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