Software PWM for Arduino/ATtiny

spwm.ino

void spwm(int freq, int pin, int spd) {
  digitalWrite(pin, HIGH);
  delayMicroseconds(spd * freq);
  digitalWrite(pin, LOW);
  delayMicroseconds(spd * (255 - freq));
}

int delay = 15;

void loop() {
    for (int i = 20; i < 240; i++)
      spwm(i, pin, delay);
    for (int i = 240; i > 18; i--)
      spwm(i, pin, delay);
}