I am trying to use an external power supply to power the STM32f0 discovery board. I used a 7805cv to regulate a 12v DC to 5v. When I connect my 5v output to the board's 5v pin, it will start to function normally for about half a minute to a minute. Then all of a sudden everything will freeze, reset button no use, the only thing moving is the LD2 onboard LED flashing red as usual. I measured the 5v pin and 3v pin onboard, each says 5v and 3v, which seems about correct. If I disconnect the power and reconnect things will start to function normally again, for a minute or so before it freezes. Does anyone have a similar experience? What am I doing wrong? I also measure the 5v pin voltage when its powered by a USB cable, it's around 4.7v instead.
PS: I tried other boards, no difference. And, after I connect the power, the board does not seem to be affected if I don't start to run the program by pressing reset. the problem appears after the program starts running.
UPDATE: I fixed the problem by dividing the 5v to 4.7v. But I still don't understand the reason behind it.