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

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Nov 19 18:03:06 UTC 2012

On 19/11/2012 09:44, Miroslav Lichvar wrote:
> On Mon, Nov 19, 2012 at 09:02:12AM +0000, David Taylor wrote:
>> On 18/11/2012 15:20, Uwe Klein wrote:
>>> what happens if you "insmod pps_ldisc" into the "not ready" system?
>> (1) I get Error: could not load pps_ldisc module: No such file or directory
> insmod needs full path to the module, it's better to call "modprobe pps_ldisc".
>> I looked at the article you referenced, but in this case the
>> Raspberry Pi is not using the DCD line, but a separate GPIO pin.
>> lsmod shows pps_gpio as present.
>  From the original post it seems you have two pps devices, one for gpio
> and the other for ldisc which is created two minutes later (some USB
> device?).
> Do you see two /dev/pps* devices and are you sure ntpd is using the
> gpio one? Perhaps there is an ordering problem?

Your help is much appreciated, folks!

On both systems, sudo modprobe pps_ldisc produces no output.

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.  How the kernel chooses which device to use I don't know.

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.

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?
Web: http://www.satsignal.eu

More information about the questions mailing list