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 _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
