#define GPIO_pin 5
uint32_t Freq=0;
void setup()
{
  pinMode(GPIO_pin, OUTPUT);
  Serial.begin(115200);
  setCpuFrequencyMhz(10);
  Freq=getCpuFrequencyMhz()
;
Serial.print("CPU Freq = ");
Serial.print(Freq);
Serial.println(" MHz");
Freq = getXtalFrequencyMhz();
Serial.print("XTAL Freq = ");
Serial.print(Freq);
Serial.println(" Mhz");
Freq = getApbFrequency();
Serial.print("APB Freq = ");
Serial.print(Freq);
Serial.println(" Hz");
}

  void loop()
  {
    digitalWrite(GPIO_pin, 1);
    digitalWrite(GPIO_pin, 0);
  }