you run off of an inverter in this country I have no idea how accurate 
the frequency is coming out.

-Dan


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