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Forum Thread Details
  • Replies 12 replies
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  • hubs
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Powered Hubs

dharr19
dharr19 over 12 years ago

Never used on before so if I want to power my RaspPI and add a wifi. How would this work out?

I know I would have to connect one port to the microUSB for power but how does the wifi fit in?

 

Cheers

David

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  • Former Member
    Former Member over 12 years ago

    What does a powered hub have to do with wifi? If you want it to run on a wifi usb dongle, check for the peripherals list (others might work too, these are identified to work so far)

    http://elinux.org/RPi_VerifiedPeripherals

     

    Same for powered USB hubs btw

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  • morgaine
    morgaine over 12 years ago

    David, I'll try to summarize it for you.

     

    A USB hub connects to a single upstream host port and expands it into two or more downstream host ports.  Full-size host port connectors are normally "type A" while a hub's upstream connector is "type B", which is why hubs require a type-A-to-type-B lead (sometimes called "A-B") to connect them to the upstream host.  The type B side is the chunky squarish end, while type A is the usual USB flat connector such as found on USB mice.

     

    USB hubs are typically powered in one of two different ways.  A self-powered hub takes its power from a dedicated external power supply, which is typically a mains wall adapter connected to the hub via a barrel connector.  In contrast, a bus-powered hub takes its power from the upstream host port to which the hub is connected, through the A-B lead.  In addition, when a self-powered hub does not have its external power cable attached (or the external supply isn't running), it typically behaves like a bus-powered hub and hence takes its power from its upstream USB connection.

     

    The Raspberry Pi model B has a micro-USB connector through which it takes power from a USB charger.  That connector does not have any data lines connected, so it is not a "USB port", just a power connector.

     

    The Pi model B also has two type A host sockets, into which you can plug USB type A plugs such as your wifi dongle, or the type A plug of a type-A-to-type-B lead such as connects to your USB hub.  If you connect your dongle directly into a Pi's type A host socket, then the dongle will take its power from this socket and hence indirectly from the Pi's power supply.  In contrast, if you plug the dongle into a self-powered hub that is properly powered from its external power adapter, then the dongle will take its power from this external supply and hence not draw power from the Pi.

     

    Because the Pi's power design is rather poor and USB chargers are typically quite dreadful compared to how a regulated power supply should behave (compounded by the high resistance of many micro-USB leads), it is not advisable to power high-current peripherals such as wifi dongles directly from the Pi.  This is where your self-powered hub ("powered hub" for short) comes in --- it provides your dongle with data access to the Pi through the upstream USB A-B lead, but powers the dongle from the external power supply instead of from the Pi's host port.

     

    A bit long-winded, but hopefully this helps to explain the relationship between dongles and powered hubs. image

     

    Morgaine.

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  • morgaine
    morgaine over 12 years ago in reply to morgaine

    If anyone's looking for what used to be the tail end of this thread, it seems to have been split off into a thread of its own over here:

     

    http://www.element14.com/community/thread/19316?tstart=0

     

    I guess this was done by administrative action, owing to the topic having diverged in part into a description of the "Pi accessory" 7-port hub not actually being Pi-compatible.

     

    Morgaine

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  • dharr19
    dharr19 over 12 years ago in reply to morgaine

    So the USB Type-A side is where data is accessed and the Type-B side is where the data is disseminated? 

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  • johnbeetem
    johnbeetem over 12 years ago in reply to dharr19

    The Wikipedia page on USB (http://en.wikipedia.org/wiki/USB) is good, but the USB 2.0 standard itself (available for free at http://www.usb.org/developers/docs/ as a .zip file) has IMO much better introductory material.  Start with Chapter 4 of the "USB Specification rev 2.0" = usb20.pdf, and skip back to Chapter 2 for terminology.

    David Harrison wrote:

     

    So the USB Type-A side is where data is accessed and the Type-B side is where the data is disseminated? 

    USB data goes in both directions on a USB cable, but only one direction at a time (half duplex).  The host controller in RasPi's BCM2835 SoC initiates all USB transactions: a USB device only "speaks when spoken to".  A USB network is a tree, with the host controller at the root.  On a standard USB A-to-B cable, the A end is closer to the host controller and either plugs into the host controller itself or to one of a hub's downstream A jacks.  The B end plugs into a USB device, either the single B jack on (for example) a USB printer or the upstream B jack on a hub.  The B plug and jack may be mini USB or micro USB instead of standard B.  There are also mini-A and micro-A, but full size A are much more common.

