// from https://docs.arduino.cc/tutorials/generic/secrets-of-arduino-pwm
// modified to be non-blocking
// https://wokwi.com/projects/371148004268653569

void setup()
{
  pinMode(13, OUTPUT);
}

void loop(void) {
  pwm();
}

void pwm()
{
  constexpr unsigned long periodUs = 1000UL;
  constexpr unsigned long onUs = periodUs * 0.10;
  static int state = LOW;
  static unsigned long last = -periodUs;
  static unsigned long interval = periodUs - onUs;
  if (micros() - last < interval ) return;
  last += interval;
  if (state == LOW) {
    digitalWrite(13, HIGH);
    state = HIGH;
    interval = onUs;
  } else {
    digitalWrite(13, LOW);
    state = LOW;
    interval = periodUs - onUs;
  }
}