Tuesday, May 5, 2009

High Speed USB Platform Design Guidelines - Common Routing Mistakes


A very common routing mistake is shown in Figure 4. Here the CAD designer could have avoided creating unnecessary stubs by proper placement of the pull down resistors over the path of the data traces. Once again, if a stub is unavoidable in the design, no stub should be greater than 200 mils.

Poor Routing Techniques

Picture below demonstrates several violations of good routing practices for proper impedance control and signal quality of high speed USB signaling.

Crossing a plane split
  • The mistake shown here is where the data lines cross a plane split. This causes unpredictable return path currents and would likely cause a signal quality failure as well as creating EMI problems.

Creating a stub with a test point
  • Here is another example where a stub is created that could have been avoided. Stubs typically cause degradation of signal quality and can also affect EMI.

Failure to maintain parallelism
  • Picture below is also a classic example of a case where parallelism was not maintained, when it could have been. The red trace (the lighter trace farthest to the right with the “x” on it) shows the wrong way to route to the connector pins. The green trace (the darker trace in the middle) shows the correct way. Failing to maintain parallelism will cause impedance discontinuities that will directly affect signal quality. In this case it also contributes to the trace-length mismatch and will cause an increase in signal skew.

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