On 2014-07-31, Rob <nom...@example.com> wrote: > William Unruh <un...@invalid.ca> wrote: >> On 2014-07-31, Martin Burnicki <martin.burni...@meinberg.de> wrote: >>> >>> Unlike otherwise stated in this thread I don't think it's a good idea to >>> leave the 1 PPS signal alone disciplining the time of the NTP server. >>> This can easily yield unforeseen problems, similarly as if you use an >>> IRIG time reference which only provides day-of-year and time-of-day, but >>> no year number. If you don't take care then such signal can be accepted >>> and provide a "valid" time which is off by an integral number of years. >> >> My point is that most of the internal clocks on computers are well able >> to maintain the time to better than a second for a long time, even if >> they were freewheeling, and if disciplined by a PPS, they are able to >> maintain the time forever (well, until the next power failure anyway). > > There are complications. > While the clock would probably be capable of maintaining the time > within a second, it cannot be set to a reasonable accuracy. > > On the system-under-test, i.e. with the locked PPS source, the LOCL > clock, and the unreachable external references, I did a reboot.
A reboot is a restart and on a restart you need an external source for the seconds. > > Result: after the reboot, the system time was off by 270ms and the > PPS was not trusted. The time remained off by 270ms as indicated by > a 270ms offset on PPS (marked with an x), and remained freerunning. Yes, not entirely surprizing, especially considering the way ntp is designed right now. This is a combination of bad ntpd design, and restart when an external source is mandatory. > > Only after I managed to get a reachable external reference again, it > stepped 270ms and then locked to the PPS again. Not surprizing. A restart is NOT a situation under which I would expect the local clock to keep resonable time, not least because when the computer is off, it does not keep time at all. And the RTC is in general a lousy source of time. > > Of course this would not have happened when the system had remained > running, as it normally would. I rebooted it for major network reconfig > and after that I was able to find NTP servers on another internet > connection so it now has 3 external references again. > > I think you should be prepared for an offset up to half a second when > saving the time to the CMOS clock, rebooting, and reading it back. > Not good. The problem is usually drift. The drift in a cmos clock is of order of 10s to hundreds of PPM. At 100PPM, the clock will be out by a second after only 3 hrs. I never claimed that relying on the RTC was a good idea. I claim that relying on the local system clock to keep time to within a second after having been disciplined by ntpd and between ntpd PPS queries is a good assumption. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions