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I read the following online: If you plug in both USB power (say from a pc) and external power via the 2.1mm jack, the Arduino chooses power source automatically.

If the external 2.1mm DC is providing greater than 6.6V, the Arduino will take power from it instead of the USB.

Firstly, is this accurate?

Secondly, my real question: If I am powering my Arduino via external DC, can I still communicate with my Raspberry Pi using serial via the USB?

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Reset to default
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Yes you can. It still leaves the data/ground lines intact, it just doesn't bridge the 5V line.

Arduino Uno Schematic v.3:

The USBVCC is directly from the 5V on the USB jack (well, technically there's a fuse there). It then goes through a transistor (T1) that only allows power through when there is no power coming from the 5V regulator (that regulates the VIN voltage).

The USB jack:

You can see pin 1 of the USB jack (5V) goes through the fuse to USBVCC. The next ones D+/D- are the data lines that go through resistors to the ATMEGA16U (used for USB to UART). The last one just ties into ground.

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    Great answer, penguin! (Voted). Does it matter which order you connect the devices? (Can you run your Arduino via USB, then connect Vin once it's running, OR power it via Vin and connect USB while it's on? It sounds like it's wired to prefer Vin power, so what happens when you are powered over USB and connect Vin? Does it switch power sources on the fly?)
    – Duncan C
    Commented Jul 28, 2014 at 12:19
  • @Duncan it should be fine to connect USB when already powered by VIN (although it will reset when the computer connects to the COM port. As far as VIN while already on USB, it looks like it will switch but I'm not sure if the transistor will let the voltage drop low enough to reset the board. Either way is fine, but it might reset a few seconds in.
    – Anonymous Penguin
    Commented Jul 28, 2014 at 12:56
  • The datasheet for the FDN340P MOSFET might provide a little more information: fairchildsemi.com/ds/FD/FDN340P.pdf
    – Anonymous Penguin
    Commented Jul 28, 2014 at 15:24

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