[time-nuts] TruePosition on the Arduino

Tom Miller tmiller11147 at verizon.net
Sun Jun 25 20:29:21 EDT 2017

This is an update to discussions off list with Ben and Chris regarding using 
the two line, 16 character display with the Packrats software on the Arduino 
to control the TruePosition GPS board.

First, the ebay seller still has a few boards left and seems to take offers 
of $40 for them.

Second, the display used with the Packrats software must use the I2C address 
of 0x3F to work. The display needs to use the PCF8574AT I2C I/O expander 
chip and not the PCF8574T. The later chip addresses between 0x20 to 0x27 and 
will not work with this software.

Third, I picked up some of the "A" chips on ebay and they were marked as A 
chips but addressed wrong. If you are going to change out this expander 
chip, get them from someone like Mouser.

Next step is to find a suitable housing for this assembly.

Thanks all,

----- Original Message ----- 
From: "Ben Hall" <kd5byb at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Tuesday, June 06, 2017 7:46 PM
Subject: [time-nuts] TruePosition on the Arduino

> 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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