[time-nuts] Net4501

[Ralph Smith]

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