On 2/4/06, Tim Shoppa <[EMAIL PROTECTED]> wrote: > Tony Hoyle <[EMAIL PROTECTED]> wrote: > > That's transmitted in plaintext over a relatively slow RS232 link, > > interpreted by the operating system, then the NTP driver.. by the time > > you get it it's way off. > > > > PPS helps (enormously.. without it it's hard to get within 200ms) > > 200ms happens with some serial clocks, but not all. Many are good > to +/-1 character (which is 1ms at 9600 baud) and a few to +/- one bit > (better than 1ms), although all of these have to be tuned via the > fudges. > > > - but > > even with that it's damned hard to get the GPS sync within 1.5 > > milliseconds of 'known' time, and the jitter is consistently 5ms or > > more... that's just inevitable in a multi-tasking OS, even a relatively > > idle one. > > Not at all inevitable. PPS support in the kernel gives me jitters of > +/- a few microseconds. (Z3801A, Linux with Ulrich's PPS patches). > > The difference between PPS out on a GPS receiver and "true" time at the > receiver is usually guaranteed to be less than a microsecond. RS-232 > smears the pulse but you can always use differential signaling instead. > Interrupt latency is the remaining uncertainty but even then there are tricks > around this microsecond-level uncertainty. > > Tim. >
I think most GPS that does 1PPS out gives few microseconds jitter, at least, my non-timing Jupiter gives me few microseconds jitter. Sometimes(2~3 per day) I see 20us spike, but I think its cause is my environment.. my antenna is glued to outside frame of window where it has limited sky view(half of frontal view is blocked by building.). _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
