32
Routing Guidelines
4.2
Crosstalk
Crosstalk is caused by capacitive and inductive coupling between signals. Crosstalk is composed of
both backward and forward crosstalk components. Backward crosstalk creates an induced signal on
victim network that propagates in the opposite direction of the aggressor signal. Forward crosstalk
creates a signal that propagates in the same direction as the aggressor signal.
Circuit board analysis software is used to analyze your board layout for crosstalk problems. Examples
of 2D analysis tools include Parasitic Parameters from
ANSOFT
*
and XFS from
Quad Design
*
.
Crosstalk problems occur when circuit etch lines run in parallel. When board analysis software is not
available, the layout needs to be designed to maintain at least the minimum recommended spacing for
bus interfaces.
•
A general guideline to use is, that space distance between adjacent signals be a least 3.3 times
the distance from signal trace to the nearest return plane. The coupled noise between adjacent
traces decreases by the square of the distance between the adjacent traces.
•
It is also recommended to specify the height of the above reference plane when laying out
traces and provide this parameter to the PCB manufacturer. By moving traces closer to the
nearest reference plane, the coupled noise decreases by the square of the distance to the
reference plane.
•
Avoid slots in the ground plane. Slots increases mutual inductance thus increasing crosstalk.
•
Throughout the design guide unbroken GND reference planes are recommended. If it is not
possible to route over an unbroken ground plane then an unbroken power plane is acceptable.
If it is necessary to use power plane referencing, it is better to reference the power plane used
by the I/O connector (if applicable). It is also recommended to add decoupling to the connector
near the pins.
•
Make sure that ground plane surrounding connector pin fields are not completely cleared out.
When this area is completely cleared out, around the connector pins, all the return current must
flow together around the pin field increasing crosstalk. The preferred method of laying out a
connector in the GND layer is shown in
B.
Figure 10.
Crosstalk Effects on Trace Distance and Height
A9259-01
Reference Plane
P
aggressor
victim
H
Reduce Crosstalk:
- Maximize P
- Minimize H
Summary of Contents for 80331
Page 1: ...Intel 80331 I O Processor Design Guide March 2005 Order Number 273823 003 ...
Page 30: ...Intel 80331 I O Processor Design Guide Terminations 30 This Page Intentionally Left Blank ...
Page 122: ...122 Intel 80331 I O Processor Design Guide Memory Controller ...
Page 136: ...Intel 80331 I O Processor Design Guide Power Delivery 136 This Page Intentionally Left Blank ...