126.96.36.199.1 General Routing and Placement
Use the following routing and placement guidelines when laying out a new design for the USB physical layer (PHY). These guidelines help minimize signal quality and electromagnetic interference (EMI) problems on a four-or-more layer evaluation module (EVM).
- Place the USB PHY and major components on the un-routed board first. For more details, see Section 188.8.131.52.2.3.
- Route the high-speed clock and high-speed USB differential signals with minimum trace lengths.
- Route the high-speed USB signals on the plane closest to the ground plane, whenever possible.
- Route the high-speed USB signals using a minimum of vias and corners. This reduces signal 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 traces by minimizing impedance discontinuities.
- Do not route USB traces under or near crystals, oscillators, clock signal generators, switching regulators, mounting holes, magnetic devices or IC’s that use or duplicate clock signals.
- Avoid stubs on the high-speed USB signals because they cause signal reflections. If a stub is unavoidable, then the stub should be less than 200 mils.
- Route all high-speed USB signal traces over continuous planes (VCC or GND), with no interruptions. Avoid crossing over anti-etch, commonly found with plane splits.