Intel 317698-001 Ethernet Component Design Guidelines, Clock Source, Magnetics for 1000 BASE-T

82575 Ethernet Controller Design Guide

3.0Ethernet Component Design Guidelines

These sections provide recommendations for selecting components and connecting special pins.

For 1000 BASE-T designs, the main design elements are the 82575 Gigabit Ethernet Controller, an integrated discrete or magnetics module with RJ-45 connector, an EEPROM, and a clock source.

3.1General Design Considerations for Ethernet Controllers

Follow good engineering practices with respect to unused inputs by terminating them with pull-up or pull-down resistors, unless the datasheet, design guide or reference schematic indicates otherwise. Do not attach pull-up or pull-down resistors to any balls identified as No Connect. These devices may have special test modes that could be entered unintentionally.

3.1.1Clock Source

All designs require a 25 MHz clock source. The 82575 Gigabit Ethernet Controller uses the 25 MHz source to generate clocks up to 125 MHz and 1.25 GHz for the PHY circuits, and 1.25 GHz for the SERDES. For optimum results with lowest cost, connect a 25 MHz parallel resonant crystal and appropriate load capacitors at the XTAL1 and XTAL2 leads. The frequency tolerance of the timing device should be 30 ppm or better. Refer to the application note, Intel Fast Ethernet Controllers Timing Device Selection Guide, AP-419, for more information on choosing crystals.

For further information regarding the clock for the 82575, see the sections about frequency control, crystals, and oscillators later in this document.

3.1.2Magnetics for 1000 BASE-T

Magnetics for the 82575 can be either integrated or discrete.

The magnetics module has a critical effect on overall IEEE and emissions conformance. The device should meet the performance required for a design with reasonable margin to allow for manufacturing variation. Occasionally, components that meet basic specifications may cause the system to fail IEEE testing because of interactions with other components or the printed circuit board itself. Carefully qualifying new magnetics modules prevents this problem.

When using discrete magnetics it is necessary to use Bob Smith termination: Use four 75 Ω resistors for cable-side center taps and unused pins. This method terminates pair- to-pair common mode impedance of the CAT5 cable.

Use an EFT capacitor attached to the termination plane. Suggested values are 1500 pF/ 2KV or 1000 pF/3KV. A minimum of 50-mil spacing from capacitor to traces and components should be maintained.

3.1.2.1Magnetics Module Qualification Steps

The steps involved in magnetics module qualification are similar to those for crystal qualification:

1.Verify that the vendor’s published specifications in the component datasheet meet or exceed the required IEEE specifications.

2.Independently measure the component’s electrical parameters on the test bench, checking samples from multiple lots. Check that the measured behavior is

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