kb...@n1k.org said: > In both cases (pulse in and pulse out) the first step is to ask NTP âwhen > was that?â. You still have a pretty big chunk of NTP in the middle of the > process â¦. If NTP only âknowsâ what is happening (or can control what is > happening) to +/- 300 ns. The guts of your data will be limited to the > same 300 ns.
NTP itself doesn't actually do any timing measurements. That is done by the OS. I'm not familiar with Windows. All my OS comments apply to Linux and *BSD. Windows is probably similar is most respects but may be missing details. The system clock runs off the CPU crystal. The kernel has a knob for fine tuning the clock speed. There is another knob that says bump the clock speed a bit (500 PPM) long enough to adjust the time by X seconds. ntpd uses both. [Many years ago, most kernels used an interrupt from the battery backed clock (RTC or TOY) for the main timing and interpolated using the CPU clock.] The kernel can capture a time stamp on a PPS pulse and on packet arrivals. I don't know of anything similar for packet departure. The kernel can support more than one PPS so you could feed a GPSDO in on one for NTP and use another for general timing measurements. Those measurements are using the kernel clock as a reference. You can turn ntpd off so the system time will have more long term drift but less short term wobble as ntpd corrects for errors.] I don't know of a good way to get a pulse out at a specified time. You can measure the time that user code sends a pulse out and setup a timer to go off every second. I don't know how to adjust or specify the offset from timer-going-off to system clock ticking to another second. The Linux kernel PPS code has an option to send a pulse out when one comes in. I haven't played with it. I think it could help with measuring the interrupt response delays. tvb: If I send you some data, can you turn it into pretty graphs? -- These are my opinions. I hate spam.
_______________________________________________ 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.