If I supply 5 volts (out of a computer power supply), what exactly are the differences between plugging into 5v vs Vin.
I didn't expect any difference, and since I know it is 5v, I figured the right thing to do was to plug it into 5v.
If 5v is connected and I turn the power supply on it puts the mega256 in some unknown state. If I then hit the reset button it works fine.
If the power supply is on, and I then connect to 5v, it works fine.
If I attach to Vin, it works either way.
What could be the issue? The power supply ramping up voltage/amps too slow or something?
Vin is the other side of the reverse polarity connection diode.
I often connect two Arduinos together by powering the second from the +5v of the first (ie. connecting both +5v lines together).
You might be right about the power supply ramping up too slowly. You could always use the +12v of the computer power supply and connecting it to the "power plug" (not Vin). Then the voltage regulator can do its stuff.
ctwo:
If I supply 5 volts (out of a computer power supply), what exactly are the differences between plugging into 5v vs Vin.
I didn't expect any difference, and since I know it is 5v, I figured the right thing to do was to plug it into 5v.
If you have a regulated 5V supply, you should just connect to the 5V pin / node.
Vin has a regulator which supplies 5V to the 5V pin / node. The regulator needs >7V to properly regulate down to 5V. Putting 5V in will get you less than 5V out, leading to unstable operation.
I suspect the issue with the Arduino not starting up properly when the supply is powered up already connected is that it's not generating a good clean rising voltage at start up, only an oscilloscope can tell.
If the supply is a standard computer supply its designed to supply much more current than the Arduino takes, this could mean its not behaving within its specification with a much smaller load (Arduino takes about 50mA or so, computers usually take some number of amps).
Its always a good idea to measure any power source connected to the +5V pin since overvoltage here will fry the board, whereas Vin has a regulator that helps to protect the board.