[time-nuts] Re: Silicom PCIe timestamping network cards

[Poul-Henning Kamp]

--------
Poul-Henning Kamp via time-nuts writes:

> The timestamping counter gets its clock from the ethernet line
> signals, and the counting frequency therefore depends on the ethernet
> speed:
>
> 	100 Mb/s	1.5625 MHz
> 	1 Gb/s		15.625 MHz
> 	10 Gb/s		156.25 MHz
>
> (The 8ns timestamping mentioned must be something outside the 82599)

I should probably expand on this to prevent misunderstandings:

The 82599 chip will timestamp with 6.4ns resolution, and since both
the frequency and the timestamp edge is derived from the ethernet
signal when you time packets, there is no noise process involved,
and you do get your full 6.4ns worth.

I understand the "8ns" number in the datasheet for the card to refer
to the PPS input and assume the extra 1.5ns to be noise in the
analog domain outside the i82599 chip.

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