| Re: [chrony-users] Multi-PPS+RTC Setup Questions |
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William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
On Tue, 29 Sep 2020, Hal Murray wrote:
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.
30us is 30000ft=6miles. I sort of doubt that his antenna cable is that long.
Of course you could say that perhaps one could use that correction to get rid
of 30us lag.
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 &
Yes, that is what I use. No gpsd getting in the way. Mind you that does not
handle the nmea sentences.
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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