David Woolley wrote: [] > A reasonable expectation of a cheap crystal is 50ppm static plus > <10ppm temperature dependent. When one gets >500ppm it suggests the > problem is rather worse than poor crystal tolerancing.
I am inclined to agree. One reference I found was this: http://www.maxim-ic.com/appnotes.cfm/an_pk/3566 including the statement about 32KHz crystals: "There is nothing inherently wrong with using an RTC for timekeeping. However, the time will only be as accurate as the reference used. Unfortunately, the typical 32.768kHz tuning-fork crystal does not provide much accuracy over a wide temperature range. Due to its parabolic nature over temperature (Figure 1), this accuracy is typically ±20ppm at room temperature (+25°C). This is the equivalent of gaining or losing 1.7 seconds of time each day, or 10.34 minutes per year. As Figure 1 shows, accuracy decreases at more extreme high and low temperatures. The typical accuracy at these temperatures is much worse than 150ppm, which is the equivalent to losing almost 13.0 seconds of time each day, or over 1.3 hours per year." Based on earlier experience, I would expect the higher-frequency crystals used for CPU-clock etc. to be rather better than this. BTW: Windows can keep quite accurate time in some versions, Windows 2000 and XP for example, however I haven't tamed Vista or Windows-7 as yet. It's not as good as FreeBSD, though. Cheers, David _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
