[ntp:questions] Stratum 0 refclocks and PPS

Mike S mikes at flatsurface.com
Tue May 8 22:57:40 UTC 2012

I've got a couple of Soekris NET4501s, with Moto M12+ GPS timing 
receivers. They've been running pretty well with nanoBSD, using the 
special high resolution elan-mmcr PPS driver.

But, there doesn't seem to be any optimal refclock driver combination. 
(ntpd 4.2.7p236)

If I use the Atom refclock driven by elan-mmcr for PPS, and the Oncore 
driver for TOD and PPS, the PPS sources seem to conflict. NTP appears to 
alternate which PPS is used, causing quite a bit of jitter (Atom is very 
stable, Oncore is subject to interrupt latencies). Not linking 
/dev/oncore.pps.0 to /dev/cuau1 causes the Oncore driver to not work at 
all. I've played with using "prefer" (prefer Oncore, prefer Atom, prefer 
both), but that doesn't seem to make things any better.

Using the Oncore driver alone would make sense, but to take advantage of 
the accuracy provided by the elan-mmcr driver, I'd have to point 
/dev/oncore.pps.0 to /dev/elan-mmcr. When I do that, the Oncore driver 
doesn't work. I'm guessing that's because elan-mmcr only signals 
asserts, and never clears, but maybe it's something else.

Things get better with just the Atom driver and an external prefer peer, 
but that means it can't be a stand-alone NTP server.


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