I'm using a Beaglebone black + BBVIEW 4.3 LCD Display + Datalogic Magellan 110i usb scanner. The scanner is connected to the usb host port but when I read a bar code the scanner goes down and then up again. The scanner datasheet says that during reading the current goes to a 400ma maximum so it should not be a problem to the USB host port. If I remove the LCD cape I have no power problems. The board is connected to an industrial adjustable power source 2.5A power source used for testing. Is the maximum current available in the USB host related to current for the LCD cape?
Electronic – Beaglebone black usb host current
beagleboardbeaglebone blackusb
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If I am not getting confused, this system of resistors is required by iPhones and other i-Devices to charge at different current levels. Otherwise an error message will pop up. If you don't have an iPhone you can quite safely remove these and replace them.
Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines. Time to sacrifice an official Apple iPhone 3Gs charger!
Taking it apart, desoldering the 4 data line resistors and measuring them on our multimeter, we found the following as shown in the schematic:
To measure a digital signal the Nyquist-Shannon theorem says you need a scope with a sampling rate at the very least twice as fast as the frequency of the signal you are sampling. An USB 2.0 bus is capable of 480Mbit/s, which means you would need a scope capable of AT LEAST 1GS/s (a billion samples per second) to be able to resolve individual bits. You have the wrong equipment for the job.
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I figured out the problem, it was not a current problem but voltage. The power cable i used was a bit long so i had a voltage drop of around 0.5v. I changed the voltage at the source to 5.5V and everything is ok now. Thanks – Rui Sebastião Nov 28 '14 at 10:22