Pci Express Layout Design Guidelines; Pci Express* Interconnect - Intel EP80579 Manual

PCI Express* Interface—Intel
Figure 86.

PCI Express* Interconnect

For more information on PCI Express, see the PCI Express Base Specification, Rev. 1.1
and PCI Express Card Electromechanical Specification, Rev. 1.0 or later.
The EP80579 provides support for x8 port or x4 port. The x8 port can be divided into
two x4 ports or two x1 ports.
For the list of PCI Express signals, see the Intel
Product Line Datasheet.
10.1

PCI Express Layout Design Guidelines

This section contains information and details for layout and routing guidelines.
One of the key points for PCI Express board design is to follow a set of rules to
minimize losses, jitter, crosstalk, and mode conversion for differential traces.
Loss is the differential voltage signal swing attenuation of the trace from transmitter to
receiver. The trace is subject to resistive, dielectric, and skin effect losses. Loss
increases as trace length or frequency increases. In addition, conductor loss increases
as trace width decreases. Vias and connectors that are a part of the interconnect also
introduce losses. Total loss allowed on the interconnect is 13.2 dB at 1.25 GHz.
Jitter, including jitter from crosstalk, consists of random and deterministic contributions
to signal edge timing that reduce the signal valid time. The total jitter on the
interconnect allowable is 120 ps.
Crosstalk is the coupling of energy from one signal trace to the other which results in
the signal voltage and phase change. There are two kinds of crosstalk, Far-End (FEXT)
and Near-End (NEXT). Whether the transmitter and receiver pairs are interleaved, the
Far-End can appear at the transmitter or receiver. Crosstalk within the differential pair
is not a concern. Crosstalk between differential pairs can be minimized by keeping a
large pair-to-pair spacing compared to spacing within a pair. Stripline traces show far
less Far-End crosstalk (FEXT) than microstrip.
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Intel EP80579 Integrated Processor Product Line
Order Number: 320068-005US
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EP80579 Integrated Processor Product Line
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