[time-nuts] TruePosition on the Arduino

[Graham / KE9H]

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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