[time-nuts] Net4501

[Chris Albertson]

No, NTP still needs to read the internal CPU cock.  The reason you read the
counter every with each PPS is not to find out when the puse occurred, no
the point is to measure the in-accuracy in the rate of the counter.

So if I direct the PS to an Ethernet card what I'm doing is measuring
the in-accuracy of the card's internal oscillator.  That would be fine if
only the OS used that as abases for system time.  But it doesn't. The OS
uses the internal CPU counter for that.

You have to look at the goal.  The goal is to GPS-disipline the rate and
phase of the OS' system time.   To do that you have to measure whatever
oscillator drive that OS' system time.

If somehow you could modify the OS to use the oscillator on the ethernet
card as a source of time then yes you could sample that oscillator ever
second but you'd have to first modify the OS to use that clock for time
keeping.


On Fri, Jun 7, 2013 at 5:35 AM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> All of the MCU based 1588 interfaces I have seen allow you to "get at" the
> internal 1588 stamping clock. You can stuff your pps in there and compare
> it directly to the stamps it puts on the incoming and outgoing packets. If
> you are on an MCU, the 1588 clock can easily be the same as your CPU clock
> (or at least derived from the same source). Stamping with the 1588 counter
> in that case is no different than stamping with the cpu clock.
>
> Doing the same thing on a pc is a bit more complex. If you can get at the
> stamping inputs and outputs, it's another layer, but still doable.
>
> Bob
>
>
>
> Get the free running 1588 counter 1 pps output to agree with your local
> clock output.
>
>
> On Jun 7, 2013, at 12:16 AM, Chris Albertson <albertson.chris at gmail.com>
> wrote:
>
> > Network time stamping is a different issue.   You are thinking of time
> > transfer over a network.   What the above is about is capture the pulse
> per
> > second from a GPS.   We actually do NOT want to time stamp the PPS.  We
> > want to capture the computer's internal clock so that it can be compared
> to
> > the PPS.  The purpose is to adjust that internal clock.
> >
> > Then one this is none is some set of stratum 1 NTP server, then you can
> > transfer the time over Ethernet.
> >
> >
> > On Thu, Jun 6, 2013 at 5:48 PM, Bob Camp <lists at rtty.us> wrote:
> >
> >> Hi
> >>
> >> 1588 compatible network cards are capable of time stamping everything
> that
> >> goes in and out. They are pretty common these days both as stand alone
> >> cards and as peripherals on MCU's. There's no real need to do hardware,
> >> just come up with drivers (and all the other  software goop) to make
> them
> >> work with NTP. More or less the same work you would have had to do once
> the
> >> FPGA was done and debugged.
> >>
> >> Bob
> >>
> >> On Jun 6, 2013, at 8:32 PM, Ralph Smith <ralph at ralphsmith.org> wrote:
> >>
> >>> On Jun 6, 2013, at 1:59 AM, Chris Albertson <albertson.chris at gmail.com
> >
> >> wrote:
> >>>
> >>>> Yes, that is exactly what I meant by "remove the temperature issue"
> >> that
> >>>> means using a clock derived from a laboratory standard like GPS
> >> disciplined
> >>>> OCXO or a rubidium oscillator.  Once you do this the next bottle next
> is
> >>>> the uncertainty in the interrupt latency and the granularity of the
> >> clock
> >>>> that is being sampled.  So practically you are limited to about
> >> microsecond
> >>>> level performance.
> >>>
> >>> The Net4501 is capable of about 1/8 microsecond performance, the
> >> limiting factor here is clock granularity.
> >>>
> >>>> I think to get better than that you need to eliminate the interrupt
> and
> >>>> have some kind of deterministic hardware where the PPS directly
> samples
> >> the
> >>>> counter.  Perhaps hosting NTP on a soft CPU inside an FPGA, then you
> >> could
> >>>> implement the PPS interrupt in gates rather then in software.   I've
> not
> >>>> read of anyone doing this yet.
> >>>
> >>> If you look at PHK's code in FreeBSD this is what is done. The PPS
> >> signal gates the timer, so no interrupt is involved in the time stamp
> >> precision. But yes, it would be interesting to do something on a FPGA.
> >> Unfortunately I wouldn't be able to get to anything like that myself in
> >> this lifetime.
> >>>
> >>> Ralph
> >>> _______________________________________________
> >>> time-nuts mailing list -- time-nuts at 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 at febo.com
> >> 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 -- time-nuts at 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 at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>



-- 

Chris Albertson
Redondo Beach, California