[time-nuts] GPS for Nixie Clock

Bob Camp kb8tq at n1k.org
Mon Jul 11 07:44:47 EDT 2016


Hi

For $15 to $45 a number of places will sell you a uBlox receiver card that runs off of 5V and has an internal antenna. They are quite sensitive and have a timing output. They are plenty good enough for what you are trying to do.  The practical issue is getting good enough GPS signals at an arbitrary location inside a house. To do that, you want a sensitive receiver. 

No matter which receiver you get, they all put out fairly complex serial strings that need to be parsed to get the time information. For a wall clock the serial data “time of arrival” will be better than your eye can see. The PPS output is overkill, but this is Time Nuts. I would do the parsing with a small MCU. 

I doubt that an older FPGA will have enough “stuff” in it to do the job. You will need a few hundred bytes of RAM for buffering and not a whole lot else. There are thousands of different MCU’s that can / will / could / might do the job. Getting one already built up on a board for < $10 is pretty easy. Getting one on a board for < $3 is possible. 

Lots of choices.

Bob

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