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.
127.127.22.0 is /dev/pps0, 127.127.22.1 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 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 flag3 1 refid PPS
I wonder whether I should be using 127.127.22.1 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.
--
Cheers,
David
Web: http://www.satsignal.eu
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