[time-nuts] TruePosition on the Arduino

Bob kb8tq kb8tq at n1k.org
Tue Jun 6 20:49:45 EDT 2017


Calibrating your GPS pulse ambiguity is one of the all time great reasons to get a
WWVB based wall clock !!!


> On Jun 6, 2017, at 8:38 PM, Graham / KE9H <ke9h.graham at gmail.com> wrote:
> Ben:
> Be careful.
> Most GPS receivers send out the serial message after the tick, that tells
> you what the time of the tick was.
> Read the manual.
> If you want to drive a clock display with a GPS, you pretty much have to
> have an independent time system that advances on the tick, then validate it
> when the serial message shows up.
> --- Graham / KE9H
> ==
> On Tue, Jun 6, 2017 at 6:46 PM, Ben Hall <kd5byb at gmail.com> wrote:
>> Good evening all,
>> There is a saying: "a man with one watch knows the time, a man with two is
>> never sure."  Clearly, this man wasn't a timenut and didn't have GPS.  ;)
>> I've been working on the Arduino code for the TruePosition boards that
>> quite a few of us have bought from the e-place.
>> It's my first real foray into both Arduino and the C language.  (About a
>> million years ago I was reasonably competent with FORTRAN...the 1977
>> version...)  It's mostly working - I can receive and display pretty much
>> everything that comes out of the unit minus a few parameters.  I can
>> display it all on three pages on a 4 line by 20 character I2C display.
>> Currently, the pages are selected by grounding out one of two pins, or
>> having nothing grounded.  Eventually, I'm going to change this so that it
>> changes display pages when a button is pressed.  I don't have lat/long
>> display yet, nor can I handle doing a survey, but those are coming.
>> My code probably would make a real programmer vomit, but hey, it works.  :)
>> Back to the man with multiple watches.  I was having a very frustrating
>> issue with my TruePosition and Arduino code being one second behind my
>> other sources of time.  I went round and round, trying to figure out why
>> the TruePosition thru the Arduino was a second slow.  In the end, it turns
>> out that it wasn't slow...it was correct...but that my other sources of
>> time have errors.
>> I finally proved this to myself by firing up an old Trimble Lassen LP GPS
>> board unit equipped with a 1PPS tick light and serial output...and it was
>> clear that it matched the TruePosition after correcting for the fact that
>> my TruePosition / Arduino code only updates the display when 1PPS is
>> asserted high...but that the Lassen LP displays the serial message before
>> it becomes valid at the next 1PPS tick.
>> I was slightly embarrassed...I should have known that the other sources of
>> time all had sources of error beyond my control.  I should have trusted the
>> TruePosition as being the purest, least complicated, and the path I knew
>> the most about between GPS and my eyeballs.
>> So for a while...the statement was true.  With my multiple sources of
>> time...I really didn't know the time.  But it was also untrue, as when I
>> got agreement between two very "pure" sources of time, I knew everything
>> else was wrong.  ;)
>> I'm getting to the point that once I've got the button logic working, I'll
>> send out my source to anyone who wants to take a look at it or use it.  I
>> will stipulate one condition - you can't make too much fun of how poorly
>> programmed it is.  ;)
>> thanks much and 73,
>> ben, kd5byb
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