[SI-LIST] Re: SERDES CDR with asynchronous reference clock?

Thank you Steve! Please let me reiterate to make sure I understand you.

"The Rx clock controls data transfer AFTER buffered CDR" is the key thing I was 
missing, the CDR will always have a PLL or some other type of clock 
regeneration recovered from the RX data stream.

In addition, to quote an off list reply I received (thanks Leonard) "When there 
is no data or no lock the internal PLL will normally frequency/ phase lock onto 
the reference clock".

Is this summary of the three PCIE RX clock architectures correct:

Common Clock
The host transmits a common reference clock (RefClk) that is used by the 
downstream device e.g. ExpressModule for both TX and RX. The downstream device 
has a PLL that is frequency locked to the RefClk and phase locked to the RX 
data.

Data Clock
It is not necessary to have a common reference clock. The downstream device has 
a PLL that is frequency and phased phase locked to the RX data. This clock 
could also be used by the downstream device for TX, or it could use a separate 
TX clock within 600ppm of the RX clock.

Separate Clock
It is not necessary to have a common reference clock. The downstream device has 
a PLL that is frequency and phased phase locked to the RX data. This clock 
could also be used by the downstream device for TX, or it could have a separate 
TX clock within 600ppm of the RX clock. In the absence of data, the RX clock is 
locked to the local reference clock.

?

Noted that (per Leonard) "There are many other ways to perform asynchronous 
sampling in the digital domain", so the each above summary method would just be 
one (typical?) way.

Comments welcome.

Thanks,

Ivor

-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx] 
Sent: Thursday, December 10, 2009 12:43 AM
To: Bowden, Ivor
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] SERDES CDR with asynchronous reference clock?

Ivor, the receive CDR phase locks onto the incoming data stream and dumps the 
data into an elastic store:  FIFO.  The Rx clock controls data transfer after 
buffered CDR.  Channel utilization is under 100% by more than 2X the clock 
tolerance.  Therefore a transmit stream will never exceed the receiving end's 
ability to absorb the data.

Steve.
Bowden, Ivor wrote:
> Hi SI people,
>  
>
> Can someone offer an explanation how a SERDES CDR can recover data with an 
> asynchronous reference clock? Example is PCI-Express (reference PCI EXPRESS 
> BASE SPECIFICATION, REV 2.1 section 4.3.7.5. "Separate Refclk Architecture"), 
> where Refclk #1 drives the TX circuit and Refclk #2 drives the RX circuit. 
> How does this CDR circuit work? I thought that even if they were separate 
> clocks they'd have to be frequency locked, and a PCI-SIG presentation 
> indicates they can be +/-300ppm each.
>
>  
>
> Thanks,
>
>  
>
> Ivor Bowden
>
>
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