Quite true, and I'm not proposing an RTC-only solution. It's just that if the constraints are loose enough, you don't have to invest much in the RTC design, and you can arrange to set the time "now and then". Like the VLF clocks that set once per day, wait until the best time to do it (2AM), and don't suffer much if they happen to miss a day. A low-ball GPS that only acquires now and then, or one that you switch on and off and it takes 5 minutes to get a lock, even that is sufficient and more than you need. And if I had a system at my home I would not be bothered to just manually correct it once a week or once a month. But then you'd find me still trying to find a clever way to "calibrate against the sun". Lots of solutions look easy but need to not be fooled by shadows, clouds, etc. It worked well for the sailors... Per the comment below, if it's a 1% error, I don't think it qualifies as an "RTC" :) On Thu, Jan 3, 2013 at 10:13 PM, William "Chops" Westfield wrote: > >> [gbarry42] > >> 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 right, a 1% inaccuracy in the timer makes your time be off by over 7 > hours after a month of operation, which is quite bad for solar tracking > purposes=85 > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .