On 27 September 2011 10:35, David Woolley <[email protected]>wrote:
> Applying a static correction to the clock frequency only helps if the > static error is close to or greater than 500ppm. However, if that is the > case, the right thing to do is to replace the motherboard, with a better one > as it is likely that the clock is unstable as well as having a high static > error. One had 180 ppm sand the other 46 ppm without fiddling with machdep.tsc_freq sysctl variable. After the correction I am getting -0.041 ppm for the first and -0.045 ppm for the second. > > performance? The machines I have at the moment are showing worse than 1 us >> performance perhaps because I am not using GPS timing receivers. >> > > Make sure the system is only used as a time server. Keep its temperature > very constant. Disable any form of power management (in particular variable > clock rates). Disable spread spectrum clocks. > Embedded machines running NanoBSD and they don't do anything else besides running NTP. They sit in a temperature controlled room at 20 ºC. Power management has been disabled. I checked the BIOS and did not see any reference to spectrum clocks. > Calibrate the systematic error by outputting a hardware pulse within the > kernel at a precisely known system clock time and using hardware to measure > the offset from the PPS input pulse. You will need to also output the > system time at which you did this, but that needn't be in real time. > > Do not try to do this on the second. Makes sense... I'll investigate a way of doing this. > > I know... :-) My cable connection is really bad for NTP... I can't get >> better than 3 us. That is why I have 2 stratum 1 GPS receivers onsite. >> > > Reading that as 3ms. The time you serve will have that sort of jitter in, > it so there is little point in being accurate to better than about 100 > microseconds of jitter/wander. You read well :-) Cheers, Miguel _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
