[LinuxPPS] Offset problem with NTP and linuxpps

[Jon Schewe]

Found it.  Turns out that ntp is looking for timepps.h instead of
linux/timepps.h, so it's not found and PPS support was turned off.  Now
I've just got a 3ms offset between the system clock and the PPS signal.
It'd be nice if I could get it better, but this will work.

On Wed, 2007-01-24 at 13:30 -0600, Jon Schewe wrote:
> I've got NTP patched for NMEA/PPS and the kernel(2.6.18) patched for
> PPS.  I've got /proc/pps/sources:
> driver:~ # cat /proc/pps/sources
> id	mode	echo	name			path
> ----	------	----	----------------	----------------
> 00	1133	no	pps_8250_0		/dev/ttyS0
> 01	1133	no	pps_8250_1		/dev/ttyS1
> 02	1133	no	pps_8250_2		/dev/ttyS2
> 03	1133	no	pps_8250_3		/dev/ttyS3
> 
> I've got a good sync according to NTP:
> driver:~ # /usr/local/ntp/bin/ntpq -p
>      remote           refid      st t when poll reach   delay   offset
> jitter
> ==============================================================================
> *GPS_NMEA(0)     .GPS.            0 l    8   16  377    0.000    3.905
> 2.678
> 
> I've got a serial board that brings the PPS signal in on the same serial
> port as the NMEA messages.
> 
> Now I've noticed some issues with time on this machine compared with an
> embedded device syncing off the same GPS, there's a 350-400ms difference
> in time.  I wrote an application that toggles a ping on my parallel port
> once a second at realtime priority to avoid scheduling issues.  I then
> hooked a scope up to that pin and the PPS signal from the GPS and notice
> that the period of the two signals is right at 1 second (the standard
> deviation is in the nanosecond range).  However the leading edge of the
> signals is off by 361ms with a standard deviation of 1ms.  Does anyone
> have any idea where this delay is coming from and why NTP isn't syncing
> the machine to the PPS signal exactly?
> 
> Thanks.
> 
> 
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*My views may not represent those of my employers