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

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Mobile Intel

Pentium

4 Processor-M and Intel

845MP/845MZ Chipset Platform

Design Guide 109

• USB signals should be ground referenced.

• Route USB signals using a minimum of vias and corners. This reduces reflections and impedance

changes.

• When it becomes necessary to turn 90°, use two 45° turns or an arc instead of making a single 90°

turn. This reduces reflections on the signal by minimizing impedance discontinuities.

• Do not route USB traces under crystals, oscillators, clock synthesizers, magnetic devices or IC’s

that use and/or duplicate clocks.

• Stubs on USB signals should be avoided, as stubs have an effect on signal quality. If a stub is

necessary in the design, no stub should be greater than 200 mils.

• Route all traces over continuous planes, (VCC or GND) with no interruptions. Avoid crossing over

anti-etch if at all possible. This increases inductance and radiation levels by forcing a greater loop

area. Likewise, avoid changing layers with high-speed traces.

• Keep USB signals clear of the core logic set. High current transients are produced during internal

state transitions, which can be very difficult to filter out.

• Keep traces at least 50 mils away from the edge of the plane. This helps prevent the coupling of the

signal onto adjacent wires and also helps prevent free radiation of the signal from the edge of the

PCB.

9.4.2. USB Trace Separation

Use the following separation guidelines.

• Recommended trace width and separation is 4-mil trace with a 6-mil space (90-Ω differential

impedance).

The goal is to have a 90-Ω differential impedance and the spacing may need to be different depending on

the stackup.

• Maintain parallelism between USB differential signals with the trace spacing needed to achieve 90-

Ω differential impedance.

• Use at a minimum 20-mil spacing between USB signal pair and other traces on the PCB. This helps

to prevent crosstalk. If possible, keep clock and PCI traces at least 50 mils from the USB

differential pairs.

• Minimize the length of high-speed clock and periodic signal traces that run parallel to USB signal

lines to minimize crosstalk.

9.4.3. USB Trace Length Matching

Use the following trace length matching guidelines.

• USB signal pair traces should be trace length matched.