On 2013-12-16, Adrian P <[email protected]> wrote: > Hi, > > In an old IBM thinkpad T22 laptop that runs FreeBSD 9.2, I have > configured a NTP server that gets the PPS signal from a Garmin GPS 18 > LVC, as described here: > http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm > > The /etc/ntp.conf contains the following generic NMEA GPS receiver > driver configuration: > > server 127.127.20.0 mode 1 minpoll 4 maxpoll 4 prefer > fudge 127.127.20.0 flag1 1 flag2 0 flag3 1 refid PPS > > I let it run for a few days, however I still have every now and then > huge peaks (between -50 and +30 us) of the clock offset as seen in the > following graph: http://goo.gl/JpSyeO > > So I am wondering: why those repeating huge clock offsets? They > allays starts with a negative peak, followed immediately by a positive > one. Is this because of the faulty laptop internal clock, that drifts > away with 30-40 us every now and then? Any thoughts?
It is hard to say, but it could be that the interrupt for the PPS is delayed because of, say, disk activity which ties up the interrupts for 50us. The next one will of course occur on time ( ie 50us early with respect to the previous interrupt). That may be what you are seeing. Are you looking at the raw graphs of offset every second? The median filter should remove those spikes. (it basically throws away about 40% of the measurements that are furthest from the median in order to eliminate "popcorn" events) so they should not affect your timing. Looking at your graph, the events are extremely regular. Unfortunately I have no idea what the lower axis is. You claim seconds, but there are no tick marks. Does that span represent a day, a year, a microsecond? If it is say minutes or hours, then the decay suggests that the computer say a huge spike for a relatively long time ( many seconds) responded by changing the clock rate, and then that anomaly disappeared and it relaxed back to its proper rate. > > Many thanks, > Adrian _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
