USB power problem

2 posts
by saty25 » Mon Jun 04, 2012 10:17 pm
I'm using raspberry pi for taking data from USB phidget 1056( and forwarding it to Google app engine. driver is working fine but pi is not powering up the sensor via USB. If i connect phidget through a powered USB hub, it is showing phidget connected when i do lsusb but directly it is not able not showing anything connected. from the datasheet phidget require only 45mA current while in pi's usb specification it is give that it can supply upto 100mA. Can anyone help me with this.Do i need to do something with software to power it up via USB??
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by mahjongg » Mon Jun 04, 2012 11:44 pm
The phidget 1056 is not a true "single current unit" USB device, so its not following official USB specs, which state that any single current unit USB device ( a device consuming less than 100mA), MUST work with just 4.4 Volt. The RPI is designed for only single current unit devices, for any device other than these you MUST use a powered hub.
That is because the polyfuses chosen for protecting the USB ports have very high internal resistances, to reach the low tripping point of just 140mA. These resistances of up to seven ohms mean that even at 50 mA there will be so much voltage drop over these fuses that there won't be the 4.75 Volt that is needed for this particular device. The solution is to either use a powered hub, or to bridge the fuse, (solder a wire over them) obviously the latter will invalidate your guarantee.
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