[SI-LIST] Re: PCI Express reference clock, is it needed?
- From: "Ransom Stephens" <ransom@xxxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 14 Dec 2011 12:44:18 -0800
Good answers, some fill:
The ref clock is in the spec so that folks can use cheaper parts in the
receiver. The low rate ref clock is multiplied up and synchronized with the
incoming data to provide the clock used for sampling the data.
If you're using a more robust clock recovery circuit, then you don't need
the ref clock - but, as Paul said, beware of SSC. If the bandwidth of your
clock recovery is large enough, you should be okay.
If, on the other hand, you don't include a ref clock from the transmitter,
whether or not you use it at the receiver, you won't be compliant - but if
you're in a closed system, who cares?
Ransom
_____________________________
Ransom W. Stephens, Ph.D.
Applied Electrodynamics Physicist
Content and Analysis
www.ransomsnotes.com
Twitting @ransomstephens
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of N. Paul Taddonio
> Sent: Wednesday, December 14, 2011 5:52 AM
> To: Pehr Andersson; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: PCI Express reference clock, is it needed?
>
> 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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