This Nixie tube clock has been designed to precisely document a particular time and place. It usually displays the number of seconds since the Common Era began (plus one year as a nod to the Doc.), but every thirty seconds it shows (with supreme accuracy) the date and time of the current location.
A stand out feature is atomic clocking of the Nixie tubes, this is achieved by flashing a custom Arduino boot loader to the clock control boards which disables the crystal oscillator in favor of an external 16 MHz clock.
The new clock source (a Stratum 0 reference clock locked to GPS with better than 50ns RMS accuracy) is directly derived from the internal Rubidium atomic frequency standard of the Symmetricom NTP server with the help of a Silicon Labs network synchronizer evaluation board.
A second Rubidium standard is configured to automatically maintain holdover timing accuracy in the event of a failure of the primary.
This approach eliminates the frequency drift associated with crystal oscillators and ensures the Nixie tubes update in lock-step with Coordinated Universal Time.
In order to guarantee the two clock control boards operate in perfect synchrony, the TinkerBoard SBC (Synced to the NTP server with kernel level PPS) runs a shell script that provides a future timestamp to the clock boards over USB, then the control boards use an interrupt to lock the timestamp and start counting on the next Pulse Per Second rising edge arriving from the NTP clock.
One difficulty that I faced was that GRA & AFCH shipped me two different revisions of their Nixie control board. They turned out to run fairly dramatically different firmware and it took some doing to strip down and standardize the code into something more manageable.
In time, I may incorporate a few upgrades that I have in mind. Although I think this is a good start.
Lastly, this will likely result in some discussion regarding NTP Stratum levels... technically Stratum 0 devices are the "reference clocks" to which Stratum 1 servers are connected. Since the Nixie tube clocks are directly derived from a GPS synchronized atomic frequency standard, and not over a network, the display itself should be able to be considered Stratum 0.