[SI-LIST] Re: PCI Express reference clock, is it needed?
- From: "Muzaffer Kal" <kal@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 13 Dec 2011 23:33:03 -0800
One always needs a good quality reference clock in a PHY so getting it
routed avoids a crystal on your board.
If on the other hand, PCIE link is coming over a cable then a local clock
source is needed.
Of course in either case, the clock recovery process in the PHY has to
multiply the reference clock to a frequency
suitable for sampling the incoming data and adjust the phase only (when the
same reference clock transmitter
is using is available) or do full clock recovery by adjusting the frequency
(as in the latter case where only PCIE
data is available over the cable.) In either case, the clock recovery has to
find the optimal place to sample the
incoming bitstream (roughly in the center of the eye) but when there are two
separate clock sources, as they are
almost always a little out of phase, this process is a little more
difficult.
Kal
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Pehr Andersson
Sent: Tuesday, December 13, 2011 4:16 PM
To: si-list@xxxxxxxxxxxxx
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.
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