Making an arduino clone circuit which drops the auto power select circuit and replaces it with a simple SPDT switch.
Now the question is about switching powering pre FTDI chip or post FTDI chip, in other words is there any reason that anyone
can think of that the external power circuit needs to power the FTDI chip, assuming the FTDI chip will always be powered by the USB when it is plugged in my circuit.
The switch only decides whether the ATMEGA is powered by external power jack or USB.
The Seeeduino mega board uses a simple two position switch for USB/External power selection. They also power the FTDI chip only from the USB voltage source, so no current draw from the chip if using external power. I mentioned this at one time and someone stated (properly) that can cause 'backdoor' current flow from any of the FTDI pins wired to powered up signals. The only two FTDI I/O pins normally used are the send and receive data, and in the standard arduino type design these have series 1k ohm isolation resistor wired to the AVR chip. Anyway I always though it was a good idea, esp if the external power is from battery source hence saving the FTDI current draw when not being used. However optimum battery conservation wouldn't be using the on-board +5vdc regulator so maybe not a real benefit in most cases. Anyway I've had the Seeeduino for a couple of years and have had no problems with it no matter how I power the board. Also the manual auto-reset on/off switch is a winner also.