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. _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
