Re: [time-nuts] verifying synchronization with PPS
On Wed, Jul 10, 2019 at 8:00 PM folkert wrote: > > > Folkert van Heusden has a driver for NTP which includes PPS output: > > https://vanheusden.com/time/rpi_gpio_ntp/ > > Perhaps this might help? > > Indeed I did! :-) > > But please note that the jitter is high, iirc around 18ms. > Personally I would use https://github.com/mlichvar/pps-gpio-poll.git and > then patch it toggle an other gpio-pin. > I did something similar using an early version of that PPS driver: https://lists.febo.com/pipermail/time-nuts_lists.febo.com/2018-April/092234.html ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
> > Folkert van Heusden has a driver for NTP which includes PPS output: > > https://vanheusden.com/time/rpi_gpio_ntp/ > > Perhaps this might help? > > Indeed I did! :-) > > But please note that the jitter is high, iirc around 18ms. > Personally I would use https://github.com/mlichvar/pps-gpio-poll.git and > then patch it toggle an other gpio-pin. It has this functionality, thus no real patching required. You only need to remove the /* around #define GPIO_ECHO in pps-gpio-poll.c and then make/install/reboot (well not literally reboot of course). ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
> Folkert van Heusden has a driver for NTP which includes PPS output: > https://vanheusden.com/time/rpi_gpio_ntp/ > Perhaps this might help? Indeed I did! :-) But please note that the jitter is high, iirc around 18ms. Personally I would use https://github.com/mlichvar/pps-gpio-poll.git and then patch it toggle an other gpio-pin. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
I wrote: > I'm guessing there are some time nuts here who might be able > to give me some pointers. And there were! Thanks for the suggestions & comments. Dana Whitlow wrote: > in the absence of a PPS or other electrical output from your > "computer", what is the nature of the time "output"? It's collecting data and coordinating activities among several other systems, each with their own clocks, some connected by serial ports and some by ethernet, some on NTP and some not, some with PPS feeds and some not, but obviously the goal is, by hook or by crook, to keep all the clocks well synchronized so that timestamped data is commensurate -- and then to positively *demonstrate* that they're well synchronized. > why not view the light output of the seconds digit with a photocell. Because I had never thought of such a thing. I could pretty easily implement, in a diagnostic window somewhere, a nice fat 1 Hz spot that would be trivial for even a crude photocell to "see". There'd be some latency, but it'd be much better than nothing. Thanks very much for the suggestion! Graham wrote: > What level of accuracy do you mean by "synchronized"? I think we'd be happy with 1 ms. > Plotting a lightly loaded Linux box, which is extracting time from > the network via timesyncd, against a GPS 1PPS signal, I observe... So it sounds like you're already doing more or less what I want to do! I'd be curious to hear more about the setup you use to make those plots. > The owner of this list has designed an excellent dual input timing > device called the TICC, which can compare two PPS signals... Ha! Thought so. Thanks for the pointer. David Taylor wrote: > Folkert van Heusden has a driver for NTP which includes PPS output: > https://vanheusden.com/time/rpi_gpio_ntp/ Great! I will read that. Hal Murray wrote: > There is code in the Linux kernel to generate a PPS output. Huh! I should have noticed that. (I'd been thinking of adding it, if I had to.) > I think it uses the printer port... But it shouldn't be too hard to modify it to use a GPIO pin. > If I wanted to try something, I would write a user-level hack > to flap a modem line and compare that to a PPS signal. That would probably be fine for my current purposes. The latent time-nut in me worries about scheduling and latency and jitter, of course, tempting me to chase an overkill solution which is not only in the kernel, but implemented in a brute-force, special- cased way, eschewing the extra function calls and indirections (and hence latency) which a properly general-purpose solution might involve. Thanks again for all the replies; this has been very helpful. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
s...@eskimo.com said: > My specific question concerns the case that one of the boxes is a computer, > for example, a Linux box with time kept using ntpd or chrony (perhaps also > listening to a PPS signal coming in on a serial port). But I've never seen a > computer with a PPS *output*. There is code in the Linux kernel to generate a PPS output. I've never tried it. I think it uses the printer port which isn't available on modern systems. You can probably get a PCI plugin card. You will probably have to build your own kernel. If I wanted to try something, I would write a user-level hack to flap a modem line and compare that to a PPS signal. The code would look something like this: t1 = time() set line low t2 = time() set line high t3 = time() log t2 and t3 wait 1/2 second set line low wait 1/2 second loop The first time/set is to warm out the cache. > (Perhaps PPS is overkill for this situation; perhaps it's the case that given > the inherent inaccuracies of software-based timekeeping, a software-based > query mechanism -- perhaps using ordinary NTP or PTP protocols -- would be > sufficient for That just gets time from one computer to another. Now you have the same "is it right?" question on another box. -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
From: Steve Summit [] My specific question concerns the case that one of the boxes is a computer, for example, a Linux box with time kept using ntpd or chrony (perhaps also listening to a PPS signal coming in on a serial port). But I've never seen a computer with a PPS *output*. = Steve, Folkert van Heusden has a driver for NTP which includes PPS output: https://vanheusden.com/time/rpi_gpio_ntp/ Perhaps this might help? Cheers, David -- SatSignal Software - Quality software for you Web: http://www.satsignal.eu Email: david-tay...@blueyonder.co.uk Twitter: @gm8arv ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
