Hi Roughly speaking, if you have a 10 MHz clock driving a timer and the pin latches data from that timer, you get 100 ns “buckets and +/- 100 ns “jitter”. You can find MCU’s that will do this for < $1. If you go crazy, you can spend < $10 and still get a very fancy MCU on a board with all the support “stuff”. That would get you into > 100 MHz clocks driving counters that might be 32 bits wide.
The Intel guys have some *very* fast timers flying around their cpu’s. They would laugh at the idea of a 10 or 100 MHz clock. If you can configure the pin to grab the data off those timer, you have way better than 100 ns at the timer. The trick is writing a driver that does that. How easy that is to do depends a *lot* on your OS and the chipset you are running. It may be trivial or it may be impossible. At some point one might ask: Is a $1 MCU a “system” or is it a peripheral? Bob > On Feb 17, 2017, at 5:58 PM, Thomas Petig <tho...@petig.eu> wrote: > > Hi, > > I was wondering whether there is some data/information available on the > claimed +/- 100 ns jitter? > > Regarding the PPS -> USB (using the CTS line of a FTDI FT232R), I > plotted, using some lines of Python, the time offset as attached. Just > to get an overview how it is 'worst case', i.e., user program, python, > etc. The 1PPS signal comes from a GPS rx. > Looks like a standard deviation of around 150 us. > y-axis: measured pps offset from full second (computer time) in us, > x-axis pps pulse number. > > On the long term it looks interesting (while measuring I played with the > NTP server on this computer) > Until ca. second 10000: ntpd synchronization via internet > Until ca. second 17000: made an additional LAN NTP server (GPS) available > Until the end: replaced ntpd with chrony (still using internet and local > servers) > > Interesting points: > -It looks surprisingly bad with using the normal ntpd (especially, there > is not really an improvement having an local GPS based server > available, did I do something wrong? Only the offset changes by ca. 3 > ms.) > -It looks surprisingly good with chrony. But there are continuously > outliers of up to 4500 us, is this a result of the chrony control loop? > The time is wandering around with ntpd, but has less jitter. > > Conclusion: > Despite the 150 us stddev, the using PPS over USB gives some interesting > inside of what the local ntp server is actually doing. It looks to me > like it would be an improvement to use it when using ntpd, but not when > using chrony. > > Best regards, > Thomas > DK6KD > SA6CID > > PS: > Raw data is here, if you want to zoom in: (1.7 MiB, one row per PPS > offset in us) > http://petig.eu/pps-usb.txt > > On Tue, Feb 14, 2017 at 07:26:23AM -0500, Bob Camp wrote: >> Hi >> >> A direct port might be a +/- 100 ns sort of thing most of the time and a >> +/-10 us >> thing every so often under some OS’s. Most desktop operating systems are not >> designed to prioritize random pin interrupts. A dirt cheap MCU coded with a >> few >> (hundred) lines of assembly code may be a better option than a typical >> desktop. >> Complicating this further is the degree to which some OS’s can be directly or >> indirectly optimized. Install *this* package and it all goes nuts. Install >> that package >> and not much happens …. >> >> Bob >> >>> On Feb 13, 2017, at 11:07 AM, Ruslan Nabioullin <rnabioul...@gmail.com> >>> wrote: >>> >>> Hi, generally speaking, what are the performance differences between the >>> following: 1. direct RS-232 (i.e., what I believe is a standard PCI card >>> offering RS-232---essentially UARTs interfaced more-or-less directly to the >>> PCI bus); 2. RS-232 via USB; 3. PPS decoding PCI cards (which might also >>> have an IRIG input or even an onboard GNSS receiver). >>> >>> Thanks in advance, >>> Ruslan >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > <overall.jpg><zoom.jpg>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
