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USB Device type

  1. Oct 22, 2008 #1
    Hi all,

    set up:
    USB devices in power mode have three types
    a. high power mode ( 500mA)
    b. Low power mode ( 200mA)
    c. Self powered mode.

    in one of our applications we are configuring the device as low power device at the descriptor level ( Software).
    Once the host has the device information it will be able to monitor the 5V USB power line.

    will the host controller play a role in controlling the amount of current to be sourced through VUSB?
    or else will the host decide the type of device by sensing the amount of current consumed?

    Thank you,
  2. jcsd
  3. Oct 22, 2008 #2


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    The host is responsible for supplying 5 volts at whatever current the peripheral happens to draw. It can't control the current!

    It will sense if the device uses more than its allocated current budget, however, and will usually disable the port.

    - Warren
  4. Oct 22, 2008 #3
    So the controller reads the descriptor type and sets up the threshold current sourcing point.
    and also is there no lower threshold level??
    what if the device is self powered?
    How should the 5V line be handled?
    If unconnected, will it cause violations?
  5. Oct 22, 2008 #4


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    Science Advisor
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    No, there is no minimum current. Small currents don't destroy things -- big currents do -- so you generally only worry about maximum currents.

    If the device is strictly self-powered, then the 5V line should not be connected. These kinds of questions are best answered by the USB specification documents, if you can find them.

    - Warren
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