power supply
I have two options for power sources for my board, which simply has an ATMega on it — one is a 24 VDC source going into a switching regulator that supplies the board with 5 V. Another source is via a USB Bus using an FT232R that could also supply the board with 5 V. Is there a way I can ensure that only one or the other powers the board and not both?
Or could both these be plugged in parallel and have no effect on the circuit? (I find that hard to imagine)
The circuit looks good and shows a lot of thought went into its design. One thing I would add is a 1K resistor from the gate of Q1 to Vin, mounted as close to the gate as possible. This reduces noise pickup by the mosfet (the gate is very sensitive to HF noise) and protects it against voltage spikes. The gate is protected by built-in diodes but current limiting protects the diodes.
Linear LTC4067 is your solution. It will "add" both the battery load and the power supply together. the example below is taken from the LTC4067 datasheet. If you install the "optional" mosfet, you will get on OUT both the battery load and the USB load.
Best Answer
Yes, the Arduino board does this. See the power section of their schematic: http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf