I'm not entirely certain this is the right forum to ask, so if I you can direct me to a better place, it would be much appreciated.
I've been looking at the schematic for the Duemilanove and it's automatic choice of power source. I notice that an LM358 is being used as a voltage comparator, diving VIN roughly by half, then comparing it to the output of the FTDI's 3.3 volt output.
I'm not entirely sure I understand why this is necessary. Two possibilities come to mind. The entire comparator circuit could be removed, leaving the +5 volts from IC4 connected to the USB bus. I can see this being an issue, depending on difference between IC4's output and the voltage output on the USB connection. There is a possibility of unwanted current flow.
Another thought, is to have VIN driving a ~5v zener diode, which itself biases the base of a transistor controlling USBVCC. Similar to the comparator setup, but without IC5.
I understand there is probably a very good reason for the current design, and I don't understand just what that reason is. Any insight the group can provide would be very educational.