[time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd

[Bob Stewart]

I'd like to comment on the idea of measuring the width of the pulse.  My experience with creating the 1PPS from an interrupt is that it's fairly straightforward how to do the set interrupt:  The interrupt happens, you execute one, maybe two instructions to raise the output pin, and you leave the interrupt routine.  But, resetting the pin is a different story.  Unless you've got a lot of interrupt vectors to play with, the reset part of the 1PPS signal is delegated to a general purpose timing interrupt where you're doing a lot of other stuff - and it has a somewhat lower priority.  So, you wind up doing a number of compares to see if you should reset the pin, which adds some ambiguity to the set/reset times.
Granted, I'm doing this on a general purpose PIC, but I have read comments about various receivers having some jitter on the reset side of their 1PPS pulse.
Bob
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      From: Chris Albertson <albertson.chris at gmail.com>
 To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Wednesday, November 9, 2016 10:41 AM
 Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd
   
On Tue, Nov 8, 2016 at 11:08 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

>
> gem at rellim.com said:
> > Who cares if the PPS is inverted?
>

If the signal is NOT inverted then the raising edge will be within
nanoseconds of the UTC seconds "tick".    The inverted PPS will have the
edge delayed somewhat.  It is not even clear if the length of the delay is
constant.

If using the PPS for timing and the PPS is inverted you will have to
measure the delay length an account for in the config file.




-- 

Chris Albertson
Redondo Beach, California
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