Tag: USB

Powering a Nexus 7 tablet through the USB port while simultaneously connecting a USB device

The links in this article are for informational purposes only. Please read the final paragraph of this article before you actually do anything based on these links.

If you have ever tried to connect a Nexus 7 tablet to an external USB device, such as a USB memory stick or a wired network adapter, you know that you need a special “OTG” cable and that the tablet will power the device. While you can purchase various kind of “OTG” Y-cables, which purportedly will allow you to use an external device (or even connect to a USB hub so you can use multiple devices) while at the same time allowing a charger/power supply to be connected, this doesn’t work with a stock Nexus 7. At best you can send power to the connected device, reducing the drain on the battery in the Nexus 7, but you cannot send power back into the Nexus 7 simultaneously, even though you can charge the Nexus 7 while you are using it if the charger is the only thing you have connected.

There is a way around this but it involves “rooting” the device. If you don’t know what “rooting” a device means, or the implications of it, you’ve come to the wrong place because we’ve never done it – the mere thought of turning our Nexus 7 into an expensive brick is enough to dissuade us from trying it. However, in the course of trying to research this, we came you with links that might be useful to Nexus 7 owners that are braver (or maybe bigger gamblers) than we are.

The key to this is “Timur’s Kernel” – if you remember nothing else from this article, remember “Timur’s Kernel”. As we searched for information on this, time and again, we were sent back to this message thread:

Timur’s Kernel – USB ROM – USB Host Power Management – USB Audio (RootzWiki)

SOMEWHERE in that thread, which as of today is 358 pages long and still growing, is the information you need. Actually, the meat of it is in the very first post on page 1, with additional explanation on page 7. If you don’t feel like wading through all of that (seriously, who reads a thread that long?), the following page pretty well summarizes what you need to know. Just keep in mind that you probably don’t have to make your own cable if you don’t want to; there is almost certainly a cable available at eBay or Amazon that would serve the purpose, although you might need to search that long thread to figure out which one(s) will work:

USB host mode power management extension for Nexus 7 (mehrvarz.github.io)

Oh, and if you have ever wondered why you need a special “OTG” cable to connect an external device, it’s because pins 4 and 5 must be shorted together at the micro USB connector — some folks have made their own (also see here), though we DO NOT recommend that because they are fairly inexpensive to purchase, and if you do it wrong you could damage your device. On some other devices, you can use a particular value resistor (which varies depending on the model) instead of a direct short, and that indicates that the device should both send data and allow charging simultaneously, but that doesn’t work with a Nexus 7. That’s why you need “Timur’s Kernel” — as we understand it, it basically bypasses the (non-existent in the Nexus 7) circuitry that detects the resistor, and acts as if the resistor were present, thereby allowing the Nexus 7 USB port to be used for charging and transferring data simultaneously.

We are not recommending you do this, nor are we attempting to discourage you, but if you don’t understand the implications of rooting your device then we suggest you don’t do this until you have read up on the subject of rooting an Android device, and fully understand and accept the risks of rooting. And if you decide to proceed, do so at your own risk, realizing that there is a very real chance you could turn your Nexus 7 into an expensive paperweight. If that should happen, don’t come crying to us, because we warned you!

If you actually do this and get it to work, please feel free to post a comment. If possible, please mention which cables or Y-adapters, etc. you used. Also, if you find any other particularly informative pages in that much too long thread, please feel free to post the links and mention why they are significant.

An alternative way to power a Raspberry Pi

Did you know that you can use a $20 USB hub (but not just any old USB hub) to power a Raspberry Pi AND use the SAME hub to add additional USB ports to the Pi? Well, you can, as demonstrated in this video:


Direct YouTube Link

Apparently not all USB hubs will work, because many only put out something close to the 500 mA that is the specification for USB 2.0, whereas the Raspberry Pi Model B requires 700 mA. So, the hub has to put out a bit of extra current or it won’t power the Raspberry Pi properly.

There is a list of USB hubs that will work with the Raspberry Pi, but pay special attention to the comments in the rightmost column. Only a few of the USB hubs shown are known to be able to power the Raspberry Pi reliably. The one shown in the video is the Plugable 7 Port High Speed USB Hub, model USB2-HUB-AG7 (Amazon affiliate link – we make a small commission if you click on it and purchase a unit).

EDIT: A few weeks after we originally posted this article, the device described in the following article was released:

PiHub from Cyntech and Pimoroni – a new powered USB Hub for the Raspberry Pi

Unless you really don’t need that many USB ports, and you absolutely fall in love with the Raspberry-shaped case of the PiHub, we suggest you stick with the Plugable 7 Port hub mentioned above. Not only are you getting more USB ports, and a hefty 3 Amp power supply, but if you’re in the U.S.A. or Canada it’s actually available here and has mostly 4 and 5 star reviews on Amazon. And if you go with the PiHub, by the time you do the currency conversion (and let your credit card company take a small bite on the exchange rate), and pay for the shipping, you’ll be paying more and getting less (only four ports rather than the seven offered in the Plugable unit). But, you will be getting that lovely Raspberry-shaped case, so there’s that.

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