As a crystal ages and undergoes temperature variation, it will have a component of drift which is continuous in one direction as well as a random walk component which still ends up going in one direction in the long term but on time scales of days or weeks can start heading backwards temporarily. Cheap crystals will have a total error of about 100ppm. Good crystals would be more like 20ppm. If you temperature compensate them and calibrate them to begin with, then can be about 1 ppm. If you ovenize them so that they do not see temperature variations (which cause part of the random walk phenomenon), then you can get to about 200 ppb. Pre-ageing for hundreds of hours can get you even further and finally you can do some modeling of the ageing of your particular crystal and get to something in the 10 ppb ballpark I think. See: http://www.prc68.com/I/Xtal.shtml Even back at the 100ppm figure, that is 0.01%, which is 4 minutes in one month. 10 ppb would take 3 years to accumulate one second of error. Sean On Fri, Jan 4, 2013 at 1:13 AM, William "Chops" Westfield wrote: > >>> I see opinions that solar tracking doesn't need that much accuracy > > yes, but the usual behavior you get with a simple microcontroller crystal= -based timer is a continuous drift in one direction. If I did my math righ= t, a 1% inaccuracy in the timer makes your time be off by over 7 hours afte= r a month of operation, which is quite bad for solar tracking purposes=85 > > BillW > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .