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

[Adrian Godwin]

But trailing isn't the same as falling. The leading edge can be of either
polarity.


On Wed, Nov 9, 2016 at 11:28 PM, Bob Stewart <bob at evoria.net> wrote:

> Tom,
> How many times have people posted here, on time-nuts, not to trust the
> trailing edge of a 1PPS pulse?
>
> Bob -----------------------------------------------------------------
> AE6RV.com
>
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>
>       From: Tom Van Baak <tvb at LeapSecond.com>
>  To: Discussion of precise time and frequency measurement <
> time-nuts at febo.com>
>  Sent: Wednesday, November 9, 2016 4:44 PM
>  Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2
> SoC serial port for ntpd
>
> Bob,
>
> The PIC's I use have essentially no jitter. If they generate a 1PPS the
> edge and the pulse width are perfect, down to picoseconds. The talk about
> "other stuff" and "priority" and "number of compares" and "ambiguity" is
> worrisome. It sounds like a design or coding flaw to me, like what happens
> when people try to do precise time with a high level language.
>
> /tvb
>
> ----- Original Message -----
> From: "Bob Stewart" <bob at evoria.net>
> To: "Discussion of precise time and frequency measurement" <
> time-nuts at febo.com>
> Sent: Wednesday, November 09, 2016 9:30 AM
> Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2
> SoC serial port for ntpd
>
>
> 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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