Le 26 juil. 2013 à 01:54, Julien Ridoux a écrit : > Hi James, > > We have done some measurements of the stability of the STC clocksource that > the kernel relies on to build its system clock. I believe this link could be > the answer to your question: > http://www.synclab.org/?post=blog/2012/11/radclock-raspberry-stability-nic-noise.html > > Please note that these measurements are made with our custom kernel patches > and bypass any kernel system clock PLL driven by ntpd. So the results have to > be interpreted in this context -- especially, they do not rely on the nominal > frequency reported by the clocksource. > > Cheers, > Julien >
Hi Julien, Most interesting. I do however have an issue with your wording. "Already, this tells us that the smallest meaningful timestamp resolution on the Pi is 1 microsecond." Timer resolution may be limited ( I haven't trawled the code), but timestamps are supported to nanosecond resolution as timespec{} is 64 bits. and clock_gettime() returns that. That said NTP limits itself to timeval{} stamps, ie usecs. from Markus Kuhn's little prog on my PI. mike@raspberrypi ~/src $ ./timelog # gettimeofday gettimeofday REALTIME MONOTONIC PROCESS THREAD 0 2013-07-26T11:50:40Z 1374839440.667447 1374839440.667508382 696669.170759074 0.008485000 0.008490000 1 2013-07-26T11:50:40Z 1374839440.916650 1374839440.916656359 696669.419906051 0.136284000 0.136289000 2 2013-07-26T11:50:41Z 1374839441.182422 1374839441.182428671 696669.685678363 0.264474000 0.264479000 3 2013-07-26T11:50:41Z 1374839441.434640 1374839441.434646527 696669.937897219 0.394819000 0.394824000 Regards > > On 25/07/2013, at 1:21 PM, James Peroulas <ja...@peroulas.com> wrote: > >> I was hoping to measure the ppm error of a Raspberry Pi's crystal using an >> NTP client running on the Pi itself. The NTP client reports a ppm >> correction that I find to be consistently (measurements performed over >> several days) off by about 10 ppm compared to what I measure using my GPS >> calibrated frequency counter (HP5328). Specifically, the Pi reports a >> required ppm correction of -33 ppm whereas I consistently measure a >> required correction of -43 ppm on my frequency counter. >> >> Any ideas on where I can look to track down the discrepancy? Perhaps the >> timers on the RPi are configured incorrectly in the kernel? Or is this the >> best I can expect from NTP? I would understand the situation if the NTP >> reported correction drifted above and below -43ppm, but it seldom departs >> from -33ppm by more than 1 or 2 ppm... >> >> Thanks, >> James >> >> P.S. I apologize if this isn't time-nutty enough :) I only need about 1ppm >> accuracy in my corrections :) >> >> -- >> *Integrity is a binary state - either you have it or you don’t.* - John >> Doerr >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.