[ntp:questions] Timing issue with Linux and kernel PPS?

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Nov 21 17:37:21 UTC 2012

On 20/11/2012 11:12, Miroslav Lichvar wrote:
> On Mon, Nov 19, 2012 at 06:03:06PM +0000, David Taylor wrote:
>> On both systems, sudo modprobe pps_ldisc produces no output.
> No message is a good message :).
>> I have no idea which device ntpd is using, I simply have the type 22
>> driver installed which, as I understood it, gets the accurate
>> timestamp from the kernel.
> is /dev/pps0, is /dev/pps1, ...
>> How the kernel chooses which device to
>> use I don't know.
> With udev the order might be random. There could be a race between the
> script which loads modules from /etc/modules and udev.
>> In /dev I see pps0 on the system without a PPS signal connected, and
>> pps0 and pps1 on the system /with/ the PPS signal active.  On the
>> system /with/ the signal active, some 25 seconds in the dmes output
>> I see: pps_ldisc registered (so ldisc does matter, I stand
>> corrected), followed by pps1 new source, and source /dev/ttyAMA0
>> added.
> You can see what pps device is actually generating events with:
> grep '' /sys/class/pps/pps*/{assert,clear}
>> So the issue appears to be that /dev/ttyAMA0 is not created until
>> the GPS receiver is sending second pulses, and by that time ntpd is
>> running and can't see the device.  Here are my lines from ntp.conf:
>> # Kernel-mode PPS ref-clock for the precise seconds
>> server minpoll 4 maxpoll 4
>> fudge  flag3 1  refid PPS
>> I wonder whether I should be using rather than .0?
> Perhaps. Do you use in ntpd the serial output from the GPS with some
> driver like NMEA?
> If you don't need the pps from /dev/ttyACM0, my suggestion would be to
> prevent loading of the pps_ldisc module, so there is always only one
> pps device. Any chance you added a udev rule to load pps_ldisc
> automatically when the serial device is created?

Thanks for your reply, Miroslav, but I don't even know what udev is, let 
alone how to edit it!  Sorry, but you are way past my level now.
Web: http://www.satsignal.eu

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