afremont:
Sounds like more of a CYA thing than anything. It's obvious that you are right in that the USB power ends up on the +5V rail, but it appears that happens from within the mega32u usb chip?No, the USB5v is routed to the AVR 5vdc bus via the FET isolation switch, which is on when no Vin is avalible. mega32U gets it's Vcc from same bus as the 328P does.
At any rate, +5V is being applied to the output of the regulator and not the input when powering from USB. The datasheet makes it clear that this shouldn't cause a problem with the regulator either. I suspect that maybe these "regulated" supplies provided an initial unregulated spike at power on.
I thought CrossRoads researched the datasheet for the regulator in question and it did show a recommendation of a 'bypass diode' wired across the regulator if there was a chance in the circuit for the output voltage to ever be higher then it's input voltage. I haven't the specific regulator part number so I've never looked over it's datasheet personally.
So what is your opinion on what to advice when people ask if they can do that? Should one mention of the 'official' arduino warning be given? I don't own a Uno, just a half dozen or so older arduino boards (mix of 328 and 1280 boards plus a few custom jobs). I have no qualm of doing it myself on my boards, but telling others to do it on a board model I haven't played with, plus with the official arduino stance on the issue, makes responding to this often asked question a little 'problematic' at least for me.
So I just tend to raise the question of why the arduino warning and sit back and watch the fun.
