General Layout Considerations For Ethernet Controllers; Guidelines For Component Placement - Intel 82541PI Design Manual

4.0 General Layout Considerations for Ethernet
Controllers
Critical signal traces should be kept as short as possible to decrease the likelihood of being affected
by high frequency noise from other signals, including noise carried on power and ground planes.
Keeping the traces as short as possible can also reduce capacitive loading.
Since the transmission line medium extends onto the PCB, special attention must be paid to layout
and routing of the differential signal pairs.
The differential signal pairs should not be routed near each other over long distances. Routing them
too close to each other induces crosstalk and can also cause common mode noise that degrades the
long cable receive bit error rate performance. To prevent these problems, separate the differential
pairs from each other by at least 5 times the height from the GND plane
separate them by 35 mils on a 7 mil thick prepreg.
Designing for Gigabit operation is very similar to designing for 10/100 Mbps. For the 82541PI(ER)
controller, system level tests should be performed at all three speeds.
4.1

Guidelines for Component Placement

Component placement can affect signal quality, emissions, and component operating temperature
This section provides guidelines for component placement.
Careful component placement can:
•
Decrease potential problems directly related to EMI, which could cause failure to meet
applicable government test specifications.
•
Simplify the task of routing traces. To some extent, component orientation affects the
complexity of trace routing. One overall objective is to minimize turns and crossovers between
traces.
The following recommendations can be used as guidelines in the layout phase of the design
implementation:
1. The Ethernet controller should be located at least one inch away from the RJ45 connector
(distance D in
2. The Ethernet controller should be placed at least two inches from the PCB edge (distance E in
Figure
Minimizing the amount of space needed for the Ethernet LAN interface is important because other
interfaces will compete for physical space on a LOM near the connector. The Ethernet LAN
circuits need to be as close as possible to the connector.
Figure 4 shows some basic placement distance guidelines.
pairs, but can be generalized for a Gigabit system with four differential pairs. The ideal placement
for the Ethernet controller would be approximately one inch behind the magnetics module. Also
note that the TX and RX termination resistors are located close to the Ethernet controller. This is
vital for meeting IEEE physical layer conformance tests.
Application Note (AP-468)
82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Figure 5).
5).
(Figure 4). For example,
Figure 4 also shows two differential
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