[time-nuts] GPS for Nixie Clock

Bill Hawkins bill.iaxs at pobox.com
Mon Jul 11 01:15:53 EDT 2016

Well, it's late and I've nothing else to do.

I sympathize with you, but my Nixie clock is a computer display made
years ago by Jag Air Clockvault. One of these Windows OS upgrades is
going to refuse to run it.

You say it has an FPGA for the display. What provides the numbers to be

A line frequency clock has no time info - it has to be set. Do you have
raise/lower buttons or just raise?

Good luck doing a simple conversion from GPS receiver time to numbers
the FPGA can convert. Much simpler to count 1 PPS pulses, but like line
frequency, there's no adjustment for leap seconds.

Reclocking the 1 PPS with 10 KHz seems like overkill for a clock to be
read by humans.

GPS reception and accuracy falls off quickly if the antenna can't see a
large part of the sky. It won't function if it can't see several
satellites. I know this empirically.

WWVB has no such requirement, and it's accuracy is a better match for
human precision.

Could you give us a link to a picture of your solid walnut Nixie clock?

Got any more walnut?

Bill Hawkins


-----Original Message-----
From: John Swenson
Sent: Sunday, July 10, 2016 10:55 PM

I'm thinking about converting a Nixie clock I built years ago into using
GPS for the time base. No real NEED, just for fun.

The clock uses an FPGA for formatting and display, using the 60Hz line
frequency as the time base. The case is a single hollowed out block of

I'm looking into a TU36-D400-020 receiver. This seems to be optimized
for timing purposes rather than navigation, it has 1PPS and 10KHz

I'd be getting it from RDR Electronics, which says it it uses the
Motorola command set. This seems fine for me, it has the information I
need, specifically UTC time so I don't have to worry about leap seconds.

I have a few questions about this receiver:
The data sheet lists two serial ports, but I don't see any information
about which to use. Are they identical, do I have to use one for some
functions and the other for other purposes?

What are the serial port parameters? 9600-8-N-1? Or something else?

Which is better to use, the 1PPS or the 10KHz? I can easily go either
way. The clock display just goes down to seconds so 1PPS would work. I
could also re-clock the 1PPS with the 10KHz.

What antenna to use? I would prefer something mounted inside the case. 
It is wood so an internal antenna will hopefully work. The board comes
with a pigtail but it is not SMA.

Any other hints for using this?

I've never done a GPS interface before so I'm not sure about how I
calibrate the time coming from the message over the serial port. Is it
something like "the time is such and such at the rising edge of the next
PPS, or the previous one? Or is there some other mechanism for
calibrating when the second changes on the display to something close to

I previously toyed with the idea of using an X72 rubidium oscillator
just for the bragging rights, but I would still need the GPS to get the
time, I decided the TU36 on its own is probably just fine.


John S.

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