[time-nuts] NTP to discipline Raspberry Pi

[mike cook]

Le 25 juil. 2013 à 05:21, James Peroulas a écrit :

> 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 consistenngtly measure a
> required correction of -43 ppm on my frequency counter.

   Could you let us know what crystal you were measuring? From the  design docs there are 2 on the board  , one at 25 MHz and one  at 19,2MHz. The 19.2MHz is the one used to derive the ARM clocks. NTP reports the system clock frequency drift ( which I guess is the pll drift), and not the crystal frequency drift, so that may explain what you are seeing. On your measuring method, how were you preventing stray capacitance from altering the crystal frequency? How about protection ? I have a PI that I would like to do the same test on but do not want to destroy.

> 
> 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
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