From looking at the schematic, it should be OK to plug in the 9v wall transformer and the USB cable at the same time. Looks like there is logic and a couple of fets to shut off the power from the USB if the power plug is used. I am about 90% sure but wanted to double check. (smoke bad!)
Yes, it's safe and designed the way you stated, auto switching voltage source if external (or Vin pin) has voltage on it >7.5vdc or so.
On the duemilanove, I find that if you plug in a wall wart and the USB cable (plugged into PC but turned off) then it will select the wall wart but it won't run properly. Power seems to go down the USB...
Slightly off-topic, but this thread is as close as I could find:
I've built a custom shield board for my Duemilanove. It has a 7805 regulator using the VIN pin for input. My intent is to power both boards with 12vdc plugged into the power jack.
However, the other day I forgot to connect the 12vdc to the power jack, but the boards ran anyway! The USB 5vdc was running the Duemilanove and I think that some power was feeding backwards through the Duemilanove's on-board regulator (MC33269D) to the VIN pin.
There was 4.3vdc on the VIN pin and after going through my 7805 it was 3.0vdc. It appears that was enough to run the PCA9540B on my shield board.
Will power REALLY flow backwards through the on-board regulator, or should I look for some other explanation?
I think that some power was feeding backwards through the Duemilanove's on-board regulator
I could be wrong, but I thought, when being powered, you could access the unregulated power through the VIN pin? :-?
TchnclFl: I was not powering through the power jack, even 'tho that was my intent. I forgot to connect the 12vdc. So does the USB power go backwards through the on-board regulator to VIN?
I was not powering through the power jack
Ah, oopsies. I need to read more carefully ;).
does the USB power go backwards through the on-board regulator to VIN?
I can't say either way. Maybe, maybe not. You'll need to wait for someone more experienced ;).
I don't own a Duemilanove but rather a RS-232 serial clone and a Seeeduino mega board. Neither of my boards has auto-voltage switching so I can't test or really comment on your problem.
I never really liked the auto voltage switching function as I prefer manual switches or jumpers for power switching, mostly because it's simpler and doesn't allow for funny things like you are seeing. Back door circuit is what one may call it, as you are obviously seeing it.