In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] wrote: > as I understand for the gps18lvc to work effectively, one needs > a fairly accurate clock. One thing I've found is with hz=100, I'm getting
Which Linux a achieves by interpolating the clock interrupts by reading the CPU TSC register. You do not need to increase the tick frequency to improve accuracy, unless your PPS interface operates on a timer tick poll, in which case you are never going to get the best accuracy. > an ntp.drift of around 40 but with a fairly good rootdispersion at stratum 2 > of approx 15. With hz=250, the frequency drift drops to 15 but dispersion Let me guess. The clock gains when uncorrected. The change in frequency is due to the increased number of lost clock interrupts. You do not want any lost interrupts at all. > So my question is should the drift be varying so much based on kernel hz and It should be independent of frequency, except in that there might be a very small increase in case temperature. > why would there be increased jitter with a reduced frequency error? Has Firstly don't think of it as frequency error; it is a frequency correction that is nulling out the original error in the motherboard crystal frequency. As long as this value is stable and less than about +/- 450 ppm (to allow some head room for short term adjustments), it doesn't matter what it is. In fact, if the sign of the correction was the opposite, and the lost interrupt hypothesis is valid, the magnitude would have increased. The reason the jitter increases is that every time you lose a clock interrupt, you get a 4ms (at 250Hz) step in the apparent time. > anyone else tried the hrtimers patch and had good results? I don't know anything except what you have written, but from that, I would think they would thoroughly confuse ntpd. The near zero correction looks very suspicious. Getting a motherboard with a clock that is accurate to 40 ppb would be very remarkable (probably less than 1% probability). _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
