void setup() {
  // put your setup code here, to run once:
  pinMode (PA7, OUTPUT);
  pinMode (PA3, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  int n = 0;
  int i = 0;
  for (; i <= 255; i++)
  { n = i;
    delay(2);
    analogWrite(PA7, n);
  }
  for (; i >= 0; i--)
  { n = i;
    analogWrite(PA7, n);
    delay(2);
  }
}