[time-nuts] GPS for Nixie Clock

Scott Stobbe scott.j.stobbe at gmail.com
Fri Jul 15 15:55:49 EDT 2016

It's fairly straight forward to build a 1 Hz NCO with a timer/counter block
using your processor's LO. Threshold - Frequency, count - Phase. You can
then DPLL to either PPS, Serial Data, or both. If your receiver drops its
NAV or timing solution you still have "time".

On Sun, Jul 10, 2016 at 11:55 PM, John Swenson <johnswenson1 at comcast.net>

> 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
> walnut.
> 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 outputs.
> 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 reality?
> 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.
> Thanks,
> John S.
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