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

Bob Camp kb8tq at n1k.org
Thu Nov 10 13:11:51 EST 2016


Well, I have yet to test a gps module that does not have a *very* accurate
pulse width out of it. Same with GPSDO’s. Yes, It’s something I look at.


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