[time-nuts] Re: Clock display on Linux systems?

[John Sloan]

I have four home-built clocks, each using a Raspberry Pi, all with slightly different designs, all running gpsd and ntpd (so all are NTP servers on my home network). Three are GPS disciplined; one is WWVB disciplined. Two of the GPS clocks use the modem-control lines on a serial port for the 1PPS signal, one uses the simulated modem-control signals on a USB-connected GPS dongle. One of the GPS clocks has a cesium chip-scale atomic clock - specifically, a Jackson Labs Technologies dev board with a Microsemi CSAC - for holdover (there’s no kill like overkill).

All four clocks use an Adafruit “Pi Plate” LCD board with a two-line display to display the date and time.

https://www.adafruit.com/product/1115

The display format could have been anything I chose, but I implemented one that looks like this (using a 24-hour time so no AM/PM).

Tue 2021-Dec-07
 06:33:49 MST

They all run the same trivial Python script that I wrote to read the system clock (which is disciplined to NTP, and hence to GPS or WWVB time) and drive the display. The script runs five times a second. Efficiency is not really an issue since this is running on a quad-core processor. An entire core could be dedicated to just updating the display. I leave that to the Linux scheduler. This approach combines the accuracy of GPS-disciplined time with support for time zones and Daylight Saving Time adjustments in the system clock.

The time on the LCD is as accurate as I need it to be, since it’s just a human-read display. The NTP time provided by each clock is at least competitive with getting the time from NTP servers across the internet. I have another Raspberry Pi that uses ntpq to query all of my clocks, along with two commercial NTP servers on my network, plus two external servers, and compares them. Based on its own measurements, it typically chooses the atomic clock as the best reference.

There is undoubtedly a lot of room for improvement in all of this, especially in my WWVB clock.

Here is a link of all of my blog articles on clock- and time-related stuff.

https://coverclock.blogspot.com/search/label/Horology

Some of those articles will include links to my GitHub repositories for those projects.

Here is an album of photographs of all of my NTP servers, both home-brew and commercial.

https://flic.kr/s/aHsmgrizkL

:John

--
J. L. Sloan             Digital Aggregates Corporation
+1.303.489.5178         3440 Youngfield Street
mailto:jsloan at diag.com  #209
http://www.diag.com     Wheat Ridge CO 80033 USA