External power supply and Serial communication?

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?

Several sources confirm you can have both plugged in safely. However I have never done this myself personally.

http://arduino.cc/en/Main/arduinoBoardUno (Power Section says it selects automatically)

However it seems that since there is a diode drop between dc barrel jack and Vin which controls comparator and mosfet to determine which power source to use. You really need probably need like 7.4 Volts on barrel jack to be safe (as forum mentions). I would just use 9V to be safe.

And I'm not sure about the communicating with Raspeberry Pi aspect. Does the raspberry pi OS have drivers for the FTDI chip that is on the arduino board which coverts the RX/TX into USB signals? If there is no driver support you will be out of luck because the arduino wont show up on the Pi as a virtual com port. Have you looked into this?

This articles says talking to Pi over USB is possible (http://blog.oscarliang.net/connect-raspberry-pi-and-arduino-usb-cable/)

c_breeez:
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?

Yes.

But you'll need more than 6.6V to get 5V out of the Arduino regulator.

c_breeez:
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?

Yes.

Brilliant thanks a lot guys, Im going to get 9V or 12V dc psu.
Good to know serial will work fine!