Bleedover in Moteino Mega power supply

[jrdoner]

I have a Moteino Mega which partly talks to 5 v. components and partly to 3.3 v. stuff.   Originally, I was going to power it at 5 v., but then I decided a 3.3 v. supply for the Mega and it 3.3 v. fellows was the better idea.

But that has led to a funny problem, which could have bad consequences.

When I power this up from my Arduino IDE, I am supplying 5 v.  As soon as I plug the connector from USB to the Mega, my onboard power rises from 3.3 to 5 v.   That seems odd, since Vin is supposed to be Vin, not out.   But be that as it may, can I just cut the 5v. pin off my FTDI input to the Mega?  Does it need the 5 v. input while being programmed, or will it program just fine without 5 v. in?   The other issue would be, though, that the output serial might then be operating at 3.3 v. levels, so perhaps I would not be able to get serial output back to the IDE serial monitor.   

Any advice concerning this quandary will be appreciated.  I don't want to cut the 5 v. input pin off until I'm sure I can get away with it.

[Felix]

You should not cut anything. All pins are there because they are needed by design. VIN is on the FTDI header as well as on the side header near GND. It can take anywhere from ~3.55v to 16v, but i would not put more than 12V on it, not less than ~3.6v.
The core operates at 3.3v no matter what, unless the regulator is damaged and outputs something other than 3.3v.

I really have no idea what you mean by this statement. Can you rephrase?

As soon as I plug the connector from USB to the Mega, my onboard power rises from 3.3 to 5 v. That seems odd, since Vin is supposed to be Vin, not out.

[jrdoner]

Felix,

Thanks for your reply.  My quandary is this.  I am powering the Mega and another 3.3 v. only device (display) using 3.3 v. regulator.  When the system is not attached to the computer USB, my local power is 3.3 v., as expected and desired.   When I plug in the USB FTDI adapter to reprogram, or use serial, the 5 v. on the USB side apparently takes over, and I find my 3.3 v. on board power rail jumps to 5 v.   Clearly the Vin pin is somehow backfeeding the USB 5 v. out onto my 3.3 v. power rail. Fortunately, I discovered this before I hooked up any other 3.3 v. components. 

The real question is, which I guess I didn't phrase well, can I eliminate the input  5v. pin on the FTDI header, thereby keeping the Mega at 3.3 v., and will serial programming from the IDE still work, and will I still have serial communications back to the IDE?

[Felix]

The VIN pin is meant as an input. But if you put 5V on it and attach another board to VIN, then that VIN will obviously pass to your other board.
I think I understand what you're trying to do now, but not knowing the entire scheme and the other board, I cannot really say you should cut the VIN from the FTDI header. I would probably just bend the pin instead before cutting anything, and try it that way.
Here is the schematic for the MEGA.

[jrdoner]

Felix,

Rather than bend a pin, I made a 2" pass-through cable for the FTDI connector which left out the Vin pin.  I am pleased to say that the programs still download successfully, my on board supply remains at 3.3 v., and serial communications also works fine.

[Felix]

Ok great, thanks for the update!