     

    Many devices don't have physical B jacks.  For example, a USB keyboard or mouse connects the B end of the cable directly to its microcontroller, so only the A end is a physical plug.  This is also true of USB hubs that have an upstream cable instead of an upstream B jack.  A USB Flash drive doesn't have a cable at all -- it just provides an A plug and the rest of the cable is just a few short wires connected to the USB Flash drive's microcontroller.

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  • morgaine
    morgaine over 12 years ago

    Updating this thread by developer request, to check date sorting.

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  • yvanttt
    yvanttt over 12 years ago

    Here are some results i ran into when using cheap powered hub.

     

    Initialy they may seem incompatible with th PI, but it could be due to some compromise in the design/production run to cut pricr down.

     

    I ordered some cheap 7 port power USB hub (like 3 for $12) with the power supply included.

     

    on my 1rst attempt i connected my PI to the hub for both power, keyboard and mouse on the hub.

    The PI would start to boot and then keept on rebooting.

    I removed the keyboard and mouse, no change.

     

    The supplied power supply was rated for 5v 1A only, clearly insuficient current rating. so i used a 5v 2A supply i had left over.

     

    I reconnected everything back, then the PI booted till it got to the USB port detection and then it was looping into the USB detection sequence. Then i noticed the micro usb plug was NOT connected on the PI ????? The PI was receiving power from the normal USB jack instead.

     

    I then connected the micro USB plug and reapplied power, now the PI booted normaly. But the keyboard and mouse where erratics.

     

    I decided to look inside the USB hub case and i found a jumper that connected the external power supply to the +5v side on the incoming usb connector from the PI.

     

    Then i proceded to remove said jumper. Now the PI would not power on from the hub, good.

    So i connected the micro usb cable to the powered hub along the keyboard and mouse.

    now they are all behaving properly.

     

    So my theory is that when the jumper wire was installed in the hub and you use the same hub for both power and expansions some current loops may be created inside the PI and cause some undesirable behaviour.

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  • johnbeetem
    johnbeetem over 12 years ago in reply to yvanttt

    Yvan Turcot wrote:

     

    Here are some results i ran into when using cheap powered hub.

     

    ...on my 1rst attempt i connected my PI to the hub for both power, keyboard and mouse on the hub.  The PI would start to boot and then keept on rebooting... Then i noticed the micro usb plug was NOT connected on the PI ????? The PI was receiving power from the normal USB jack instead.

     

    I then connected the micro USB plug and reapplied power, now the PI booted normaly. But the keyboard and mouse where erratics.

     

    I decided to look inside the USB hub case and i found a jumper that connected the external power supply to the +5v side on the incoming usb connector from the PI.

     

    Then i proceded to remove said jumper. Now the PI would not power on from the hub, good.

    So i connected the micro usb cable to the powered hub along the keyboard and mouse.

    now they are all behaving properly.

     

    So my theory is that when the jumper wire was installed in the hub and you use the same hub for both power and expansions some current loops may be created inside the PI and cause some undesirable behaviour.

    Interesting.  I can't figure out why USB would be erratic with micro USB connected and jumper installed.  I'd put a 'scope on TP1 and see if 5V is unstable.

     

    The behavior of your first attempt is well known: the hub powers your RasPi through polyfuse F1 or F2, depending on which RasPi host port is connected to the hub.  F1/F2 is low enough resistance to start the boot process, but when it gets underway and RasPi needs more current the resistance of F1/F2 causes RasPi's 5V plane to drop and the boot to fail.  But then the SoC's current needs drop so 5V comes back and the process repeats.

     

    Hubs should never supply current upstream.  Some hubs use a Schottky diode to provent this, but it adds a small voltage drop if the upstream host or hub is powering the hub.  Your hub uses a mechanical solution -- I hadn't seen this before.  You obviously must install the jumper if the hub is powered from the upstream connection (bus-powered) and remove the jumper if the hub is powered from an external source (self-powered).  So as long as the hub is configured correctly (is there anything is the hub's user instructions about this?) it works fine.  

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  • oadesida
    oadesida over 12 years ago in reply to johnbeetem

    Updating this thread by vendor request, to check date sorting.

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  • oadesida
    oadesida over 12 years ago in reply to oadesida

    Updating this thread by vendor request, to check date sorting.

     

    Updating this thread by vendor request, to check date sorting -082212

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