[SI-LIST] Re: Termination of unused PCIe pairs
- From: Joseph.Schachner@xxxxxxxxxx
- To: otter30 <otter30@xxxxxxxxxxxxx>
- Date: Wed, 8 Sep 2010 09:14:19 -0400
This is one of the few questions on SI-LIST that I feel qualified to
answer.
The PCIe base spec has a section titled "Channel AC Couping Capacitors".
The relevant sentence for this case is: "Capacitors must be placed on the
Transmitter side of an interface that permits adapters to be plugged and
unplugged." So, since you said you have a video card (that plugs in), you
can expect that the card's TX pairs will be AC coupled on the card. On
your board, all you need are 50 ohm resistors to ground, one on each side
of each unused pair.
This should be clear, but just to make sure you realize: On your board's TX
lines going to the card, your board needs the AC Coupling caps.
Here's the entire section, from the PCI Express Base Specification Rev.
2.1. This text has not changed in years, it is the same in the latest
draft of the PCIe Gen3 base spec.
4.3.5.1. Channel AC Coupling Capacitors
Each Lane of a PCI Express Link must be AC coupled. The minimum and maximum
values for the
capacitance is given in Table 4-9. Capacitors must be placed on the
Transmitter side of an interface
that permits adapters to be plugged and unplugged. In a topology where
everything is located on a
single substrate, the capacitors may be located anywhere along the channel.
External capacitors are
assumed because the values required are too large to feasibly construct
on-chip.
-- Joe S.
From: otter30 <otter30@xxxxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Date: 09/07/2010 05:21 PM
Subject: [SI-LIST] Termination of unused PCIe pairs
Sent by: si-list-bounce@xxxxxxxxxxxxx
Hello Friends,
I am designing a carrier pcb that provides interconnect between a CPU
module and
a video processing card.
The video card supports 4x PCIe, but the CPU module has only 2 PCIe lanes
available.
Since PCIe pairs are specified to have 100ohm differential impedance, 50
ohm
resistors to ground ought to
work on the unused TX pairs. Do I also need a .1u cap in series with the
resistors for the RX pairs?
Thanks,
Jim
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