power pi from usb hub connected to pi.

Can i power the raspberry PI using a usb to mini usb cable from a powered usb hub that itself is connected to the usb port of the Pi..

So i end up with the pi and the hub powered from one plug?

Chris

[abishur]

That's an interesting (and often reappearing) question

The answer basically boils down to "how smart is the hub?"

See the USB standard says that a usb port provides a minimum of 100 mA, if a device desires more power it makes a request for it and the port provides up to 500 mA.

If you get a hub that's too smart, then it will follow this specification exactly and you will not be able to power the pi from the hub because the pi doesn't ask for more power, it just needs the power it needs to run.

If, however, you get a hub that's a little more dumb and each port can automatically draw as much power as it wants to, then yes you can power the r-pi from the hub then turn around and plug the hub back into one of the USB ports.

It also helps to remember that the power port may be shaped like a micro-usb port, but it has no communication abilities.  Only the power lines are connected so it's not as if the hub will be caught in a communication loop if you plug the r-pi into it and then plug it back into the r-pi

sorry wrong thread [stupid db errors]

Initially, the only way my RPi would work in a consistent manner was if I used the above mentioned double connection, one from the externally powered hub into the PI's micro USB port and the second in reverse order (that is, from one of the PI's USB ports into the powered hub's mini USB port, therefore called the 'data' cable). I got to such config by pure chance, because whan I first got the PI I wanted to have it connected to three USB devices, which of course would be impossible without and external USB hub, since the PI only sports two USB ports. From my previous experiences with PC's and Mac's, I figured that such an arrangement would allow me to me to use the remaining hub's ports to connect a keyboard, mouse, Wifi dongle to the PI, while at the same time getting 5v power from the USB port connected to the micro USB on the PI (therefore called the 'power' cable).

I was quite surprised to learn that if I disconnected the 'data'cable (the one going from the PI into the mini USB port on the Hub), the PI would not work consistently. I tried 3 different 5v power supplies...
I was puzzled to learn that connecting a multimeter into the probe points into the PI resulted varying in values, all below 4.5v...

Later, I found out that one of the same switched 5V/2A PSU's I had previously tried would provide higher voltages (between 4.8-4.9v) if, instead of using a USB<->micro USB cable to connect the HUB to the PI's micro USB power input, I connected it to pins 02 and 06 of the PI's GPIO. But that required a secure connection into the GPIO pins. But where would I find such a cable?

After some thinking, I decided to to try and modify a regular USB extension cable, by removing the female connector and replacing it with a connector that would fit into four pins on the GPIO. I found out that old CD-ROM cables had the just the perfect connector (even though it had 4 pins and only 3 were connected), on both ends.

I cut one 4 pin connector out, soldered it into the USB extension cable that had the female connector removed and used the multimeter to test for positive 5V on the red connector and ground on the black of the 4 pin connector. I was happy to verify I had done a good soldering job and indeed correct voltage was coming from the appropriate pins on the 4 pin connector. So, after I double checked everything, I connected the 4 pin connector into the PI, making sure red was on pin 02 and black was on pin 06 of the GPIO, and connected the switched PSU into the mains. It worked as expected. I waited until Raspbmc was up and running and connected the multimeter to P1 and P2 on probing points in the PI. I was very satisfied to verify that a steady 4.89v of DC power was being read by the multimeter. I then fired a few 720p files and verified that voltage was steady during playback.

I'm not sure what was causing the voltage loss and fluctuation when using the micro USB power connection on the PI, but I suppose I had a poor quality USB <-> micro USB cable. But I really don't care much, because now I only need on cable to power my PI, while at the same time getting proper voltage levels.

I power my pi through a cheap 7 port usb hub from Ebay and it works with no problem.
I also power a keyboard, mouse, usb hdd and a memory stick from the same hub.
My hub has a 2.5 amp power supply or at least that's what it says on the label.