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Good USB Routing Practices

A quick summary of Good Usb Practices

Trace length Matching

  • Intel suggests that USB traces be matched to 150 mils within each other in length.
  • Trace spacing is debated.
    • Intel suggests 20 mils spacing between the USB traces.
    • Texas Instruments suggests a minimum of 30 mils.
    • Other sources suggest 5x the trace widths as trace spacing.
  • To forcefully match trace lengths, bend traces in a “zigzag” pattern at the mismatched end.
    • Diagram from Texas Instruments PDF
      mnf-9-uui-7-rpwfntkz-0-ctxca.png

Signal Integrity

  • Run USB lines in parallel at all times.
  • Avoid jumping voids in ground planes.
  • Lines may have to be routed non-parallel near connectors and controllers. If so, make sure the non-parallel area is within 0.25 inches of the connector and controller.
  • Don’t bend USB signal lines at right angles.
  • Try keeping the traces at least 100 mil from the edges of ground planes.
  • Don’t run the USB signals in parallel to other pulsating traces such as the keyboard matrix or LED backlighting. If this is required, keep the traces at least 50 mils away.
  • When the traces must go through parts (I.E. the 22-ohm resistors), keep the layout symmetric
    • Diagram from Texas Instruments PDF
      errlmphstbkuv-4-reodumla.png

Sources

http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf
http://www.ti.com/lit/an/spraar7g/spraar7g.pdf

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