[time-nuts] TruePosition on the Arduino

Graham / KE9H ke9h.graham at gmail.com
Tue Jun 6 20:38:49 EDT 2017


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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