// #define SERIAL_ON

#define PWM_PIN LED_BUILTIN
int i, freq = 1000;  // 1Hz peak to peak
bool j;

void setup() {
#ifdef SERIAL_ON
  Serial.begin(115200);
  Serial.println("PWM on LED_BUILTIN");
#endif

  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
#ifdef SERIAL_ON
  Serial.print(j);
  Serial.print(" ");
  Serial.print(i);
  Serial.println();
#endif

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

  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));  // invert state

  switch (j) {
    case (0):
      delayMicroseconds(freq * j + i);  // % duty cycle
      digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
      delayMicroseconds(freq * !j - i);
      break;
    case (1):
      delayMicroseconds(freq * j - i);
      digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
      delayMicroseconds(freq * !j + i);
      break;
  }

  i++;  // increasing to 100% duty cycle
}