void (*func)();

void func_1(){
  Serial.println("func_1");
  func = *func_2;
}

void func_2(){
  Serial.println("func_2");
  func = *func_1;
}
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("START");
  func = *func_1;
}

void loop() {
  // put your main code here, to run repeatedly:
  func();
  delay(1000);
}