void set_output(uint8_t pwm_lvl) {
  if (pwm_lvl > FAST_PWM_START && pwm_lvl != 255) {
    TCCR0A = PWM_FAST; // fast-PWM
  } else {
    TCCR0A = PWM_PHASE; // phase corrected PWM
  }
  PWM_LVL = pwm_lvl;
}