9880 datasheet(65/118 Pages) INTEL

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Advanced 10/100 Repeater with Integrated Management — LXT9860/9880

Datasheet

Document #: 248987

Revision#: 003

Rev Date: 08/07/01

4.3.2

MII Terminations

The LXT98x0 MIIs have high output impedance (250-350

Ω). To minimize reflections, serial

termination resistors are recommended on all MII signals, especially with designs with long traces

(>3 inches). Place the resistor as close to the device as possible. Use a software trace termination

package to select an optimal resistance value for the specific trace. Proper value = nominal trace

impedance minus 13

Ω. If a software package cannot be used and nominal trace impedance is not

known, use 55

Ω.

4.3.3

Twisted-Pair Interface

The LXT98x0 transmitter uses standard 1:1 magnetics for both receive and transmit. Nonetheless,

system designers should take precautions to minimize parasitic shunt capacitance and meet return

loss specifications. These steps include:

Figure 21. Power and Ground Connections

RBIAS

VCC

GND

VCCR

GND

.01

µF

22.1 k

Ω1%

GND

VCC

0.1

µF

Ferrite

Beads

µF

+3.3V

LXT9880

VCCT

GND

µF

.01

µF

Digital Supply Plane

Analog Supply Plane

.01

µF

To Output Magnetics Centertap

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