[ntp:questions] NTP.ORG MEINBERG KEEP TIME ACCURATE to 10MS

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Nov 15 08:12:33 UTC 2012


On 15/11/2012 07:22, unruh wrote:
[]
> Actually more like 2 microseconds accuracy should be possible, unless
> the Raspberry Pi's servicing of the GPIO interrupt is really very bad.
>
>>
>>     http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
>
> Nice page.

Glad you liked the write-up.  Just some notes for myself, really!

If you search for "latency" here:

   http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=13883

"jbeale" says: "My experiments with NTP-PPS timing in the 3.1.9 kernel 
with a relatively unloaded system indicate that my R-Pi has an interrupt 
latency ranging between 10 and 80 microseconds, and occasionally gets as 
long as 150 microseconds (see below)"

and provides a graph of some values he has seen.  I haven't made any 
direct measurements.  On my system here, the offsets reported by NTP are 
less, but it would not show any fixed interrupt latency.  It's all well 
within the 10 milliseconds requirement, of course.

   http://www.satsignal.eu/ntp/2012-11-01-02-03-raspi1_ntp-day.png
   http://www.satsignal.eu/ntp/2012-11-08-09-10-raspi1_ntp-day.png

with glitches due to restarts.  Zero offset is plotted as 20 microseconds.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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