int redDuty = 0;
int greenDuty = 0;
int blueDuty = 0;
int pwmPhase = 0;
int state = 0;
int cycles = 50;
int curCycle = 0;

void setup() {
  pinMode(PC13, OUTPUT);
  pinMode(PC14, OUTPUT);
  pinMode(PC15, OUTPUT);
  digitalWrite(PC13, HIGH);
  digitalWrite(PC14, HIGH);
  digitalWrite(PC15, HIGH);
}

void loop() {
  if(blueDuty > 0) digitalWrite(PC13, LOW); else digitalWrite(PC13, HIGH);
  if(greenDuty > 0) digitalWrite(PC14, LOW); else digitalWrite(PC14, HIGH);
  if(redDuty > 0) digitalWrite(PC15, LOW); else digitalWrite(PC15, HIGH);

  for(pwmPhase=0;pwmPhase < 255; pwmPhase++) {
    if(pwmPhase == blueDuty)
      digitalWrite(PC13, HIGH);
    if(pwmPhase == greenDuty)
      digitalWrite(PC14, HIGH);
    if(pwmPhase == redDuty)
      digitalWrite(PC15, HIGH);
    delayMicroseconds(20);
  }

  if(curCycle++ >= cycles) {
    switch(state) {
      case 0:
        if(++redDuty >= 255) {
          curCycle = 0;
          state = 1;
        }
        break;
      case 1:
        if(++greenDuty >= 255) {
          curCycle = 0;
          state = 2;
        }
        break;
      case 2:
        if(--redDuty == 0) {
          curCycle = 0;
          state = 3;
        }
        break;
      case 3:
        if(++blueDuty >= 255) {
          curCycle = 0;
          state = 4;
        }
        break;
      case 4:
        if(--greenDuty == 0) {
          curCycle = 0;
          state = 5;
        }
        break;
      case 5:
        if(++redDuty >= 255) {
          curCycle = 0;
          state = 6;
        }
        break;
      case 6:
        if(++greenDuty >= 255) {
          curCycle = 0;
          state = 7;
        }
        break;
      case 7:
        greenDuty--;
        if(--blueDuty == 0) {
          curCycle = 0;
          state = 1;
        }
        break;
    }
  }
}