Hi
> On Apr 10, 2018, at 9:59 AM, Dan Kemppainen <d...@irtelemetrics.com> wrote: > > Hi, > > Don't know how good they are, but there are two functions in the kernel32 lib > in windows that are related to a cpu performance counter, > QueryPerformanceCounter and QueryPerformanceFrequency. (Maybe Linux has > similar?) > > Anyway, on most systems the frequency reported is the raw cpu clock. (Couple > of Ghz Range numbers, My current system is reporting 3,320,458 Hz, windows7.) > Supposedly these are low latency functions. It may not offer a perfect > solution, but at least it gives you 'low latency' access to a high speed > counter. > > Maybe it's possible to timestamp incoming PPS pulses with this (assuming > they're triggering an interrupt), and learn something neat. > > Some of this is subject to change with windows versions: > > https://msdn.microsoft.com/en-us/library/windows/desktop/dn553408(v=vs.85).aspx > > <https://msdn.microsoft.com/en-us/library/windows/desktop/dn553408(v=vs.85).aspx> NTP already looks at incoming pulses and reports what it thinks is going on with them. The desire here is to get a pulse *out* of the device. Then you can toss it into a conventional set of gear. From the data you can independently evaluate what’s going on. So more or less: 1) Generate pulse 2) Work out when the pulse went out 3) Compare that to what NTP thinks is going on 4) Generate a message to describe the delta in time No, not trivial …. Bob > > Dan > > > > > On 4/10/2018 8:01 AM, time-nuts-requ...@febo.com wrote: >> kb...@n1k.org said: >>> The kernel clock comes from the CPU clock. That CPU clock is phase locked to >>> a crystal. If you have a CPU that is driven by a VCXO that is a*very* >>> unusual CPU board. The crystal runs at an arbitrary frequency. That gives >>> you edges that are unlikely to happen ���right on the second���. >> I was assuming the CPU clock was fast enough that reading a cycle-count >> register and converting to ns would be within a ns which is the resolution of >> the clock. >> That's obviously not true for low end SOC type setups. A Pi-1 runs at 700 >> MHz. The Pi 3 is up to 1.4 GHz. > _______________________________________________ > 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.
