[ntp:questions] ntpd connect gpsd shared memory driver

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Jun 18 18:56:52 UTC 2013

On 18/06/2013 19:48, David Taylor wrote:
> Progress!
> Now you can take the average of the offset values, say -171
> (milliseconds) in round numbers, and change the ntp.conf to read:
>    server minpoll 4 maxpoll 4
>    fudge time1 0.171 refid GPSD
> This will make the serial data be approximately in sync with the
> external servers.  Ideally, average over a few minutes, but it doesn't
> need to be that exact unless you will have no PPS signal.
> I just did the same on my Raspberry Pi test server:
>    http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#user-mode
> How are you feeding the PPS data into the system?

The above assumes that time1 was initially set to zero.

If you have PPS support in the kernel you need a type 22 driver as well 
as the type 28.0 driver.  Type 28.1 assumes you have some software 
poking timestamps into the second shared memory segment provided by 
gpsd, as my user-mode Raspberry Pi system has.

Something like:

# Kernel-mode PPS ref-clock for the precise seconds
server minpoll 4 maxpoll 4
fudge  flag3 1  refid PPS

Web: http://www.satsignal.eu

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