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Re[2]: NTP with only GPS/PPS flags PPS as falseticker


From: Nick Burkitt
Subject: Re[2]: NTP with only GPS/PPS flags PPS as falseticker
Date: Thu, 19 Mar 2020 00:56:42 +0000
User-agent: eM_Client/7.2.37929.0

If I disconnect the Ethernet cable off my Pi 3B+, NTPsec 1.1.8 continues to show the GPSd PPS shm as (+) and the PPS refclock shows (o). This is a significant difference from you, I use the Linux kernel PPS refclock as a "prefer". It's activated by a 

dtoverlay=pps-gpio,gpiopin=18,assert_falling_edge=off,capture_clear=off
in the Pi's config.txt file, and takes the MAX-M8Q's PPS pulse and sends it to the kernel. So, my configuration may be different enough not to help much.

Still, here's most of my ntp.conf (whole file attached):

# flag2 0 | 1    - Specifies PPS capture on the rising (assert) pulse edge if 0 (default) or falling (clear) pulse edge if 1.
# flag3 0 | 1    - Controls the kernel PPS discipline: 0 for disable (default), 1 for enable.
refclock pps unit 0 	refid kPPS flag2 0 flag3 0 minpoll 4 prefer

# time1 time     - Specifies the time offset calibration factor, in seconds and fraction, with default 0.0. 
# flag1 0 | 1    - Skip the difference limit check if set. Useful for systems where the RTC backup cannot keep the time 
# 		over long periods without power and the SHM clock must be able to force long-distance initial jumps. Check the difference limit if cleared (default). 
refclock shm unit 1 	refid gPPS minpoll 4
refclock shm unit 0 	refid GNSS stratum 11 flag1 0 time1 0.060 	# fudged so it stays positive and minimum +10ms offset

Hi Martin.

I'm using NTPsec 1.1.8+, but the pps driver doesn't seem to be available:
2020-03-18T23:40:49 ntpd[1468]: CONFIG: Unknown driver name pps

I'm using my own kernel driver instead of the pps-gpio driver, but it does the same thing, only with a hardware interrupt line instead of a GPIO pin.
But I don't think the problem lies with PPS. It's the fact that the GPS clock is marked as a falseticker, nearly always. I've tweaked the time1 value to get the offset (as shown by ntpmon) to around 10-15 ms, with a jitter of around 5 ms, but still ntpd doesn't like my GPS

-Nick

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