[time-nuts] GPS for Nixie Clock

Vlad time at patoka.org
Mon Jul 11 17:28:28 EDT 2016

Last year I created GPS-based Nixie clock for myself. After all the 
considerations I decide to use GPS receiver with external antenna. Not 
so aesthetic, but more reliable for me. I was using DS32xx chip as a 
clock. And GPS plus 1PPS output from GPS to set/sync that DS32xx. So, 
GPS module is not critical there, as clock will do its work without GPS. 
Also I implemented "OR" gate to be able to do 1PPS sync with external 
1PPS source. Just in case.
The DS32xx is OK for this project. Since its very unlikely it will drift 
so much.

Late, I found myself thinking that NTP protocol instead of GPS will be 
better solution for that project. For example, I found that sometimes 
the GPS clock is set the time by 16 second off the UTC. Likely its 
because GPS getting the report without of "leap seconds". Its still 
puzzle for me why its happens. I saw it couple of times for the past 
year. So, its rare, but it happens.

Here is my clock "during" and "after":

On 2016-07-10 23:55, John Swenson wrote:
> 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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