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Thread: Tricky USB power question - who can tackle it?

  1. #1
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    Tricky USB power question - who can tackle it?

    Here's the deal. I added a radioshack USB hub to my system because I was running out of ports and I wanted to hook up my USB wireless adapter. The hub can be self powered (can't support the wifi adapter in this mode - too much current) as well as take 5v from the supplied adapter. I hacked off the AC end of the cable and hooked it up to the 5v rail from the opus, so it's drawing its 5v at up to 2.4A from the Opus.

    However, I have one problem. The wifi adapter and most other devices will power on and then immediately shut off when they detect that there is not enough power available to them. I don't know how they do this, but they do, and it's a hardware thing not software - this all happens within a split second.

    When I turn on the system, the hub comes right on, as do the USB devices, then they power off just like they would if the hub was self-powered (not enough power). Now, if I unplug all the devices and plug them back in everything works fine. Best I can tell, here's what's happing. The OPUS is powering up the USB 5v a split second before it powers up the 5v rail that is supplying the extra current to the hub, and in that split second, the USB devices have time to power up, think there's not enough power (5v rail hasn't been energized) and shut off. Shortly after, the 5v rail comes on and it can fully support everything once I unplug and replug all of the devices.

    Is there any way I can make this work? I figure I just need to make the hub wait until the 5v rail is powered up before it turns on. I figured I would do this by cutting the 5v+ from the usb cable so it didn't have power until it got it from the 5v rail, but when I do this Windows doesn't even recognize the hub. For some reason it needs to be drawing power from the USB 5v line to be recognized. I verified this on other computers as well.

    Can anyone figure out a solution?

    Thanks in advance,
    Dan

  2. #2
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    I figured I would do this by cutting the 5v+ from the usb cable so it didn't have power until it got it from the 5v rail, but when I do this Windows doesn't even recognize the hub.
    This doesn't make sense. You said the 5V line comes up just after the 5V in the cable, how has windows loaded so fast and already detecting devices at this point?

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    Maximum Bitrate binary.h4x's Avatar
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    are you using WinXP or Win2k?
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    Quote Originally Posted by MrPerfectionest
    This doesn't make sense. You said the 5V line comes up just after the 5V in the cable, how has windows loaded so fast and already detecting devices at this point?

    It hasn't. This all happens within a split second. Like I said, this is a hardware problem, not software. There is not enough power for all the USB devices once the 5v USB bus comes up, so they shut off. Nothing to do with Windows turning them off. And they won't come back on (and stay on) until the extra current is available from the 5v rail. Once it's there, they'll stay on fine. I just don't want to have to unplug and replug every USB device every time I turn the car on...

    Does that make sense?

  5. #5
    Newbie Montanaja's Avatar
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    If you don't have enough power then add in a POL...

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    Quote Originally Posted by Montanaja
    If you don't have enough power then add in a POL...
    Ugh. Did you read my post? I HAVE enough power, it's just not getting supplied right.

  7. #7
    Newbie Montanaja's Avatar
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    Again... I think the easiest fix would be a 5v POL.

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    Again, I ALREADY HAVE enough power. The USB bus would STILL COME ON before the 5v with a POL. Ugh...

  9. #9
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    You could try wiring your 5v from your opus directly to the usb connection for your usb hub (obviously disconnecting the existing usb power supply first..)??

    It seems a bit weird that the hub doesn't work when the 5v usb supply is disconnected and the hub is running on an external supply (never tried it before).

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  10. #10
    Maximum Bitrate zPilott's Avatar
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    it is kind of a bandaid, but you could use devcon to disable and reenable the hub (once you get into windows) which is basically a software unplug the plug. Not the best solution, but it might work until you find something better.

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