I built a project with an Atmega328 on a copper protoboard and all the circuitry on it works out fine except this one anomaly...
I have two leads coming off the board that attach to a 5v regulator which powers the whole project.
I have an arduino board I attached to a 9v power supply (9v 650mA, sparkfun).
When I plug the leads from my project into the vin and ground of the arduino, it comes to life no problems.
When I touch the leads to the + and - of the barrel jack it comes to life no problems.
When I touch the leads to the + and - of the power supply plug I get nothing.
When I touch the leads to a 12v 7Ah battery the leds in my project blink quickly like everything is resetting over and over.
So the question is this...
What does the arduino do between vin and ground that the power supply doesn't do?
I quick look at the schematic (http://arduino.cc/en/uploads/Main/arduino-uno-rev2-schematic.pdf
) shows a 47uf decoupling cap.
I added a 47uF capacitor both before and after the 5v regulator, to no avail.
I'm really stumped... any ideas?