How are you powering your board?
How are you measuring to say "I have tried to make loop , the voltage is fall down a bit then rise up ."? Multimeter? Oscilloscope? LED?
Be more specific.
Be aware that interrupting power for even the tiniest moment will cause the processor to reset because microcontrollers are absurdly fast. What is too small for you to spot is a lifetime for one of them!
How long are the cables from your power-supply? What is the voltage?
Is it a battery (that might be about to go flat).
Please hang a voltmeter between Vdd and Vss and tell us what it reads - while you try to run your code.
Similarly at the input of the voltage regulator.
And on the Reset pin of the microcontroller.
We (probably) can help. But you do have to cooperate and answer all the questions we ask as fully as you can. We ask specific questions as the answers give us clues as to what might be wrong.
"It doesn't do what I want" is useless without telling us (a) what you're trying to do and (b) what it actually is doing - as quantitatively as you can.
