Okay, a little background: I am designing a micro controller daughter board (uCDB) that is nominally powered at 3.3V by a voltage regulator on a mother board (MB). Also I will have a programmer board (PB) that I can plug directly into the uCDB. As an alternative to powering the uCDB from the MB, I could instead draw power from the PB at 3.5V. This would be very convenient for instances where I wanted to power and program the uCDM even when it was not plugged into the MB. What happens if I try to do this and the uCDB is already powered by the MB? Is it a problem?
An obvious fix would be to put in a jumper that selects what voltage source to use, but requires more thought from the user each time he/she hooks it up; "Okay, what state am I in so I don't burn things up?" Any thoughts on how to allow both voltage sources to be on without burning things up and without mechanical switches/jumpers. Perhaps it is okay to just connect the two lines together either directly or through a resistor or something? Maybe the jumper is the best solution? I attached an image that hopefully summarizes my problem.