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