USB data detection circuit

I'm looking for a circuit to detect a USB signal (activity, no activity). When there is activity at the USB port I want an LED connected to port to turn on (or start blinking), when there is NO activity the LED will turn off.
I would really appreciate if someone can help me find on solving this problem
Tico_technologiesAsked:
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dbruntonConnect With a Mentor Commented:
This is a counter for USB activity.  Most of the other stuff I can find for activity requires wiring to a USB chip.

http://www.bobhobby.com/2008/04/22/usb-physical-hit-counter-based-on-avr-attiny25/
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dbruntonCommented:
There are these cables which might do what you want.

http://www.pctoys.com/usbacflcaaco.html
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Tico_technologiesAuthor Commented:
Yes, that is correct, this cable does what I need it to do, however, I'm actually looking for the schematic for this type of circuit or at least how I can make one myself. I'm currently working on a project that will use that flashing action (activity) to be fed into another circuit.
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nobusCommented:
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Tico_technologiesAuthor Commented:
Nobus:
Thanks for the link, however, it is just a very basic tutorial of what a USB cable is. I'm looking for a circuit schematic, something that actually works, that I can use in my project. Or least and idea of how I can do this.
Thanks
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nobusCommented:
sorry, it's all i found.
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stoop_edConnect With a Mentor Commented:
What kind of activity are you trying to detect?
This is more difficult than you probably expect because of the way USB works. If the device is connected, the host (computer usually) will continuously send Start of Frame packets to the device even if it is not actively being used. So if you are looking at the electrical signals, you will always see packets going to the device. The only time there is no activity on the lines is if the device is in "USB suspend" which usually only happens if the computer is asleep or if the device is disabled in device manager.

The easiest way might be to monitor the amount of power going to the device. Most devices will consume more power from the USB bus if they are active. (assuming the device is not externally powered) But this would be very device specific behavior.
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Tico_technologiesAuthor Commented:
Stoop ed:
Thank you for strearing me in the right direction. I was not aware of this USB behaviour, I was expecting that there would be no activity at the port when there was no data transfer between PC and USB device.
I'm trying to develop a customized USB switch. I want to share a USB flash drive between a PC and another USB device. The PC initially would load some piece of software into the flash drive, then external device would read this flash drive and upgrade from it; so basically once the write operation is done on the flash drive form the PC, a relay would switch or onnect the USB flash drive (power and data lines) to the USB device.
I have not found any device (USB switch or bridge) that does what I need. These devices require installation of drivers on the PC and the USB device I have. My USB device can only read flash drives and upgrade from them, I cannot do any windows driver type of installation.
So I'm trying to create a custom switch, but I'm just brainstorming with sensing the data lines.
Thanks for any asistance you can offer me.
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tnndesignConnect With a Mentor Commented:
Have you examinined the USB/USB2.0 specification?
They have an electrical doc which documents the electrical parameters of the USB bus whilst communicating. A simple micro-based solution would probably due to the trick.

Like what has been mentioned, you could sense the power drain on the port and/or presence of bus pull-ups (indicating speed of device) as well. But this wouldn't give an accurate indication of whether the device is transmitting, but rather the presence of the device only.

See:
http://www.beyondlogic.org/usbnutshell/usb2.htm
http://www.usb.org/developers/docs/
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compfixer101Commented:
This question has been classified as abandoned and is being closed as part of the Cleanup Program.  See my comment at the end of the question for more details.
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