Saturday, July 11, 2009

Gigabit Ethernet Controller Design --> General 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.

Clock Source

All designs require a 25 MHz clock source. The 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.

Magnetics for 1000 BASE-T

Magnetics for the Gigabit Ethernet 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 pairto- 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.

Magnetics Module Qualification Steps

The steps involved in magnetics module qualification are similar to those for crystal
  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 consistent from sample to sample and that measurements meet the published specifications.
  3. Perform physical layer conformance testing and EMC (FCC and EN) testing in real
  4. systems. Vary temperature and voltage while performing system level tests.

Modules for 1000 BASE-T Ethernet

Magnetics modules for 1000 BASE-T Ethernet are similar to those designed solely for
10/100 Mbps, except that there are four differential signal pairs instead of two. Use the
following guidelines to verify specific electrical parameters:
  1. Verify that the rated return loss is 19 dB or greater from 2 MHz through 40 MHz for 100/1000 BASE-TX.
  2. Verify that the rated return loss is 12 dB or greater at 80 MHz for 100 BASE-TX (the specification requires greater than or equal to 10 dB).
  3. Verify that the rated return loss is 10 dB or greater at 100 MHz for 1000 BASE-TX (the specification requires greater than or equal to 8 dB).
  4. Verify that the insertion loss is less than 1.0 dB at 100 kHz through 80 MHz for 100 BASE-TX.
  5. Verify that the insertion loss is less than 1.4 dB at 100 kHz through 100 MHz for 1000 BASE-T.
  6. Verify at least 30 dB of crosstalk isolation between adjacent channels (through 150 MHz).
  7. Verify high voltage isolation to 15000 Vrms. (Does not apply to discrete magnetics.)
  8. Transmitter OCL should be greater than or equal to 350 μH with 8 mA DC bias.

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