[ntp:questions] 500ppm - is it too small?
David J Taylor
david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Fri May 8 06:47:16 UTC 2009
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:
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.
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