Here is a patent on a method for receiver detection on a PCIe bus using
these ac coupling caps.
https://patents.google.com/patent/US7222290
Praveen
On Mon, Aug 13, 2018 at 8:42 AM, <leeritchey@xxxxxxxxxxxxx> wrote:
Thanks for all of the replies.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf
Of Chuck Corley
Sent: Sunday, August 12, 2018 11:32 AM
To: leeritchey@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCI Express AC coupling caps
Sorry, scratch that last one from the 82573 layout checklist (about
coupling
caps being near the transmitter), that was for Ethernet, not PCIe. My bad.
Chuck Corley, National Instruments
On 2018-08-12 11:29, Chuck Corley wrote:
I believe it's written up as being always required in the PCIe spec forthis reason:
presence of a physical link and channel width. The specification exploits
_Receiver detection:_ PCIe uses an ingenious means to recognize both the
the fact that an un-terminated, ac-coupled transmission line will have a
very different charge time when the line is terminated versus open. Each
PCIe transmitter, at the commencement of linkup, produces a low-frequency
"ping" on each of the differential TX outputs. The transmitter includes a
simple detection circuit to monitor the line response to this ping. With no
receiver attached, the edge rate (and amplitude) of the line change is much
higher than when a receiver is present. Because the specification has a
defined range of coupling capacitance and the receiver termination, a
distinct, detectable time constant range defines when a receiver is present
or not.
Express link be AC-coupled between the driver and receiver. The actual AC
From: PCI Express And The PHY(sical) Journey To Gen 3, Reginald
Conley | May 18, 2009
From a different reference:
AC COUPLING CAPACITORS
The _PCI Express Base Specification_ requires that each lane of a PCI
coupling capacitors can be located either on or off the die/component. In
most cases, the AC coupling is separate from the component, and you must
use
discrete capacitors on the PCB board itself to satisfy the AC coupling
requirement of the specification. The following guidelines apply to AC
coupling capacitors.
located on the card for each of the TX pairs originating from the add-in
*
For add-in cards and ExpressCard modules, the AC coupling capacitors are
card PCI Express device. You do not need to place any AC capacitors on the
system or host board for these signals.
*pairs originating from the system board PCI Express device and traveling to
For the system board, the AC coupling capacitors are required on the TX
the respective connector interface.
*have AC caps located on them somewhere along the interconnect.
For chip-to-chip connections, all PCI Express differential pairs should
*component placement, all TX pairs originating on the system board are
Because most four-layer system boards allow only single-sided, top layer
likely
to be routed on the top layer. Knowing this, you should plan to route all
the RX pairs on the bottom layer of your board. This separation of the RX
and TX pairs also helps to reduce near-end crosstalk (NEXT), which can
severely limit the performance of PCI Express signals. If necessary, TX
signals routed...
seen checklists that say to put them near the transmitter:
From: From PCI Express Electrical Interconnect Design: Practical
Solutions for Board-level Integration and Validation
Some people say they can be anywhere along the link path, however I've
possible to the 82573E/V/L but not further than 250 mils.
Place the AC coupling capacitors on the PCIe* Tx traces as close as
From: Intel(R) 82573E/V/L Layout Checklist (version 1.5)
Chuck Corley
National Instruments
On 2018-08-12 09:47, leeritchey@xxxxxxxxxxxxx wrote:
Does anyone have a writeup describing when and where to use AC
coupling caps in PCI Express lanes? And why and when they are needed?
Lee Ritchey
Speeding Edge
P.O. Box 817
Bodega Bay, CA
94923
408-781-0253
I took the energy needed to be mad
And wrote some blues.
Count Basie
Worry is like rocking chair
It keeps you busy
But it doesn't get you anywhere.
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