[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Chuck Swiger cswiger at mac.com
Wed Mar 14 00:24:43 UTC 2012

On Mar 13, 2012, at 5:05 PM, Ron Frazier (NTP) wrote:
> I don't have PPS on this computer.  It has no serial port.  When I get my Sure board, assuming I can solder it without killing it, I'm going to try bringing the PPS signal through a Prolific chipset based serial - USB converter which is supposed to pass handshaking signals.  I don't know if that will work.

You should be able to get reasonable time via USB, but not of the quality that a serial or parallel connection would provide, especially one where the ISR does timestamping.

> Of course, this thread was started because of a mysterious 50 second jump in my system clock.  That's still a mystery.

Yes.  I've seen that sort of thing with VMs; some older Linux kernels also had some fairly bad timekeeping issues.

> Regarding the NMEA though.  If I can get those data sentences to my PC with a jitter of 3 ms through USB, which I can, I don't see why the start and end time of the sentence should vary by 120 ms over 2-4 days.  I'm trying to figure out where that variance is coming from.  If I can consistently keep time with this device, or some other USB device, to within + / - 6 ms or even 10 ms of "true time", that's plenty good enough for my purposes.  If I can only keep time to within + / - 70 ms, then the device is more or less worthless, no better than the internet servers.  So, I'm trying to figure out whether it's good enough, or worthless.

You shouldn't have too much difficulty getting timekeeping accuracy to within ~6 ms using public internet NTP servers which are nearby, although if you just use the global ntp pool and get some servers which are not very close, that might be on the order of +/- ~20 ms instead.


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