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

[Bob kb8tq]

Hi

> On Jul 4, 2022, at 10:04 AM, Poul-Henning Kamp via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> --------
> 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.

Could be. They also mention a 25 MHz clock on the card. That could
get you to a 125 MHz time base with a 8 ns resolution. Again, without
a deep dive into what they did … who knows.

Bob

> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
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