[ntp:questions] u-blox reference clock driver

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Aug 11 15:02:36 UTC 2018

On 11/08/2018 12:29, Terje Mathisen wrote:
> David Taylor wrote:
>> This was posted by lukas at 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.


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