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TUSB8044A: USB_VBUS pin connection purpose

Abhishek singh
Expert 2000 points
Part Number: TUSB8044A

Hi  Team 

In our  desig we  are  connecting  USB HUB  upstream port  to  Nvidia  ( SSTX, SSRX, DP and DM )  there  is  no  connection  of  USB  VBUS  to   Nvdia . I jhave  following  query 

1) Does  volatge  on VBUS  is  must ?

2) What is  purpose  of this  piin

3) How  we  should terminate  this  pin  incase  we  are  not  connecting this  to VBUS  

4)  Can we  connect  VBUS  of  Downstram port  to  this  pin? 

  • 0
    TI__Guru 60870 points

    Hi Abhishek,

    USB_VBUS is a detection input and should not be connected directly to a power rail.  It should be driven by a signal that will indicate that the host is powered and ready to connect to the hub.  The upstream port of the hub will use this input to help determine its behavior. The resistor values can be adjusted for various input levels so that the pin is ~500 mV when input is asserted.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Expert 2000 points

    Hi JMMN '

    We  are  connecting USB hub  with  NVIDIA on same board . It will be  used  as  self  powered  non detachable  hub ie  UP stream  port  will be  always  connected  to the  Nvidia . In  this  case how  we  should make  connection to  this  pin.  

  • 0 in reply to Abhishek singh
    TI__Guru 60870 points

    Abhishek,

    Can it be controlled by the NVIDIA host?  My concern is that if USB_VBUS is on before the host is ready and the host leaves its receiver terminations on by default (some hosts do), then the hub upstream port will enter compliance mode.  I would suggest have some control over the USB_VBUS or the GRSTz to the hub to prevent interoperability issues.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Expert 2000 points

    Hi  JMMN Sorry somehow I missed this reply. Yes, GRSTz can be controlled by  NVIDIA if it must. In our current configuration power, supply of  HUB will be enabled by NVIDIA only. My doubt is since there is no  VBUS  pin on NVIDIA  to supply power to  VBUS  pin (Only superspeed pairs and DP and DM ) are connected. Shall  I  connect  5V  to VBUS  through some controllable  load  switch or  MOSFET .

  • 0 in reply to Abhishek singh
    TI__Guru 60870 points

    Hi Abhishek,

    If there is any voltage rail in the system or output that will only be high when the host is on and ready to connect to the hub, that can be used to drive USB_VBUS.  It doesn't need to be a 5V input, the resistor values in the divider network can be modified to accommodate different levels while still allowing the pin to be ~500mV when high.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Expert 2000 points

    Thanks, JMMN

    You can mark this issue as resolved.