[SI-LIST] Re: regarding PCI-Express clocking scheme - dazzled & confused

Roy,

A link is composed of lanes, each operating at 250 MB/(s*lane).  The x16 
link has a link bandwidth of (16 lanes x 250 MB/s*lane), or 4 GB/s. 

Bytes become 10-bit entities for transmission.  Each byte is encoded 
into a 10-bit symbol, transmitted on a lane at 2500 Mb/s, for a 
bandwidth of 250 MB/s*lane.  The 8b-10b encoding provides attributes of 
run-length limiting and DC balance to the encoded data stream.  The 
Clock Recovery Unit (CRU) at the receiver relies on the limited run 
length for proper operation.  CRU effectively clocks the serial stream 
at 2500 MHz and presents the recovered serial stream to symbol alignment 
logic, which recovers 10-bit encoded symbols, and a byte clock, at a 
rate of 250 MHz, which are then decoded into 8-bit bytes at the same speed.

The reference clock is transmitted to the link logic to facilitate 
spread spectrum clocking (SSC).  Were it not for SSC, I think the 
reference clock would not be needed.

Hope this helps.  It just skims the surface.

Regards

Mike
- - - - -
Roy M wrote:

>Hi all,
>I'm working on a PCI-E board for quite a while now, and I'm trying to
>figure something out here, regarding the clock scheme of the PCI-E.
>I've looked through the spec. of PCI-E and from wondering around the
>net i found a few things that don't match up:
>
>1. I understand that the transmission rate is 250 MB/S/lane, hence, if
>I have x16link, then the tx rate will be 4GB/S/lane, BUT, i've read in
>few places (including the Spec.) that the idea in PCI-E is that one
>does not transmit clock, but rather 8b10b encoded data, and from this
>the other end re-create the recieve clock. how does that settle?
>
>2. also, there's a 100MHz clock which is called a reference clock,
>that IS transmitted... how does that settle with remark 1) ?
>
>3. furthermore, what is the 2.5Gb/s tx rate, which ussually, when
>people talk about PCI-E, they through it in the air...? 2.5Gb/s is not
>250MB/s/lane...
>
>if anyone will be able to explain all that to me ,it will be great,
>cause i'm all messed up around here...
>Thanks guys and girls!
>Roy
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