profharris
Posts: 5
Joined: Thu Oct 04, 2012 8:48 pm

Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:16 pm

I had a 4-port powered hub attached to my Type 2 Raspberry Pi.
I recently purchased a small compact 7-port powered hub off eBay.
I'm powering the hub with a 5V 2.4A power supply. I'm powering the
Raspberry Pi from one of the powered USB ports. I also connected
the "host" cable from the powered USB hub to the bottom USB port
on the Raspberry Pi. Now for the surprise...

When I unplugged the micro-USB power cable from the Raspberry Pi
to swap it out for a shorter one... my Raspberry Pi stayed on!

I unplugged the powered USB hub from the wall (to power down my
Pi) then plugged it back in. My Raspberry Pi will power itself,
boot, and run from power from the powered USB "host" cable, with
nothing plugged into the micro USB power port.

Is the normal? ... with this damage my Raspberry Pi?

Prof. Michael P. Harris
[email protected]

PS1981
Posts: 446
Joined: Wed Dec 12, 2012 4:11 pm

Re: Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:19 pm

You are not the only person to have had this experience. As far as i know it may be considered normal.

Cloudcentric
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Joined: Fri Sep 14, 2012 9:13 am

Re: Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:24 pm

USB On-The-Go (OTG) http://en.wikipedia.org/wiki/USB_On-The-Go

dmesg lists the USB Controller as OTG

[ 1.896975] usb usb1: Product: DWC OTG Controller
Last edited by Cloudcentric on Tue Feb 12, 2013 10:38 pm, edited 2 times in total.
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PS1981
Posts: 446
Joined: Wed Dec 12, 2012 4:11 pm

Re: Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:29 pm

i didn't check the schematic, but is it not simply a case of the power supply and the usb power supply being the same circuit? I heard that the pi can also be powered over the 5v pin on the gpio headers but that would be bypassing a fuse. I cannot verify this however.

Cloudcentric
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Joined: Fri Sep 14, 2012 9:13 am

Re: Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:39 pm

"As the 5V rail is brought out in the GPIO pins, you can power the Rpi from there too. You should mind however, that those are behind the power protection circuitry, so you should provide your own."

http://elinux.org/RPi_Hardware
I know everything about nothing"

PS1981
Posts: 446
Joined: Wed Dec 12, 2012 4:11 pm

Re: Raspberry Pi powered by the "host" cable.

Tue Feb 12, 2013 10:42 pm

that's what I meant, thanks. But is this also the same 5v circuit as the usb ports?

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rpdom
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Re: Raspberry Pi powered by the "host" cable.

Wed Feb 13, 2013 3:40 am

PS1981 wrote:that's what I meant, thanks. But is this also the same 5v circuit as the usb ports?
Once past the the polyfuse (F3) on the microUSB input, there is only one 5V circuit on the Pi. It connects to the input of the 3.3V regulator, the GPIO 5V pins and the USB port 5V lines.

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tonyhughes
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Re: Raspberry Pi powered by the "host" cable.

Wed Feb 13, 2013 3:53 am

Does that make this a worthwhile temporary suggestion for those with blown polyfuses?

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rpdom
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Re: Raspberry Pi powered by the "host" cable.

Wed Feb 13, 2013 4:05 am

tonyhughes wrote:Does that make this a worthwhile temporary suggestion for those with blown polyfuses?
Yes. (Although blown polyfuses "should" recover in time)

"Backpowering" from a USB hub is a well known and often utilised "feature" of the Pi and badly designed USB hubs. Others method are powering from the GPIO pins and soldering wires to the test points TP1 and TP2.

What you have to remember is that by using any of these methods you are bypassing F3 and therefore any protection circuits on the Pi.

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