Tuesday, May 5, 2009

High Speed USB Platform Design Guidelines - Layout Guidelines

General Routing and Placement

Use the following general routing and placement guidelines when laying out a new design. These guidelines will help to minimize signal quality and EMI problems. The high speed USB validation efforts focused on a four-layer motherboard where the first layer is a signal layer, the second layer is power, the third layer is ground and the fourth is a signal layer. This results in placing most of the routing on the fourth layer closest to the ground layer, and allowing a higher component density on the first layer.

  1. Place the high-speed USB host controller and major components on the unrouted board first.
  2. With minimum trace lengths, route high-speed clock and high-speed USB differential pairs first. Maintain maximum possible distance between high-speed clocks/periodic signals to high speed USB differential pairs and any connector leaving the PCB (such as, I/O connectors, control and signal headers, or power connectors).
  3. Route high-speed USB signals on bottom whenever possible.
  4. Route high-speed USB signals using a minimum of vias and corners. This reduces signal reflections and impedance changes.
  5. 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.

  6. Do not route USB traces under crystals, oscillators, clock synthesizers, magnetic devices or ICs that use and/or duplicate clocks.
  7. Stubs on high speed USB signals should be avoided, as stubs will cause signal reflections and affect signal quality. If a stub is unavoidable in the design, no stub should be greater than 200 mils.
  8. Route all traces over continuous planes (VCC or GND), with no interruptions. Avoid crossing over anti-etch if at all possible. Crossing over anti-etch (plane splits) increases inductance and radiation levels by forcing a greater loop area. Likewise, avoid changing layers with high-speed traces as much as practical. It is preferable to change layers to avoid crossing a plane split. Refer to Section 3.6 Plane Splits, Voids and Cut-Outs (Anti-Etch) for more details on plane splits.
  9. Separate signal traces into similar categories and route similar signal traces together (such as routing differential pairs together).
  10. Keep high-speed USB signals clear of the core logic set. High current transients are produced during internal state transitions and can be very difficult to filter out.
  11. Follow the 20*h thumb rule by keeping traces at least 20*(height above the plane) away from the edge of the plane (VCC or GND, depending on the plane the trace is over). For the suggested stackup the height above the plane is 4.5 mils. This calculates to a 90-mil spacing requirement 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.

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