Jeff Epler's blog

6 June 2019, 12:19 UTC

Yet another Raspberry Pi Stratum 1 NTP server


There are lots of instructions for setting up a Stratum 1 NTP server on your Raspberry Pi. A lot. After much research, I found a simple configuration that uses a single ntp reference clock and does not involve gpsd, but uses both NMEA and PPS for the most accurate timekeeping possible. NMEA+PPS is a mode of NTP's "Generic NMEA GPS Receiver".

Here's what worked for me:

Start with a pi3 and raspbian stretch. Add a GPS with TTL-level PPS and 9600 baud NMEA outputs.

Hook up you GPS: GND and +3.3V or +5V (according to the specifiics of your GPS) to any matching pin, the GPS's TX output to the Pi's RX input on pin 10 of the header, and the GPS's PPS output to the Pi's pin 12. (You can hook up your GPS's RX input to the Pi's TX output on pin 8 if you like, but I don't think this is necessary)

Using raspi-config, disable the serial console and enable the serial port hardware.

Manually edit /boot/config.txt to add: dtoverlay=pps-gpio — If you have to (or prefer to) use a different pin for pps, you can apparently specify it as dtoverlay=pps-gpio,gpiopin=##, where ## is the internal numbering of the GPIO pins, not the number on the 40-pin header.

Manually edit /etc/modules and add a line that reads pps-gpio — According to some sources, this step is not necessary.

Install ntpd with apt-get.

Delete all the content in /etc/dhcp/dhclient-exit-hooks.d/ntp, leaving an empty file.

Remove the file /run/ntp.conf.dhcp if it exists.

Edit /etc/udev/rules.d/99-com.rules. On each of the two lines that ends , SYMLINK+="serial%c", append , SYMLINK+="gps%c"

Create /etc/udev/rules.d/99-ppsd.rules with the content SUBSYSTEM=="pps", GROUP="dialout", MODE="0660", SYMLINK+="gpspps%n"

In /etc/ntp.conf, add a stanza to access the GPS:

# gps clock via serial /dev/gps0 and /dev/gpspps0
server 127.127.20.0 minpoll 3 maxpoll 3 mode 16 burst iburst prefer
fudge 127.127.20.0 refid GPS time2 +.250 flag1 1
(leave the "pool" line(s) or other ntp server lines; if your pi doesn't have a battery-backed RTC, you need a way to get the correct time initially, before a GPS fix may be available!)

"time2" doesn't seem critical with this setup, because the PPS time is preferred over the serial reception time. "minpoll", "maxpoll", "burst" and "iburst" may be superstitious and unnecessary.

Reboot now and have a look at `ntpq -c peers`. You should see something like this:

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .GPS.            0 l    1    8  377    0.000   -0.001   0.003
+ntp.u           .GPS.            1 u    2    8  377    1.104    0.013   0.064

"GPS_NMEA" is selected as peer and is using PPS (this is what "o" in the first column means). delay, offset, and jitter should all be extremely small (Here, .001 is 1 microsecond). "ntp.u" is my other local stratum-1 NTP server. The "+" indicates it's in good agreement with the local GPS. If you use pool, you will see multiple lines here; some may have "+" and some may have "-".

If something's not working, you will get, " " (blank), "-" or "x" next to GPS_NMEA. If it's got "*" then NMEA is working but PPS isn't. Now you get to do things like debug whether the PPS signal is working properly according to ppstest, whether NMEA messages are actually coming in at 9600 baud, etc. Or you can follow one of those other guides. :wink:

Here's how the local time to GPS offset has looked over the last 10 hours or so — I find it awesome that my computer appears to be synchronized to GPS to within ±5 microseconds almost all of the time:

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3 June 2019, 1:17 UTC

Precision vs Accuracy: A Clock


I was inspired by this watch face design (I think that's the original version) and by the arrival of a "OCXO", a very reliable time keeping circuit, to finally make an electronic clock.

Accuracy: Between hardware and software tuning, the OCXO keeps time with an accuracy of possibly better than 100 microseconds per day (loses or gains well less than a half second per year) (Yes, I'm deliberately ignoring a lot about crystal aging here!)

Precision: The time displayed is to the nearest minute, and the touchscreen setting mechanism is (deliberately?) poor, making it hard to set the time closer than +- 2 minutes or so. Oh, and it takes a good fraction of a second to update the screen anytime it changes. (The best way to set it seems to be to wait until a few seconds before 6AM/6PM and plug it in, since it boots with that time showing)

The clock consists of: .. all in a 3d printed enclosure that's not quite the right size.


The dial as an animation (1 revolution = 12 hours)
Along the way, I added an even more accurate time source to a Raspberry PI (GPS with PPS) so that I could even measure the accuracy of the OCXO, and discovered I even have a GPS module which was negatively affected by the GPS rollover that occurred in April of this year (the second 1024-week rollover). This leads to a surprising sequence of clock arithmetic, and finally gpsd decides the GPS is returning a date sometime back in 1963.

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23 November 2013, 18:19 UTC

SNTP from Python: getting server's esimate of time quality


Anders Wallin recently discussed generating graphs of server time error using a timestamp from the Date: header of an http request.

NTP servers themselves have estimates of time error, and it's possible to get the local server's idea of the error by sending SNTP packets to the local machine. You can also see the last time this NTP server successfully communicated with a peer or upstream, and see whether the server considers itself desynchronized by checking the stratum and leap second fields.

Based on my tinkering, the root_dispersion field or the derived LAMBDA value is the easiest to look at for estimating how bad the NTP time is. Between communications with a peer, root_dispersion and LAMBA increase at a rate of 15µs/s, ntp's arbitrary estimate of the accuracy of the local undisciplined clock. (YMMV and you can appaerntly tweak CLOCK_PHI in your NTP configuration file)

The attached program is a basic Python SNTP client which by default polls the local server every 5 seconds and prints some information. License: GPLv2+

Typical output:

12:25:58  0.06233  228.95875 ST-SYNCH LI-SYNCH
12:26:03  0.06241  233.96404 ST-SYNCH LI-SYNCH
12:26:08  0.06247  238.96745 ST-SYNCH LI-SYNCH
12:26:13  0.06255  243.97141 ST-SYNCH LI-SYNCH
12:26:18  0.06262  248.97673 ST-SYNCH LI-SYNCH
12:26:23  0.06270  253.98213 ST-SYNCH LI-SYNCH
12:26:28  0.06277  258.98746 ST-SYNCH LI-SYNCH

Files currently attached to this page:

ntpsynch.py4.9kB

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