Um, do you have a definition somewhere (like in comments) of the definition of the accuracy you're using? So multiple people can add consistent values? Is this the standard deviation (67% confidence), 2 standard deviations (95%), 3 (99%), or something else? What averaging time is this computed over? Usually clock stability (which I recognize is not accuracy) is expressed using the Allan deviation, http://tf.nist.gov/general/glossary.htm#allandeviation This is because real clocks have unbounded asymptotic error. Over long time intervals, have a random frequency walk characteristic, where the frequency error after time T is proportional to sqrt(T). So an oscillator which is stable to 1ppb over a T second interval will be stable to 2ppb over a 4T second interval, 3ppb over a 9T second interval, and so on. Since accuracy is limited by stability, and there's no upper bound on instability, there's actually no upper bound on inaccuracy, either. (Admittedly, your typical crystal oscillators have their stability limited by environmental instability, particularly temperature, which dwarfs the frequency flicker noise limit.) On the flip side, some systems are synchronized to UTC by various means. This means (if we either neglect UTC's far sub-ppb instability, or just define it as "perfect") that the inaccuracy over a long enough averaging interval is zero. But if it only synchronizes once a day, there can be significant inaccuracy between synchronization. Should the accuracy specification reflect that shorter-term instability? Finally, you can't specify *too* short an interval, because clocks also have increasing error over small time intervals. Below 10 seconds or so, white noise cycle-to-cycle jitter dominates, and the fewer cycles you average over, the larger it appears to be. A clear definition would help people understand what numbers to put in. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
