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USB interface question.

Discussion in 'Electronic Design' started by siliconmike, Jun 10, 2006.

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  1. siliconmike

    siliconmike Guest

    I'm newbie at USB, if someone can help it would be wonderful:

    I'm envisioning a gadget that would take input from a USB port on a PC
    (or any USB host). And it would act as a UPS for four downstream USB
    devices, providing currents upto 1A.

    In other words, the gadget would have 4 USB sockets for output, capable
    of providing upto 1A to devices connected to it.

    I have done some reading on USB but would like to ascertain whether the
    gadget needs to have a controller / or can it be a passive device, as
    far as the USB interface circuitry is concerned ?

    (of course, the gadget would have its own separate supply, from where
    it will derive the UPS power)

  2. Mike,

    Buy a simple externally powered USB-hub. The ones I have simply connect the
    5V to their power outputs, so they pass all of the current the power supply
    can provide to the USB-devices. I even saw some (older) PCs that has
    connected their 5V power directly to the USB. (Way beyond spec and risky of

    petrus bitbyter
  3. Well, any USB hub with a wall wart would do this. The only problem is that
    1A is far beyond the USB specs. Any device is not allowed to consume more
    than 100mA from the start and after negotiation with the host, it may
    consume up to 500mA. If you draw more, you'll get a "USB overcurrent"
    message. This means that a hub can measure the current of each port and
    shuts down a port that is loaded to heavy.

  4. siliconmike

    siliconmike Guest

    The gadget I mentioned is part of a bigger project. I mentioned it
    separately only to simplify conversation.

    -Can I just short the D+ pin from input to output ports
    -Then short the D- pin from input to output ports
    -Provide a separate 5V VBUS+ to output port
    -Provide a common shorted GND.

    Or is this a disaster eh?
  5. siliconmike

    siliconmike Guest

    Yes, the electronics world be a simpler place, if everyone followed the
    specs correctly ;)
  6. quietguy

    quietguy Guest

    Ahhh you just need a powered hub - like the one I bought 4 years ago

  7. Start at the beginning. UPS, is a _point to point_ interface. There can
    only be one USB device on a 'leg'. To have multiple USB devices from one
    host, requires a hub.
    Now other people have then pointed out that 1A, is more power than the
    target device is allowed to draw off USB. 0.5A max, and even this requires
    negotiation (though a hell of a lot of hubs, will supply this power
    without comment). You could potentially use two USB sockets, and draw 0.5A
    off each, but realistically, the simplest solution, will be a seperate
    power connection for the devices. However it actually sounds as if the
    devices will be normal USB units, in which case you might as well reduce
    your power capabilities to 0.5A each...

    Best Wishes
  8. Mike,

    USB connectors are designed to make and break contacts in the right order.
    The hub I think about has its own 5V (wall wart) power supply that has been
    connected to the 5V power pins of the output directly. So the USB hub can
    provide whatever current its power supply provides... as long as the
    conductors and connectors can handle it. Which, of course, comes with its
    own risks. A short in one of the connected (USB)devices may fry the internal
    organs of your hub and damage other connected devices as well.

    IMHO this scheme is a consequence of a free interpretation of the specs. The
    USB host should be able to provide 100mA at first contact and possibly 500mA
    after negotiation. If you interprete these values as *at least* 100mA and
    *at least* 500mA respectively, you can possibly use even a 100A power
    supply. No need to say it's little bit beyond the maximum current your USB
    cables can handle ;)

    petrus bitbyter
  9. siliconmike

    siliconmike Guest

    And is there a way to make this hub a CPU-less device?

    Or are there available hub ICs to minimize homework ?

  10. Any number of hub IC's.
    Alcor, FTDI, Atmel (AT43301), Cypress, NEC, Philips, SMSC, TI (most of
    their's have some extra functions built in)....

    Best Wishes
  11. Yes, the ones I talked about are made that way.
    Yes, the hubs a talked about contain one of these.
    petrus bitbyter
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