wiiguy
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usb hub power ?

Tue Jul 10, 2012 5:10 pm

can i just power the rpi trough a usb hub ?

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mahjongg
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Re: usb hub power ?

Tue Jul 10, 2012 5:21 pm

Yes, in fact this is the preferred method (instead of having two power supplies, which must be turned on and off simultaneously).
One provision is that a single port of the hub (the port you power the R-Pi with) is able to provide at least 500mA. (700mA minus the 200mA that otherwise would be used by USB devices).

wiiguy
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Re: usb hub power ?

Tue Jul 10, 2012 5:41 pm

so i can just buy a 4 usb hub
use 1 of those usb to power the rpi and the other 3 i can use for other usb devices to connect on teh rpi ?
while still using teh second usb on teh rpi

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mahjongg
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Re: usb hub power ?

Wed Jul 11, 2012 8:53 am

In short, yes.

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RaTTuS
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Re: usb hub power ?

Wed Jul 11, 2012 8:58 am

wiiguy wrote:so i can just buy a 4 usb hub
use 1 of those usb to power the rpi and the other 3 i can use for other usb devices to connect on teh rpi ?
while still using teh second usb on teh rpi
well one to power the RPi and one to connect to the hub for data
7 port hubs are probably better
1 to power RPi
1 upstream to RPI
1 keyboard
1 mouse
1 hd [may be 2 because of power]
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Beerhunter
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Re: usb hub power ?

Wed Jul 11, 2012 10:14 am

Why two USB connections to the Pi?

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Re: usb hub power ?

Wed Jul 11, 2012 10:16 am

Beerhunter wrote:Why two USB connections to the Pi?
One to provide power to the Pi and the other to connect the hub so it is visible at the USB level.
Last edited by itimpi on Wed Jul 11, 2012 10:17 am, edited 1 time in total.

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Re: usb hub power ?

Wed Jul 11, 2012 10:16 am

Beerhunter wrote:Why two USB connections to the Pi?
one for the power i.e. usb to microusb
and one for data i.e. from your usb port to the hub then you use the hub for all conenctions
the micro usb does not supply data and the normal usb [should not ] supply power
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Re: usb hub power ?

Wed Jul 11, 2012 10:18 am

(I hit go too quickly.) In addition to my question above, my four-port hub has a dedicated upstream connector leaving four free ports.

So: connection to pi via dedicated port and the remaining four: keyboard, mouse, USB hard disk.

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Re: usb hub power ?

Wed Jul 11, 2012 10:22 am

Beerhunter wrote:(I hit go too quickly.) In addition to my question above, my four-port hub has a dedicated upstream connector leaving four free ports.

So: connection to pi via dedicated port and the remaining four: keyboard, mouse, USB hard disk.
That does not leave one for powering the Pi (one would not normally expect power to flow over the dedicated upstream port) unless the HD can get away with using only one port for power.

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Re: usb hub power ?

Wed Jul 11, 2012 10:24 am

mahjongg wrote:In short, yes.
Is that entirely right? That may work for non-compliant hubs, but compliant ones will only output 500mA per port without negotiating more (the pi doesn't) which is too low to power the pi. I worked round this by soldering a micro usb lead to the the hub's power input. A "Y" cable seemed too expensive and would have taken up two hub ports, and now I have all my hub ports free.

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Re: usb hub power ?

Wed Jul 11, 2012 10:53 am

Doesn't the 700 mA for RPi include feeding the USB ports ? That would be taken over by the hub now ? Bottomline: are the power requirements for RPi still 700mA min even if you have a powered hub ?

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Re: usb hub power ?

Wed Jul 11, 2012 10:58 am

The Pi still needs to power one port (if you use it) and the uplink to the hub!

However you are correct in that it is highly likely that you would get away with a 500mA feed from the hub. Stll, since power problems can be a bit deceptive to identify it might still be a good idea to play safe.

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Re: usb hub power ?

Wed Jul 11, 2012 12:35 pm

500mA should be enough for a PI, (700mA minus 200mA no longer needed for USB) but I also don't expect the hub to fail when trying to provide 600mA! If there is any current limiting its never that precise.

Powering though the upsteam port will fail, even when the hub simply connects it's 5V to the upstream port, because the USB polyfuses, (rated for 100mA) are rated something like at least 5 Ohm, so when 500mA flows through them there would be 5(ohm) x 0.5(A) = 2.5 Volt over them, which is simply impossible, and also the fuse would trip.

grahamed
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Re: usb hub power ?

