On 6/28/11 10:03 AM, neutron spin wrote:
32.768KHz has a nice property that it oscillates 2^15 times per second which makes the timer math easy...I made a calc using the average tolerance of these crystals and came out to an accuracy of around 0.002 % accuracy or better - about 2 seconds a day. This of course does not account for temperature drift....wow...are we splitting hairs or seconds?...lol...regards...
The standard 32768 Hz watch crystal has a parabolic frequency/temperature curve, which tops out in frequency at 25C (77F) and gets slower in either hotter or colder weather.
Once adjusted, it's good to a couple PPM at ~20C to 30C (68F to 86F). One PPM is 30 seconds a year, half a second per week.
I highly recommend adjusting a crystal if you're going to use it for timekeeping. The hairs you can split with it are much finer.
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