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Page created July 7, 2012
My Raspberry Pi board arrived June 5, 2012. I had already purchased extra hardware needed to connect it up. I played with various configurations, and this page describes my current hardware configuration as at July 7, 2012.

From a brief scan of posts on the RP Forum, many people are experiencing problems with the hardware setup, so this page might be helpful.

My drawing skills are somewhat limited, anyway, here is a sketch of my setup:

I purchased a cheap 7-port USB powered hub, which of course comes with a mains adaptor and a USB cable. As you can see above, the mains adaptor plugs into the powered hub, and the cable, labeled "USB input to hub" connects the input-port on the hub to one of the USB sockets on the RP board.

I purchased a standard-USB-2.0 to micro-USB cable, connected from one of the hub sockets to the power-input socket on the RP board.

A USB socket has a 5V power pin that is often current-limited at 500mA maximum. Some USB hubs have individual current limiting on each socket, however, my understanding is that the cheaper ones don't. As the Model-B RP needs at least 700mA, a cheaper USB hub is an excellent power supply for the RP!

The polyfuse problem

However, even if each socket on the hub is current-limited to 500mA, the hub is also powering the RP board via the other cable -- into one of the standard-USB sockets. This might seem a bit weird. Consider this diagram:

The 5V rail powers all the chips, etc., on the board. It is connected directly to the micro-USB socket. It connects to the standard-USB sockets via polyfuses, one for each socket. This means that 5V supplied to one of the standard-USB sockets can power the board, except that the polyfuse will trip (or drop too much voltage ) if the current gets beyond 100mA, so it cannot power the board.

Reading the Forum, these polyfuses are causing grief for some people. If you were to connect a 5V mains adaptor to the micro-USB socket, and in addition a powered hub is connected via one of the standard-USB sockets, which is what many people have, at power-on there will be a momentary in-rush of current through the polyfuse. This could trip the polyfuse, and your hub will not work.

These polyfuses are solid-state devices that do recover. You turn off the power and wait for a few hours, and the polyfuse will work again. In fact, the polyfuses in the RP have very high resistance and you cannot pass any more than 100mA through them, as explained here: http://elinux.org/Polyfuses_explained

...the voltage-drop across the polyfuse may be the cause of many problems with keyboards, especially if the "5 volts" supply on the board is already a bit low, say down at 4.7 or 4.6 volts. Then, if you "trip" the polyfuse, your peripheral won't work at all, until the polyfuse recovers -- which is going to baffle many people.

Anyway, for people who have both a powered hub and a mains adaptor, I recommend turn on the power to the hub after powering up the board with the mains adaptor. Horrible advice to have to give, considering that people are likely to have these both plugged into the same mains power multi-socket board, with one power switch.

Do not ever have the hub powered-on and the mains adaptor turned off! You will trip the polyfuse.

My configuration avoids the problem. There is only one power source, so the voltage on both sides of the polyfuse will be the same during power-up, so very little in-rush of current through the polyfuse. My solution is also cheaper. Actually, there will be a momentary spike of current through the polyfuse, due to electron transmission delay through the cables,

This potential problem of power flowing backwards through a polyfuse, is further described here (with much nicer pictures than my mediocre artwork!):

Finding the hardware

If you are not sure whether your hub has individual current-limiting on each port, I recommend buy a cable with dual standard-USB plugs on one end and a micro-USB plug on the other end. I went out today hunting for one, but these are not an item that stores stock, at least not here in Perth, Australia. However, they are available online, and cheap, for example (Note, I have not actually bought one of these):
There are many 7-port USB hubs available, but make sure that you get one with a mains adaptor rated at 2A (or more). Here is information on the one that I bought:

Technical notes

When I first acquired my RP, my digital TV connected via HDMI cable from the RP, was not recognised unless I unplugged the USB keyboard prior to powering-on -- then plugged the keyboard in after bootup!
Issues like this cause a lot of confusion for newcomers!

I solved the problem of non-recognition of my TV by hard-coding the video output to be HDMI -- this is done in a file named 'config.txt', and you can read about this file here:

My Puppy Linux distro has this hard-coding, as it gives positive outcome (that is, it "just works") for a newcomer, as long as you are using HDMI of course. Otherwise, you will need to edit 'config.txt' before booting up on your RP.

Puppy Linux is introduced at http://puppylinux.com (very x86-centric though!), and releases for the RP are announced on my blog http://bkhome.org/blog/ and on the Puppy Linux section of the RP Forum http://www.raspberrypi.org/phpBB3/viewforum.php?f=52

Regarding suitable hardware, there are many reports of troublesome peripherals. This is a good place to check on compatibility:

Regarding usage of a powered hub as the only power supply, that is, no separate power-adaptor for the RP, you may have read in some places that this should not be done. But, as this page explains, it is OK, as long as you use a cheap "dumb" powered hub:

How about this for the adventurous:

Here is a hardware troubleshooting page:


I have given advice on this page with a disclaimer that I accept no responsibility if your board explodes or there is any other mishap. If anyone thinks that there is something wrong with my advice, or needs further explanation, please post to my blog: http://bkhome.org/blog/

© Copyright Barry Kauler 2012 bkhome.org All rights reserved
See FAQ for legal statement.