Several comments: What level of accuracy do you mean by "synchronized"? Plotting a lightly loaded Linux box, which is extracting time from the network via timesyncd, against a GPS 1PPS signal, I observe typical time excursions within +/- 10 ms, and extreme excursions over a 24 hour period of up to 20 ms. So, I would not count on network based time on a Linux box to be better than +/- 20 ms, probably worse with heavy loading of the OS. A simple navigation GPS receiver PPS output, without any additional corrections can provide accuracy of less than 0.1 microsecond. The owner of this list has designed an excellent dual input timing device called the TICC, which can compare two PPS signals with resolution of 8 or 9 decimal places, for projects like this. https://tapr.org/kits_ticc.html --- Graham == On Mon, Jul 8, 2019 at 9:03 AM Steve Summit wrote: > This is a different sort of question than what seems to be > discussed here usually, and I apologize if it's wholly off-topic, > but I'm guessing there are some time nuts here who might be able > to give me some pointers. > > If I have two boxes with clocks that are supposed to be perfectly > synchronized, and I need to verify this, and they're physically > right next to each other, and they both have PPS outputs, one > way to verify the synchronization would be to compare the two PPS > outputs with a 'scope. (I'm guessing there's probably a dedicated > instrument out there specialized for the task of comparing two > such PPS signals for frequency, phase and jitter, and if I were > a proper time nut myself I guess I'd know that.) > > My specific question concerns the case that one of the boxes is > a computer, for example, a Linux box with time kept using ntpd > or chrony (perhaps also listening to a PPS signal coming in on a > serial port). But I've never seen a computer with a PPS *output*. > > Is this a reasonable thing to be thinking about, or am I going > about it wrong? How would *you* positively verify synchronization > of such a system? Me, I'm pursuing this idea because too much > of the time (at least in my own, perhaps parochial experience) > synchronization seems to be "verified" either by saying "Yes, > we configured it properly", or by observing a status output > from ntpd or chrony claiming the requisite synchronization > has been achieved, but in neither case by making a definitive, > independent, external, empirical determination. > > (Perhaps PPS is overkill for this situation; perhaps it's the > case that given the inherent inaccuracies of software-based > timekeeping, a software-based query mechanism -- perhaps using > ordinary NTP or PTP protocols -- would be sufficient for > externally assessing synchronization.) > > I guess it's not a *completely* unreasonable thing to be thinking > about, because I've found a couple of web pages (for example > https://www.raspberrypi.org/forums/viewtopic.php?t=115554 and > https://github.com/jsln/pps-gen-gpio) describing how to implement > a PPS output under Linux. And I do realize that trying to > minimize the jitter and latency in this situation (given that > the principal drivers for the hypothetical output are inherently > software-based) presents considerable difficulties. But taking > all of that into consideration, I'm wondering what others think > of the approach. Thanks for any comments. > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] verifying synchronization with PPS
Steve, I'm curious: in the absence of a PPS or other electrical output from your "computer", what is the nature of the time "output "? If it's just visual via a display, why not view the light output of the seconds digit with a photocell. Most of the transitions should produce an easily-detectable step or glitch in the light. Dana On Mon, Jul 8, 2019 at 9:03 AM Steve Summit wrote: > This is a different sort of question than what seems to be > discussed here usually, and I apologize if it's wholly off-topic, > but I'm guessing there are some time nuts here who might be able > to give me some pointers. > > If I have two boxes with clocks that are supposed to be perfectly > synchronized, and I need to verify this, and they're physically > right next to each other, and they both have PPS outputs, one > way to verify the synchronization would be to compare the two PPS > outputs with a 'scope. (I'm guessing there's probably a dedicated > instrument out there specialized for the task of comparing two > such PPS signals for frequency, phase and jitter, and if I were > a proper time nut myself I guess I'd know that.) > > My specific question concerns the case that one of the boxes is > a computer, for example, a Linux box with time kept using ntpd > or chrony (perhaps also listening to a PPS signal coming in on a > serial port). But I've never seen a computer with a PPS *output*. > > Is this a reasonable thing to be thinking about, or am I going > about it wrong? How would *you* positively verify synchronization > of such a system? Me, I'm pursuing this idea because too much > of the time (at least in my own, perhaps parochial experience) > synchronization seems to be "verified" either by saying "Yes, > we configured it properly", or by observing a status output > from ntpd or chrony claiming the requisite synchronization > has been achieved, but in neither case by making a definitive, > independent, external, empirical determination. > > (Perhaps PPS is overkill for this situation; perhaps it's the > case that given the inherent inaccuracies of software-based > timekeeping, a software-based query mechanism -- perhaps using > ordinary NTP or PTP protocols -- would be sufficient for > externally assessing synchronization.) > > I guess it's not a *completely* unreasonable thing to be thinking > about, because I've found a couple of web pages (for example > https://www.raspberrypi.org/forums/viewtopic.php?t=115554 and > https://github.com/jsln/pps-gen-gpio) describing how to implement > a PPS output under Linux. And I do realize that trying to > minimize the jitter and latency in this situation (given that > the principal drivers for the hypothetical output are inherently > software-based) presents considerable difficulties. But taking > all of that into consideration, I'm wondering what others think > of the approach. Thanks for any comments. > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
