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Intel
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820E Chipset
R
Design Guide 107
2.22.2. General LAN Routing Guidelines and Considerations
2.22.2.1. General Trace Routing Considerations
Trace routing considerations are important to minimize the effects of crosstalk and propagation delays on
board sections where high-speed signals exist. Signal traces should be kept as short as possible to
decrease interference from other signals, including those propagated through the power and ground
planes.
Comply with the following suggestions, to help optimize board performance:
• The maximum mismatch between the length of the clock trace and the length of any data trace is
0.5 inch.
• Maintain constant symmetry and spacing between the traces within a differential pair.
• Keep the signal trace lengths of a differential pair equal to each other.
• Keep the total length of each differential pair under 4 inches. (Many customer designs with
differential traces longer than 5 inches have had one or more of the following issues: IEEE phy
conformance failures, excessive EMI, and/or degraded receive BER.)
• Do not route the transmit differential traces closer than 100 mils from the receive differential traces.
• Do not route any other signal trace both parallel to the differential traces and closer than 100 mils
from the differential traces (300 mils recommended).
• Keep the maximum separation between differential pairs to 7 mils.
• For high-speed signals, the number of corners and vias should be minimized. If a 90° bend is
required, two 45° bends should be used instead. Refer to Figure 68.
• Traces should be routed away from board edges by a distance greater than the trace height above the
ground plane. This allows the field around the trace to couple more easily to the ground plane,
rather than to adjacent wires or boards.
• Do not route traces and vias under crystals or oscillators. This will prevent coupling to or from the
clock. And as a general rule, place traces from clocks and drives at a minimum distance from
apertures, at a distance greater than the largest aperture dimension.
Figure 68. Trace Routing
45