Mobile Intel Pentium 4 Processor - M and Intel 845MP/MZ Chipset Platform Design Guide

ManualsBrandsIntel ManualsOtherIntel Pentium 4 Processor supporting HT Technology 3.00E GHz, 1M Cache, 800 MHz FSB

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Mobile Intel

Pentium

4 Processor-M and Intel

845MP/845MZ Chipset Platform

Design Guide 139

The most sensitive signal returns closest to the chassis ground should be connected together. This will

result in a smaller loop area and reduce the likelihood of crosstalk. The effect of different configurations

on the amount of crosstalk can be studied using electronics modeling software.

9.10.4.6.1. Terminating Unused Connections

In Ethernet designs it is common practice to terminate unused connections on the RJ-45 connector and

the magnetic module to ground. Depending on overall shielding and grounding design, this may be done

to the chassis ground, signal ground, or a termination plane. Care must be taken when using various

grounding methods to insure that emission requirements are met. The method most often implemented is

called the “Bob Smith” Termination. In this method a floating termination plane is cut out of a power

plane layer. This floating plane acts as a plate of a capacitor with an adjacent ground plane and couples

capacitively to the ground plane creating the required 1500 pF of capacitance. The signals can be routed

through 75-

Ω resistors to the plane. Stray energy on unused balls is then carried to the plane.

9.10.4.6.2. Termination Plane Capacitance

It is recommended that the termination plane capacitance equal a minimum value of 1500 pF. This helps

reduce the amount of crosstalk on the differential pairs (TDP/TDN and RDP/RDN) from the unused

pairs of the RJ45. Pads may be placed for an additional capacitance to chassis ground, which may be

required if the termplane capacitance is not large enough to pass EFT (Electrical Fast Transient) testing.

If a discrete capacitor is used, to meet the EFT requirements it should be rated for at least 1000 Vac.

Figure 72. Termination Plane

N/C

RJ-45

Magnetics Module

RDP

RDN

TDP

TDN

Termination Plane

Addition Capacitance that may need to be

added for EFT testing

9.10.5. 82562ET/82562EH Dual Footprint Guidelines

These guidelines characterize the proper layout for a dual footprint solution. This configuration enables

the developer to install either the 82562EH or the 82562ET/82562EM components while having only

one motherboard design. The following are guidelines for the 82562ET/82562EH Dual Footprint option.