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[FAQ] TUSB212: Why does USB 2 (HS) stop working when multiple USB 2 redrivers are enabled?

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Part Number: TUSB212

I have a system where I have implemented USB 2 that uses long traces and cables. To accommodate for the long traces and cables and still pass the USB 2 eye mask compliance test I used two USB 2 redrivers. When both USB 2 redrivers are enabled my system can pass the USB 2 High Speed (HS) eye mask test but when I plug in a USB 2 HS device the USB 2 data drops out, is there an explanation for this? 

  • In the case when two or more USB 2 redrivers are necessary to compensate for long traces and cables high AC and high DC boosting may be selected. When more than one USB 2 redriver is used with high DC boost the USB controller and/or USB endpoint may see a false USB High Speed Disconnect disconnect state on the bus. This will occurs when the peak to peak voltage of the high speed USB 2 data exceeds ~525 mV.

    When DC boost is desired it is recommended to use the lowest DC boost possible on both USB 2 redrivers and ensure that 45 termination is seen on both DP and DM to avoid this issue. Higher than nominal USB 2 HS termination can also contribute to this issue. For more specifics on the USB 2 High Speed Disconnect State please refer to Table 7-3. High-speed Signaling Levels of the USB 2 specification.