What are you measuring? Seriously. What is it you need to know, is it? 1) The time between the raising edge of the PPS and when the OS samples the time
2) The time it takes between the PPS edge and when a user land process is notified. There are other things you can measure but if you want to see #1 above you can't use a TIC. And you can't have the user space process set s GPIO bit. The reason is that the PPS interrupt handler dramatically shortens the time removing ALL of the kernel process or latency. Look at the interrupt code. The clock is sampled there. The edge triggers the interrupt then while inside the handler the internal clock is sampled and stored and a flag is set to indicate the PPS was received. Som tie MUCH later the flag is checked and the user-land process is told the PPS has detected The delay does not matter because the clock was sampled with very low latency even if the user process was not notified right away. I think the details are platform dependent, hardware on a PC is not the same as a Raspberry Pi. So you need to look at the source. On Tue, Aug 25, 2015 at 10:53 AM, Andrew Symington <[email protected]> wrote: > Hi Folkert > > If you have a board with a hardware timer that supports load/match/compare > then you can schedule an external interrupt to be generated at a > predetermined point in the hardware count. Thus, if you know the transform > between your disciplined clock and the hardware counter of the timer that > drives it, then you should be able to do this. I have spent some time > working with the (pretty neat) timers on board a beaglebone black, and I've > written some code to setup input capture and compare on up to 4 timers: > https://bitbucket.org/rose-line/roseline/src/35d551bf29e4bfec80f8ba667b199c8aa333b87f/core/modules/roseline.c?at=master > > Cheers > Andrew > > > On Tue, Aug 25, 2015 at 8:24 AM, folkert <[email protected]> wrote: > >> Hi, >> >> Not sure if it is interesting for you guys but I wrote a simple program >> for e.g. Linux (or any other system with the pps api implemented) that >> listens on a pps source waiting for a pulse and then toggles a gpio >> pin. That way you can measure the latency introduced by the the kernel >> when listening from userspace. Note that there's a little extra latency >> due to the gpio-pin handling. >> >> It is on github: https://github.com/flok99/pps2gpio >> >> >> Folkert van Heusden >> >> -- >> MultiTail cok yonlu kullanimli bir program, loglari okumak, verilen >> kommandolari yerine getirebilen. Filter, renk verme, merge, 'diff- >> view', vs. http://www.vanheusden.com/multitail/ >> ---------------------------------------------------------------------- >> Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com >> _______________________________________________ >> 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. -- 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.
