[LinuxPPS] PC Clock Offset

[Andrew Hills]

Hal V. Engel wrote:
> I am not sure why you are seeing this.  A well setup Oncore on a LinuxPPS 
> machine should see offsets that approach the microsecond range.  You didn't 
> include your /etc/ntp.conf or your /etc/ntp.oncore* files so it is difficult 
> for us to tell what you may have done.  Are you using the correct edge of the 
> pulse?  If the PPS line is a direct ttl connection from the GPS to the DCD pin 
> then you should be using the ASSERT edge but if this is inverted you should be 
> using CLEAR.  If you are using the wrong edge then it should be off by about 
> 200ms.

I've attached my ntp.conf and ntp.oncore.0 files (please ignore the 
strange naming scheme). I believe I should be using ASSERT, but I'm 
really only basing that off of the knowledge that the person who built 
the system had ASSERT rather than CLEAR in his ntp.oncore.0 file. I will 
experiment with CLEAR later today.

> Also when the serial "time" information arrives from the GPS is not 
> deterministic.  What is deterministic is when the PPS pulse arrives.  In other 
> words the time data transmitted by the GPS is to number the seconds and is 
> always after the top of the second being numbered (actually I think it is 
> after the PPS pulse has completed but I may be wrong).  The PPS is to signal 
> the exact top of the second (+- sawtooth).

That makes much more sense. I suppose I was confused about the PPS 
method of timekeeping. Thanks.

> In 
> any case you can NOT use the serial time data for anything other than for 
> numbering the seconds since it is not transmitted at the exact top of the 
> second and the Motorola specs say that the timing of this data is not 
> guarantied and it should not be used for anything other than numbering the 
> seconds.

Well, I'm pretty sure that's my error, then. Is there another way to 
determine the offset between my PC clock and the GPS clock?

--Andrew Hills