Hi 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.
Bob > On Nov 10, 2016, at 10:22 AM, Chris Albertson <[email protected]> > 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 <[email protected]> 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" <[email protected]> >> To: "Discussion of precise time and frequency measurement" < >> [email protected]> >> 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 >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