Wed Jul 11, 2012 1:28 pm

Hi all

It seems there is a constant theme in this forum - power supplies. Any problem and someone will sate that it is due to a power supply problem - and sometimes it is.

I have spent an hour or so experimenting with powering the Pi in many ways. The only time It have found it to be reliable is direct power to the Pi from a *good* supply - preferably a well-specified laboratory bench 3A unit. I have had no success what-so-ever when there has been a powered hub present.

When I say present I mean connected in the conventional A-B manner with separate power to hub, or the same but with the red wire in the A-B cut, or by powering the Pi by back-feeding into the micro-USB from one or even two hub ports.

[As an aside I suspect that most hubs will not provide the full 500mA of which they are capable without a proper negotiation]

The only time it has worked anything like consistently with a hub has been when the hub is un-powered - with all the limitations that implies.

Strangely I notice that, though the Pi with powered hub will boot OpenElec, in addition to the hub connected peripherals not working the network also fails - the clock is not set.

Of course all this could be a problem with my single Pi and as the boot diagnostics fly past I can't really say what is going wrong - of course if I had a working keyboard....or a working network....

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Re: usb hub power ?

Wed Jul 11, 2012 11:36 pm

As an aside I suspect that most hubs will not provide the full 500mA of which they are capable without a proper negotiation
You do expect too much power management from a simple hub!
In almost all cases it has been shown that powering the R-Pi from a hub works fine.

powering the R-Pi from its own PSU and the hub from another would also be fine, if it were not for the fact that when you power the PI off, it will still receive al little bit via the Pi's 100mA fuses, and a little bit can lead to problems. So simply turn both off at the same time and avoid such problems!
Another solution would be to break the +5V connection from the Pi to the hub, only some hubs monitor that +5V and will only switch their own power on when they see the Pi's +5V.
The most "universal solution" is still to power the Pi from the hub.

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Re: usb hub power ?

Thu Jul 12, 2012 12:39 am

Many people, including myself have had good success with the R-Pi using powered hubs, which are connected in the normal way to wall-wart type 5V switching power supplies.

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Re: usb hub power ?

Wed Aug 01, 2012 9:00 pm

I said "Strangely I notice that, though the Pi with powered hub will boot OpenElec, in addition to the hub connected peripherals not working the network also fails - the clock is not set."

All I am saying is that there is a fundamental problem here. Somehow USB is killing ethernet - and it is not a problem caused by a simple lack of proper power.

Having only one Pi does limit the scope of my investigation somewhat.

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r4049zt
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Re: usb hub power ?

Thu Aug 02, 2012 1:00 pm

I have not tested the upper of two usb sockets on the pi, but the lower got 3V not 5V when powered from the rPi board with a 5V 1A rated micro-usb power supply. Therefore I'd expect issues with trying to power an rPi from an external usb powered hub via the lower of its two usb sockets.

I have a 7-way usb hub with a mechanical switch on it to switch off the usb power connection between the rPi board and the hub. So far I have copied >2GB from one usb memory stick to another in Debian6 whilst that external usb hub was separately powered from four NiCad rechargeables at 5-ish volts. File transfers became unreliable when battery voltage fell below 4.4 Volts.

Good luck.

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Re: usb hub power ?

Thu Aug 02, 2012 1:09 pm

r4049zt wrote:I have not tested the upper of two usb sockets on the pi, but the lower got 3V not 5V when powered from the rPi board with a 5V 1A rated micro-usb power supply. Therefore I'd expect issues with trying to power an rPi from an external usb powered hub via the lower of its two usb sockets.

I have a 7-way usb hub with a mechanical switch on it to switch off the usb power connection between the rPi board and the hub. So far I have copied >2GB from one usb memory stick to another in Debian6 whilst that external usb hub was separately powered from four NiCad rechargeables at 5-ish volts. File transfers became unreliable when battery voltage fell below 4.4 Volts.

Good luck.
you will not get anywhere back powering the RPi from it's own normal usb sockets
YMMV but mine would boot about half way when getting power from the USB sockets only
you use the HUB with a usb-> micro usb cable and power the RPi with that, then use another lead from the RPi normal USB to the HUB to get data out/in - you can "cut the red lead" on this cable to prevent back powering the RPi if the hub does this - [it's up to you if you want to depending on the design of your kit
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grahamed
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Re: usb hub power ?

Thu Aug 02, 2012 8:31 pm

I have just read an interview of Eben/Li z in the latest Magpi which seems to refer to this very problem..

Just to be clear my Pi does not work, at all, if there is a powered hub connected in any way, either supplying power or just being a hub.

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