Re: [chrony-users] Multi-PPS+RTC Setup Questions

[Hal Murray <hmurray@xxxxxxxxxxxxxxx>]

> Just a note, at least in my case, the PPS outputs are not variable at 30us,
> they are fixed at a 30us offset from each  other, 

Many GPS systems have an option to correct for the length of the antenna 
cable.  It's worth checking on that.

An oscilliscope is the right tool for that sort of difference.

Another thing to investigate is the other edge of the clock.  Some PPS pulses 
are 100 or even 500 ms long.  Many are short 10-50 usec.  Triggering on the 
wrong edge of a short pulse could get you numbers like like a 30 usec offset.

On Linux, you can see that with command line hacking.   The PPS timing is 
available via
  cat /sys/class/pps/pps0/assert
  cat /sys/class/pps/pps0/clear
You will get something like:
  1601420619.999999124#1112896
The number after the # is a sequence number.  The number before is the time - 
seconds and nanoseconds.

ntpd disables the edge it isn't using.  I don't know what chrony does.  You 
may have to disable chrony and reboot the system.  (or write the code to turn 
it back on)

I thing gpsd sets up the PPS devices.  If nobody does, you need something like:
  ldattach 18 /dev/ttyS0 &
I think gpsd tries to dance around a possible extra/missing inversion by 
assuming the pulse is narrow and using the edge that arrives first.

That's likely a rathole, but it doesn't take a lot of time/effort to explore.


-- 
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