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TUSB8020B: Is AC coupling needed for internal upstream connection?

Ryotaro Fukui
Intellectual 2345 points
Part Number: TUSB8020B

Hello team,

  • If the upstream port is connected to an internal CPU, do we need to have these signals ac coupled?
  • If coupling is needed, should only TX side(CPU->USBHUB) be OK, or RX side(USBHUB->CPU) needs coupling too.

From datasheet 9.2.2 graphs, it could be read that AC coupling capacitor is needed between USB_SSTXx pins and connector.

Also, the E2E thread below was about similar case but was about downstream port.

> TUSB8020B: AC coupling of internally used downstream tx signals

Best Regards,

Ryotaro Fukui

  • 0
    TI__Guru 64326 points

    Hi,


    Your issue is being assigned to the appropriate engineer. Due to the US Holiday there may be a delay in response to your post. We apologize for any inconvenience.

  • 0
    TI__Guru 60870 points

    Hello,

    Yes, the AC caps are required on an internal USB 3.x connection.  There will be caps on both sets of SS lines, the TX lines of the hub connect to the RX lines of the CPU and the TX lines of the CPU connect to the RX lines of the hub.

    Regards,

    JMMN

  • 0 in reply to JMMN
    TI__Intellectual 2345 points

    Hello JMMM,

    Thank you for your feedback. I understand that coupling capacitor is required for both SS lines.

    I would also like to confirm about the placement. Would it be OK to place it near the correspondent node, in this case the CPU side?

    Since the EVM implemented it near the connector, the downstream sending side, will it be reversed in the upstream case? Or should it always be near the correspondent side? (Connector, CPU side)

    Best Regards,

    Ryotaro Fukui

  • 0 in reply to Ryotaro Fukui
    TI__Guru 60870 points

    Hi,

    We recommend that the AC caps be placed closer to the transmitter on each side of the SS connection, however, in an embedded system the traces may be short and placement can be varied if needed.

    Regards,

    JMMN