On 11/08/2018 12:29, Terje Mathisen wrote:
David Taylor wrote:

This was posted by lu...@fridolin.com on the NTP hackers list, just
in case you missed it.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Hi,

I've been writing a reference driver for u-blox GPS receivers as
part of my master's thesis and thought ntp hackers might be
interested in it. Additionally to normal PPS operation this driver
can make use of the u-blox's "Timemark" functionality.

It does this by enabling the PPSAPI echo, which generates an echo
pulse right after receiving a PPS pulse. This echo is connected to
EXTINT0 on the u-blox, where it is timestamped. So now I have the PPS
timestamp as a local time and the timemark as the receiver time to
calculate an offset. The cool thing about this is, that the offset
does not include the local interrupt latency anymore, which leads to
less jitter.

This is indeed cool, do you have any graphs/stats for the actual/remaining jitter?

Terje

There are some histograms in his thesis for various conditions of the Raspberry Pi which I saw on a quick glance. Take a look at Chapter 4, page 35. Graphs on p.40 onwards.

  https://wwwcip.informatik.uni-erlangen.de/~in18otin/thesis.pdf

--
Cheers,
David
Web: http://www.satsignal.eu

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