[SI-LIST] Re: PCI Express reference clock, is it needed?
- From: "N. Paul Taddonio" <paul.taddonio@xxxxxxxxxxxxxx>
- To: "Pehr Andersson" <andersson.pehr@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 14 Dec 2011 08:52:05 -0500
Pehr
It all depends on your SERDES and the qualities of the serial link signal.
For example, many SERDES require a reference clock from the transmitter to
receive links modulated with spread spectrum clocking (SSC).
That is because the clock frequency is varying with time and is hard for the
receiver to track.
PCI Express does include spread spectrum operation.
If there is no SSC then a local oscillator may be substituted for the
external reference clock connection, providing
that both oscillators are accurate enough in frequency for your SERDES to
tolerate (they must be within a certain PPM difference in
frequency).
The newest technology SERDES, within the last couple of years, can receive
spread spectrum modulated serial signals without
a reference clock connection. This is implemented in the DisplayPort
serial link standard.
-Paul Taddonio
----- Original Message -----
From: "Pehr Andersson" <andersson.pehr@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, December 13, 2011 7:16 PM
Subject: [SI-LIST] PCI Express reference clock, is it needed?
> Hello dear readers, I have maybe a silly question that has been
> bugging me for a while, hopefully it will get answered so it's not
> bugging me anymore :).
>
> For PCI Express it is stated that the clock signal is embedded within
> the data signal.
>
> Why do we then need to route reference clock (REFCLK), that is an
> external clock signal for PCI Express?
>
> Would PCI Express data communication work even if we didn't have
> REFCLK? Say it we go over a cable for instance.
>
> Thank you for you time,
> Best regards, Pelle
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