[time-nuts] Best Chance GPS module

David J Taylor david-taylor at blueyonder.co.uk
Sun Mar 19 05:13:41 EDT 2017

> On Dec 1, 2016, at 7:44 AM, David J Taylor <david-taylor at blueyonder.co.uk> 
> wrote:
> Venus838LPX-T.   Seems to go into a non-locked state for a proportion of 
> the time but still emits a PPS signal, which increasingly deviates from 
> true UTC.  That's using a 25 mm square 28 dB active patch antenna, similar 
> to the antennas on other similarly located GPS receivers.  This was 
> indoors, on the top floor of a two-storey building, with just the power 
> fed to it, watching the LED flash.
> Maybe I was unlucky?  I wonder what experience others have?

From: Nick Sayer via time-nuts

Was the LED on the PPS pin or on the FIX pin?

The FIX pin is nothing like the PPS output. Frankly, I’m not 100% sure what 
the rules are for it. When things are working properly, it blinks at 0.5 Hz, 
but the leading and trailing edges are something like 120-150 msec after the 
second. It’s very, very loose. My own interpretation is that the controller 
flips the bit when it’s got nothing better to do.

I’ve not witnessed the actual PPS pin being toggled when GPS isn’t 
available, but it’s not a failure mode I’ve expended a lot of effort to 


I had to dig out the module to check, but as far as I can tell the LED is 
connected to pin 7 on the chip, listed as GPIO0/LED on the data sheet. 
Nominally, navigation status.

The PPS output which I was monitoring comes from the PPS pin (40) on the 
chip - GPIO3/P1PPS1

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