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

  • From: Roy M <roymesi@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 27 Sep 2005 11:25:28 +0200

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

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!
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