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I'm working on a project using Sparkfun's IOIO for Android (https://www.sparkfun.com/products/10748) where power consumption is a concern. The IOIO board provides the phone with 500 mA charging if I'm correct, which is too much for continuous operation.

There's a trick I have in mind where I want to power the phone and the IOIO board separately from the same external power source (Of course the phone gets its 3.7v while the IOIO gets 5v). The catch here is that I want to cut off the 5v line on the cable from the IOIO to the phone, leaving the GND, Rx and Tx (Which are good enough for any serial transmission).

schematic

simulate this circuit – Schematic created using CircuitLab

(Middle lines are those in the micro-USB cable)

I'm kind of limited on USB cables so I don't want to make a mistake. Is my theory feasible with the IOIO? Or will the Android OS not detect it?

(NOTE: My project uses a magnetometer, so I can't resort to bluetooth which might interfere with the readings)

Ahmed Farid
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  • Please make a drawing / circuit diagram of what you are trying to do. It is not clear from the text. – jippie May 26 '13 at 17:41
  • ok wait. I'll draw it right away – Ahmed Farid May 26 '13 at 17:42
  • Take a look now – Ahmed Farid May 26 '13 at 17:50
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    i have IOIO board and connected my phone by bluetooth.is it possible to use bluetooth on your project – yogece May 26 '13 at 17:57
  • That's a clever way of doing it, but I'm afraid bluetooth might interfere with the magnetometer readings. – Ahmed Farid May 26 '13 at 17:59
  • much clearer now. – jippie May 26 '13 at 18:14
  • What magnetometer? – Andy aka May 26 '13 at 20:11
  • My project uses the phone's magnetometer for readings. I don't want to use bluetooth which might interfere with the readings. – Ahmed Farid May 27 '13 at 02:23
  • What is the basis for you thinking that the Bluetooth will interfere with the magnetometer? We have successfully designed a wireless magnetometer sensor that used the 2.4GHz spectrum and saw no problems. – Amoch May 27 '13 at 02:37
  • Thing is I didn't study wireless signals at college, but my humble understanding is that anything wireless is based on a magnetic field, which in my opinion could cause an interference (Maybe I'm wrong). STILL, my main question remains unchanged in terms of wired connections. I still want to know an answer for my proposed technique. – Ahmed Farid May 27 '13 at 09:37

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USB doesn't have the notion of TX and RX lines, it's not a UART. The data lines are D+ and D- used for bidirectional differential signaling. The VBUS line (which you refer to as Vcc) is essential for detection the presence of the host (IOIO in this case), so what you're suggesting will not work. The best you can do is use the current limiting trimmer that's on the IOIO. Start with it fully clockwise, then, with the Android connected, turn it counter-clockwise until the connection drops, then back some. The minimum current that can be achieved like that varies by phone model. Also, turning the screen and comms off on the Android will reduce current significantly provided the phone battery is fully charged.

Hint: in the future, you're most likely to find answers to IOIO-related questions on the ioio-users Google group.

Ytai
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  • @AhmedFarid Ytai's answer is correct. The VBUS/VCC on the usb cable is required for the connection to work. The trimmer is the only way to change how much current draw is allowed. But the real question is, if you are powering both from the same external supply, why? Are you regulating the 5v down to 3.7v? The phone would be more efficient at doing that itself from it's normal 5v charging. – Passerby May 29 '13 at 05:36
  • Never thought the creator himself would answer my question! I never noticed the limiter thing. This is best way to go for me. Thanks Ytai! – Ahmed Farid May 29 '13 at 08:19
  • @Passerby Input directly to the phone is a better option indeed, but I have the issue of having a single battery system supporting every component in my project. In other words, I'm not using the phone's battery. This I believe leads to the need to connect both of them separately and reduce current draw as much as possible. My project is kind of like a robot: remote and autonomous. – Ahmed Farid May 29 '13 at 08:25