Tuesday, May 5, 2009

High Speed USB Platform Design Guidelines - USB Trace

High Speed USB Trace Spacing

Use the following separation guidelines. Picture below provides an illustration of the recommended trace spacing.

  1. Maintain parallelism between USB differential signals with the trace spacing needed to achieve 90 ohms differential impedance. Deviations will normally occur due to package breakout and routing to connector pins. Just ensure the amount and length of the deviations are kept to the minimum possible.
  2. Use an impedance calculator to determine the trace width and spacing required for the specific board stackup being used. For the board stackup parameters, 7.5- mil traces with 7.5-mil spacing results in approximately 90 ohms differential trace impedance.
  3. Minimize the length of high-speed clock and periodic signal traces that run parallel to high speed USB signal lines , to minimize crosstalk. Based on EMI testing experience, the minimum suggested spacing to clock signals is 50 mils.
  4. Based on simulation data, use 20-mil minimum spacing between high-speed USB signal pairs and other signal traces for optimal signal quality. This helps to prevent crosstalk.

High Speed USB Termination

Use the following termination guidelines.

  1. High-speed USB designs require parallel termination at both the transmitter and receiver. For host controller designs that use external termination resistors, place the termination resistors as close as possible to the host controller signal pins. Recommend less than 200 mils if possible. Follow the manufacturer’s recommendation for the termination value needed to obtain the required 45 ohm-toground parallel HS termination.
  2. For downstream ports, a 15 kW pull down resistor on the connector side of the termination is required for device connection detection purposes. Note that this pull down might be integrated into the host controller silicon. Follow the manufacturer’s recommendation for the specific part used.
  3. A common mode (CM) choke should be used to terminate the high speed USB bus if they are need to pass EMI testing. Place the CM choke as close as possible to the connector pins. See Section 5.1 for details.

Note: Common mode chokes degrade signal quality, thus they should only be used if EMI is a known problem.

High Speed USB Trace Length Matching

Use the following trace length matching guidelines.

  1. High-speed USB signal pair traces should be trace-length matched. Max trace-length mismatch between
  2. High-speed USB signal pairs (such as, DM1 and DP1) should be no greater than 150 mils.

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