unsigned long lastPrinting;
bool operationStarted = false;

void setup() {
  Serial.begin(115200);
  Serial.println(F("type 1 to start printing every 200ms and 0 to stop"));
}

void loop() {
  int r = Serial.read(); // will be -1 if nothing is available to read
  if (!operationStarted && r == '1') {
    operationStarted = true;
    lastPrinting = millis() - 200;
    Serial.println(F("STARTING"));
  }
  else if (operationStarted && r == '0') {
    Serial.println(F("STOPING"));
    operationStarted = false;
  }

  if (operationStarted) {
    if ((millis() - lastPrinting) >= 200) {
      Serial.println("Tick");
      lastPrinting = millis();
    }
  }
}