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For connecting external hard drive I want to use an active USB hub.
I found some USB hubs which seem also to power the upstream ports.
While this is under normal circumstances very bad I was wondering if this is actually a nice feature for powering also the Raspberry Pi device just via the Type A connector where I connected the USB hub.
Is this possible? Or can I power the Raspberry Pi only via the micro USB port?
From the Raspberry Pi wiki:
Back-Powering; (powering the Raspberry Pi from a USB hub through the uplink/data port, single cable) Back powering is possible on the Raspberry Pi.
Revision 1.0 boards have to be modified to back power, this is due to the 140ma "polyfuses" that are installed in the USB port circuit.
Revision 1.1 boards do not need modifications to back-power, they have replaced the polyfuses with 0ohm resistors in their place.
Revision 2.0 boards do not need modification, they have neither resistors nor polyfuses. It is advised that short (12" (.3 meter) or less) USB cables be used for back-powering a Raspberry Pi. Cable resistance plus connector resistance can quickly reduce operating voltages below the proper range(5.25V to 4.75V).
1Aper port. Both the mini and large usb connectors are only connected to5+vandGND- For various reasons it is better to power using the micro usb port at all times - but not mandatory500mAper port and connect two of them. And even one can be enough if you don't have to power anything but RPi itself.