void setup() {
  noInterrupts();
  CLKPR = _BV(CLKPCE);  // enable change of the clock prescaler
  CLKPR = _BV(CLKPS3);  // divide frequency by 256
  interrupts();

  Serial.begin(1000);
}

void loop() {
  Serial.write((uint8_t)0xf0);
}