On 5 Mar 2020 at 17:01, Jacob Keller wrote:
> On 3/5/2020 1:11 AM, Frantisek Rysanek wrote:
> > And another sideways question is: in the i210 hardware, there's a
> > register called SYSTIMR, supposedly holding the fraction of a
> > nanosecond (= sub-nanosecond resolution). And this register is
> > ignored by the igb driver in Linux - first and foremost because the
> > internal timestamping infrastructure only supports nanosecond
> > resolution. I know that a "ns fraction" field is present in the PTP
> > frames, but everybody except the White Rabbit just leave that field
> > empty (all zeroes). I'm wondering if this SYSTIMR register in the
> > i210 hardware has some practical use, or is always zero, or what.
> > Well for my practical purposes, the SYSTIMR does not get reflected in
> > the two AUXSTMP registers - so I can probably just ignore SYSTIMR
> > too.
>
> So, the SYSTIMR field is not "used" directly, but it holds and maintains
> fractional nanoseconds. When you adjust the increment value slightly,
> these get added to the SYSTIMR field of the system time. As that slowly
> increments and eventually overflows, it will then increment the SYSTIML
> register.
>
> Essentially we always round down by cutting off SYSTIMR.
>
Mr. Keller thanks for your polite explanation :-)
I have to say that the i210 is a very nice piece of silicon to play
with :-) I'm aware that at 1 GHz / fractions of a ns, it takes some
cunning design to make a synth with this kind of fine adjustment,
immediate response and nanosecond resolution,
with hardly any jitter/phase noise. It's a job well done.
The fact that timestamping external events is granular at 8 ns
("only") does not offend me. I'm aware that it's difficult
to run counters and atomic latches that fast.
I've read something about WhiteRabbit's phase detector,
called the DDMTD - which supposedly can measure phase
differences down to the picosecond range. If I understand correctly,
that comparator block depends on having isochronous clocks
(the reference and the measured input) much faster than 1PPS
and hinges on down-mixing those clocks, then phase comparison
and statistics (filtering) to arrive at that fine-grained result.
Hence also WhiteRabbit's dependence on SyncE,
and the BC-only nature of their PTP-aware switches...
I understand that this is what it takes to run time-sync
over a WAN at ns/sub-ns level preecision...
Frank Rysanek
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