#define GPIU 5
uint32_t freq =0;

void setup() {
  pinMode(GPIU, OUTPUT);
  Serial.begin(115200);
  setCpuFrequencyMhz(10);
  freq = getCpuFrequencyMhz();
  Serial.print(("freq = "));
  Serial.println((freq));
  freq = getXtalFrequencyMhz();
  Serial.print("xtal =");
  Serial.println(freq);
  freq = getApbFrequency();
  Serial.print("apb = ");
  Serial.println(freq);
}

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