Mistery: I2C not working, blink working

Projects Networking, Protocols, and Devices
16

Hello again. I bought oscilloscope today and tried to measure what's going on.

Same programm on arduino uno and arduino mega. Right connection to both.

This is oscillogramm from uno(it's good):
On mega no impulses appear.

I also made an experiment. I wrote this code to mega and use oscilloscpe with it:

void setup()  { 
  // nothing happens in setup 
} 

void loop()  { 
 while(true){
    analogWrite(21, 0);         
    delay(1000);
    analogWrite(21, 100);         
    delay(1000);
    analogWrite(21, 255);         
    delay(1000);    
  } 
}

But i saw only one impulse at 5V with 1sec duration and then 2sec 0V.

My theory is that DAC is broken, but I still don't know why I2C isn't working.

Anyway powering from battery doesn't give any results.