// fade LED_BUILTIN

int i, speed = 1000; // 1kHz
bool j;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  if (i > speed) { // end of cycle
    j = !j; // change phase
    i = 0; // restart cycle
  }

  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));  // start with HIGH level
  switch (j) {
    case (0):
      delayMicroseconds(speed * 0 + i);  // % duty cycle @ 1KHz
      digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
      delayMicroseconds(speed * 1 - i);
      break;
    case (1):
      delayMicroseconds(speed * 1 - i);
      digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
      delayMicroseconds(speed * 0 + i);
      break;
  }
  i++;
}