lxt9781 Intel Corporation, lxt9781 Datasheet - Page 44

• Manufacturer Part Number: lxt9781
• Description: Fast Ethernet 10/100 Multi-port Transceiver With Rmii
• Manufacturer: Intel Corporation

LXT9761/9781 — Fast Ethernet 10/100 Multi-Port Transceiver with RMII
3.0
3.1
3.1.1
3.1.2
44
Application Information
Design Recommendations
The LXT97x1 is designed to comply with IEEE requirements and to provide outstanding receive
Bit Error Rate (BER) and long-line-length performance. To achieve maximum performance from
the LXT97x1, attention to detail and good design practices are required. Refer to the LXT97x1
Design and Layout Guide for detailed design and layout information.
General Design Guidelines
Adherence to generally accepted design practices is essential to minimize noise levels on power
and ground planes. Up to 50 mV of noise is considered acceptable. 50 to 80 mV of noise is
considered marginal. High-frequency switching noise can be reduced, and its effects can be
eliminated, by following these simple guidelines throughout the design:
Power Supply Filtering
Power supply ripple and digital switching noise on the VCC plane can cause EMI problems and
degrade line performance. The best approach is to minimize ground noise as much as possible
using good general techniques and by filtering the VCC plane. It is generally difficult to predict in
advance the performance of any design, although certain factors greatly increase the risk of having
problems:
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Fill in unused areas of the signal planes with solid copper and attach them with vias to a VCC
or ground plane that is not located adjacent to the signal layer.
Use ample bulk and decoupling capacitors throughout the design (a value of .01 F is
recommended for decoupling caps).
Provide ample power and ground planes.
Provide termination on all high-speed switching signals and clock lines.
Provide impedance matching on long traces to prevent reflections.
Route high-speed signals next to a continuous, unbroken ground plane.
Filter and shield DC-DC converters, oscillators, etc.
Do not route any digital signals between the LXT97x1 and the RJ45 connectors at the edge of
the board.
Do not extend any circuit power and ground plane past the center of the magnetics or to the
edge of the board. Use this area for chassis ground, or leave it void.
Poorly-regulated or over-burdened power supplies
Wide data busses (32-bits+) running at a high clock rate
DC-to-DC converters
Datasheet