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TUSB2551A USB transceiver in upstream facing port versus downstream facing port applications

CapnStatic
Prodigy 10 points
Other Parts Discussed in Thread: TUSB2551A

I am currently working with the TUSB2551A USB transceiver and would like to ask for clarification regarding the need for external 15K ohm pulldown resistors in a downstream facing (USB host side) application.  Although the spec sheet indicates that this part is USB 2.0 compliant, it is ambiguous as to whether the part has these 15K ohm pulldown resistors internally.  On an upstream facing (USB client) application, these pulldown resistors should NOT be part of the circuit (you would only want the 1.5K pullup resistor which is configurable on this part through the SOFTCON line).  The 15K ohm pulldown resistors, if they are there, do not seem to be configurable.  I would like to better understand how this part can seemingly be used for both upstream facing port versus downstream facing port applications (if it can at all).

  • 0
    TI__Guru* 76130 points

    Hello,

    From a USB HUB perspective, the 15k pull-down is not required on its upstream port but on its downstream ports.

    So, if you are using the PHY within a USB host, then you don't need the 15k pull-down on the D+/- signals of the PHY.

    If you are using the PHY within a downstream device, then the USB host/hub/controller with it is gonna be communicating with should implement the 15K pull-down on its downstream ports which will be facing to the D+/- lines of the PHY.

     

    Regards.