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

Scott Stobbe scott.j.stobbe at gmail.com
Thu Nov 10 11:10:05 EST 2016

When 1PPS is implemented as a time mark signal narrow width (versus 1 Hz
50% duty cycle) and you time stamp both edges, you can always tell which is
supposed to be the leading edge. Either the assert and deassert are close
together in the correct case or far apart when using incorrect polarity.

On Thu, Nov 10, 2016 at 10:22 AM Chris Albertson <albertson.chris at gmail.com>

> The problem here is "real world".  Yes in theory you can do it perfectly
> but in the real world do all makes and models of GPS receivers get it
> right?   I would not bet on it.
> This is just like the argument over using NMEA only for timing.  Some GPS
> receivers push out NEMA sentences with very little jitter and others only
> follow the NMEA spec with only requires plus or minus one second accuracy
> and then there is every shade of grey between.
> My conservative engineering background tells me that unless the variation
> of the pulse width is species you should assume the width is not controlled
> at all.  It may even look good in testing but a firmware update would kill
> that.  Likely in there is no regression test for not specified features.
> People now days who write software (or firmware especially) will build an
> automated test suit that lets them checks f the software still works after
> making changes.  They are careful to test each written requirement.
> The UART on the PC can interrupt on either raising or facing edges so if
> the PPS is inverted you interrupt on the falling edge.   But you have to
> get this right too
> one IMPORTENT thing to  get right is to remember that on modern control
> line under RS232 the logic is "active low".  TTL logic is "active high"
> where a higher voltage means "one" and allow voltage means "zero" but this
> is backwards for modern control under RS232   So, it is WAY-EASY to get it
> wrong when connecting a TTL PPS to a modern control line.     You might
> need an inverter because many level shifters invert the signal
> The one thing that helps is the at 1Hz the signal is slow enough to see on
> a volt meter, Even an old VTVM reacts fast enough
> On Wed, Nov 9, 2016 at 2:44 PM, Tom Van Baak <tvb at leapsecond.com> wrote:
> > 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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> --
> Chris Albertson
> Redondo Beach, California
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