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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