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USB Issues

Dave Pulvermacher
Intellectual 870 points
Other Parts Discussed in Thread: AM3517

We have a custom pcb built using the AM3517 EVM reference design.  We have the vast majority of the issues worked out but a couple of bugs remain.  Concerning the USB, when we plug in a USB device, the board resets.  Also, we occasionally see the USB driver detach and reattach.  Any ideas?  I believe our routing is solid.  There are a couple of resistor values that COULD be suspect...we are using a 1K VBUS resistor (pin 22 on USB3320), a 8.06K RBIAS resistor, and a 0 ohm on the ID pin.  

Thanks

  • 0
    TI__Guru 63145 points

    I assume the USB port in question is operating as a host since you mention the SMSC USB3320 which can only be connected to one of the AM3517 USB ports.  The USB specification requires a USB host to provide a VBUS power source that can provide a minimum of 100mA with a minimum 120uF capacitor.  A USB peripheral device must have a VBUS capacitor that is less than 10uF and must consume less than 100mA when initially connected to the host.  Since the initial connection of the USB peripheral to the USB host is a hot plug event, there will be an inrush of current from the host VBUS capacitor to the peripheral VBUS capacitor.  The capacitor limits were specified to prevent the peripheral connection from sagging the host supply voltage and causing a problem similar to your description.

     

    Please verify the USB host and peripheral being used meet the requirements defined in the USB specification.

     

    The resistor values you mentioned should be good for your design.  At one time the SMSC USB3320 data sheet recommended a 10k ohm resistor on VBUS, but I found this resistor needed to be 1K before the USB3320 would report the correct VBUS voltage to the AM3517.  The USB3320 also has a sensitivity to power supply noise, so you may need to power this device from a dedicated low noise power source.

     

    Regards,

    Paul

  • 0 in reply to peaves
    Intellectual 870 points

    Paul,

    Changing our 100uF cap to 120uF seemed to solve the problem.  Thank you for the feedback.  

    Dave