Self-powered USB "pass-through"?

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russosv
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Self-powered USB "pass-through"?

Post by russosv » Mon Jan 07, 2008 7:17 am

Hi All!

I have a laptop on which I damaged the USB port, about 1 1/2 years ago when I was learning about the PIC 18F4550 and USB devices. In short, the USB ports no longer supply power, but they work with self-powered USB devices.

The laptop is now my wife's, and I wanted to make a device for her that would supply power to small, port-powered USB devices (like data sticks), so she can use the port with other devices.

I created a simple PCB that provides power to a device using a step-up converter for 2 AAA batteries (MAX1676), and simply passes D+/D- to the USB port.

The circuit does provide power to the device, however, nothing happens when I plug in a data stick (tested it on a good computer, and the device works fine on its own).

Since then, I've read that you need to "sense" a cable connection when a self-powered USB device is plugged in.

Does anyone know the details on this? Do I have to use a microcontroller for this type of app? And, is this type of device even possible?

Thanks in advance for any assistance!!

dyarker
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Post by dyarker » Mon Jan 07, 2008 10:39 am

A long time since I looked at USB spec, but yes there is some handshaking between USB port and port powered device. (Though not as complicated as POE power over ethernet.)

I would get a mini USB hub that is powered by wall wart or battery.

Sorry I be more help.
Dale Y

russosv
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Post by russosv » Mon Jan 07, 2008 10:56 am

Hi Dale,
I would get a mini USB hub that is powered by wall wart or battery.
Thanks for the suggestion... I have thought about getting a self-powered USB hub, but I'm short on money right now, and I'm also interested in the electronics of the application. It's much more satisfying to "do it yourself" if it's possible! :smile:

If the handshaking is simple, I would think that it shouldn't be too hard to build a basic device to do something like this with a microcontroller. I just need to find out what the exact "handshake" is... Any ideas on where to start?

Steve

dyarker
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Post by dyarker » Mon Jan 07, 2008 12:52 pm

"I'm short on money right now" That makes at least two of us :sad:

"and I'm also interested in the electronics of the application" Yeah :grin:

www.wikipedia.org is useful at times like this. Not for what's on their USB page, but for the links in the reference section at the bottom. Item 10 ? looked good, but is multi-meg, so I stopped the download.
Dale Y

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jollyrgr
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Post by jollyrgr » Mon Jan 07, 2008 10:15 pm

Can you swing $5, and an old keyboard cable? Walmart has this device:


http://www.walmart.com/catalog/product. ... id=5280907

If you are not using the PS/2 port on the back of the laptop, tap off the 5V using a PS/2 plug. Splice this into the USB hub's +5V wire.

If you feel comfortable working inside the laptop, you could bridge the 5V of the PS/2 port, through a diode maybe, to the +5V for the USB port. This would "fix" the laptop in a fashion.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

russosv
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Post by russosv » Tue Jan 08, 2008 7:57 am

Hi Jollyrgr,

Thanks for the suggestion. I may end up going this route because I couldn't get my solution to work.

Thanks,
Steve

russosv
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Post by russosv » Tue Jan 08, 2008 9:06 am

Oh , Dale - I forgot to say -- thanks for the link!

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