[time-nuts] Net4501

[Chris Albertson]

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
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-- 

Chris Albertson
Redondo Beach, California