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Question for Dual input XOR Power Switch and Battery

Discussion in 'Embedded' started by cf.isys, Apr 18, 2007.

  1. cf.isys

    cf.isys Guest


    I have a question: In my design I have 2 power input: USB (5V) and
    Serial+Power Jack (9V). I see in differents computar that the voltage
    in the USB port not is 5.0V exactly. I want to know which is the best
    way for implement a automatic xor and power switch regulate for ouput
    5V 500mA. The idea is that is the user connect the USB the hardware
    have power and communication for the USB PORT and if connect the Serial
    +Jack the hardware have power for this last. And is connect the two
    together the priority is the USB.


    The other point is that I use a SuperCap for battery operation, is the
    user disconnect the USB and/or the SERIAL+JACK the hardware can finish
    the operation. How to put the SuperCap in the this design.

    Which is the best design for this problem???????

    Thank you very much!! for help me.

    Best Regards,
    cf.isys, Apr 18, 2007
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  2. cf.isys

    Tom Lucas Guest

    The first thing to bear in mind is that the USB will only release 100mA
    until you specifically instruct it to allow 500mA.
    Tom Lucas, Apr 19, 2007
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  3. cf.isys

    cf.isys Guest

    I not understant that you say. Please check this link
    http://www.usb.org/developers/usbfaq/#pow1 that say:

    1. How much power does a system in S3 need to supply to USB?

    A: 500mA per USB port. See section 7.2.3 for the details of device
    behavior during suspend and resume. The Device Working Group is
    working on power management extensions to the common class definition
    that should alleviate most of the S3 power supply issues with USB.
    Contact for more info.
    cf.isys, Apr 19, 2007
  4. cf.isys

    Tom Lucas Guest

    The USB port certainly _can_ supply 500mA (assuming it really meets the
    specs) but only after a negotiation with the host to request the extra
    current. With no handshaking and just using the port as a voltage source
    then the port will only permit 100mA to be drawn.
    Tom Lucas, Apr 20, 2007
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