[quote author=Nick Gammon link=topic=68963.msg513372#msg513372 date=1313296532]
Well I presume from the original post that this is battery powered and not connected via USB (then he wouldn't need the battery power).
But I take your point that, if you connect Vin to less than 6.6V, and then connect the board up to a USB port as well, then the USB is not properly isolated.
Isn't there a design problem here though? It is recommended to connect the power plug on the Arduino to 7 to 12V DC, so if you connect it to 7V, then after the diode drop of around 0.7V, Vin will be 6.3V. This is too low for the auto-voltage selector which will also attempt to connect it to the USB. Also the hardware page for the Uno says:
Input Voltage (limits) 6-20V
So connecting the power port to 6V would mean Vin would be 5.3V, which is definitely under 6.6V.
Yes, it is a pretty 'fragile' design and consumes too much board space and uses too many components for the problem trying to be solved, IMHO. The first original arduino USB board used a simple 3 male pins with a jumper clip. You placed the jumper at one side for USB power and the other side for external power. Simple, fool proof, and effective. They might have considered upgrading to a manual two position switch like the Seeeduino designs uses, but they went with an over-engineered design that adds more costs then required for the function needed. Plus if you power the board with an external regulate +5vdc wired to the shield +5vdc pin (as many users do), there is no isolation from the USB port when uploading or using the serial monitor. I know, Artist don't deal well with jumper clips, and OMG they may lose the jumper clip. ;)
The Seeduino design went one step further and powers the on board FTDI chip from only USB power, cleaver as if you have nothing plugged into the USB port why would you want the FTDI to consume +5vdc board power? That would help in battery powered projects. And speaking about battery powered application, maybe have a trace cut pad so the on-board power led can be disable for power sensitive applications. There is still room for the Arduino to continue to improve the design of the basic board instead of just concentrating on adding new features.