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TUSB8020B: Power switch

Mariem Chaabani
Intellectual 255 points
Part Number: TUSB8020B

Hello everyone, and Merry Christmas!

I have an MCU to which I have only one USB3.0 interface, so I decided to use a HUB. But I have some concerns and would like to confirm a few things about TUSB8020B.

1- The MCU is the one that must be common point. The other 2 USB sources must go to the MCU, so the MCU is the one that is linked to the upstream port, right?

2- The MCU's USB communicates with both a Module through USB connection but without VBUS on one hand, and an external USB-A connector that comes with a USB_VBUS on the other hand. So I attached the USB_VBUS of the MCU to an external source (5V power supply on the board). In this case, we do not need to use the power switch, do we? So no PWRCTL, no OVERCURxZ, pull-up on FULLPWRMGMT (power switch not supported), and I would put a pull down on GANGED or NC?

3- I saw on an other thread this answer: "TUSB8020B is an active part of the USB system, so RX of the upstream port goes to TX of the host and TX of the upstream port goes to the RX of the host. Likewise, the RX of the downstream port goes to TX of the device and TX of the downstream port goes to RX of the device. It would look like the bottom drawing in the photo that was attached." Here is the link to the thread to see the image they are talking about: https://e2e.ti.com/support/interface/f/138/t/755697?tisearch=e2e-sitesearch&keymatch=TUSB8020B%252525252520power%252525252520switch

But it is not the case from what I see on the Dev board, TX is attached to TX and RX to RX on both upstream and downstream. Are they talking about the internal function of the hub?

Thank you, and happy holidays!

  • 0
    TI__Guru 60865 points

    Hello Mariem,

    1.  Yes, the MCU would be connected to the upstream port of the hub.

    2.  The hub does not require power switching to be enabled, but the USB_VBUS input of the hub is a detection input and should not be pulled high unless the host is active.  We do recommend overcurrent protections be enabled on any exposed ports.

    3.  On the development board, the hub is being routed to connectors and the swap between TX and RX will be handled inside the external cable.  For a on-board hub to host or hub to device connection, the swap must be done as part of the design.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Intellectual 255 points

    Thank you JMMN! 

    This has been very helpful.

    Just a clarification about the second point please:

    "but the USB_VBUS input of the hub is a detection input and should not be pulled high unless the host is active."

    I've put 5V on the USB_VBUS of the hub (with a divider with resistors 90.9k and 10k as recommended). This 5V being also the power supply of the MCU (host), is it ok or should I introduce some sort of delay for the USB_VBUS of the hub, controlled by the MCU for example?

    Thank you again.

  • 0 in reply to Mariem Chaabani
    TI__Guru 60865 points

    Hi Mariem,

    It is best for interoperability to have the USB_VBUS input controlled by the host.  If that is not possible, adding a delay would be the next best option.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Intellectual 255 points

    Hello JMMN,

    Thank you so much for your help JMMN.

    And happy new year!