How to Check Whether a USB 2.0 Hub is Multi-TT or Single-TT - KBA93279

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    Question: After the USB 2.0 hub has enumerated, how do you check whether it is a multiple transaction translator (multi-TT) or single transaction translator (single-TT) hub?



    The value of the bDeviceProtocol field of the USB device descriptor and the bInterfaceProtocol field of the USB interface descriptor indicate whether a USB 2.0 hub is single-TT or multi-TT and its current working mode.

    Table 1. bDeviceProtocol and bInterface Fields of Single-TT and Multi-TT USB 2.0 Hubs

         bDeviceProtocol field of
         Device descriptor
         bInterface protocol of
         Interface descriptor 1
         Interface descriptor 2

    For a single-TT USB 2.0 hub, there will be only one interface descriptor. For a multi-TT USB 2.0 hub, there will be two interface descriptors. When a USB 2.0 mutli-TT hub is configured with an interface protocol of one ‘1’, it will operate as a single-TT organized hub. When a USB 2.0 multi-TT hub is configured with an interface protocol of ‘2’, it will operate as a multi-TT organized hub. The TT organization must not be changed while the hub has full- or low-speed transactions in progress.

    Note: Once the device has enumerated, you can view the descriptors using USB View or USBLYZER free software tools.

    If you want to know whether the multi-TT hub enumerated as single-TT (with interface descriptor ‘1’) or multi-TT (with interface descriptor ‘2’), capture the trace during its enumeration (using USB analyzers like Ellisys™ or Lecroy®) and check the SET INTERFACE control transfer.