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TUSB2046 Resets With ESD

Ivon Cullumber
Prodigy 20 points
Other Parts Discussed in Thread: TUSB2046B, SN75240

I have a USB hub that I have put together that utilizes the TUSB2046B in order to multiplex the upstream port to 3 downstream ports and a FT232RL USB to 
UART converter that is used for a RS485 port.  Everything on it works great with one exception: Occasionally the TUSB2046B will reset when there is a static electricity discharge near the system.  It doesn't happen every time, but it can average anywhere from 1 out of 100 to 1 out of 20 times.  

We have the hub connected to a hand held tablet; however, this problem has also been reproduced when connected to a desktop.  The signal going from the host to the hub indicates that the reset is software driven.  The host tells the TUSB2046B to reset.  That, in turn, resets all devices that the TUSB2046B is connected to on the downstream ports.  

Does anyone have any explanations for this behavior?  And, more importantly, does anyone have any suggestions on how to fix it?

  • 0
    TI__Mastermind 18655 points


    Hello Ivon,

    The TUSB2046B passed the standard ESD testing and does not incoroporate any additional IEC61000-4-2 ESD protections. The testing levels were:
    HBM 4000 V
    CDM 1500 V

    Do you have any external ESD protection in the circuit?  Something like the SN75240 might fix this.

    While it is not absolutely imperative to have additional ESD protection on differential pair, depending on the kind of implementation and the amount of ESD sensitive circuits in the design, customer have used them in their implementations. However, it is important to make sure the impedance requirement on the differential pair meets the USB 2.0 specification even with the additional ESD circuit.

    Regards

  • 0
    Prodigy 50 points
    Hello Ivon,
    Software reset can be caused by OVRCUR signal. Do you have pull-up resistor/resistors on pins 10, 14, 18, 22?
  • 0
    Prodigy 20 points

    Thanks for the replies so far.  

    Here it is on May 5th and I just received e-mails stating that this question was replied to.  TI might want to take a look at their notification system.

    Concerning this board: I have been able to greatly alleviate the problem by optically isolating my RS485 circuit from the rest of the board.  In addition I rearranged the power configuration such that the TUSB2046B, FT232RL and all other parts associated with those chips are now powered from the usb host instead of being self-powered.  While the system still exhibits the problem it has been reduced to a less than 1% failure rating.

    Thanks for your suggestions!

    Ivon Cullumber