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Re: [gpsd-dev] A detail in your ntpoffset script

From: Hal Murray
Subject: Re: [gpsd-dev] A detail in your ntpoffset script
Date: Wed, 23 Oct 2013 12:52:38 -0700

address@hidden said:
> On one host I have two GPS/PPS.  They differ by a consistent 1.5 uSec. 

Is that just the expected synchronization?

What would happen if both PPS pulses happened at exactly the same time?  The 
interrupts would get merged into one.  The interrupt processing would 
consistently check one first.  How much time would you expect between first 
check and the second?  1.5 uSec seems like a reasonable ballpark to me, 
depending on your CPU.

It's slightly more complicated with USB.  The hardware will go through a 
polling cycle every time the clock ticks.  It will test one first, then the 
second.  I'd expect a bit more than 1.5 uSec in that case, and occasionally 
the PPS pulses will happen after the hardware has checked the first and 
before the second, so the first will be one polling cycle later than the 

These are my opinions.  I hate spam.

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