uint8_t state = 1;                    // current state (start in last state == default == Datum)
uint32_t previousMillis = 1;              // time management
constexpr uint32_t interval = 1500;   // default periode between steps

void setup() 
{
  Serial.begin(115200);
  
  Serial.println("Setup");
    
}

void loop()
{
  // Serial.println("loop");

  OLED();
 
}

void OLED()
{
  // Serial.println("OLED");
    
  switch (state) 
  {
    case 0:                // Kalenderwoche            // wenn wir in Schritt 0 sind
      Serial.print("Millis: ");Serial.println(millis() - previousMillis);
      if (millis() - previousMillis >= interval)       // und das Intervall abgelaufen ist
      {     
        // previousMillis = millis();
        previousMillis += interval;                    // merken wir uns die Zeit
        state++;                                       // drehen den Status weiter

        Serial.println("case 0");                              // und führen ab hier eine Aktion aus
      }
    break;
    
    case 1 :                             // Temperatur
      Serial.print("Millis: ");Serial.println(millis() - previousMillis);
      if (millis() - previousMillis >= interval) 
      {
        //  previousMillis = millis();
        previousMillis += interval;
        state++;

        Serial.println("case 1");
      }
    break;
          
    case 2 :                             // Luftfeuchte
      Serial.print("Millis: ");Serial.println(millis() - previousMillis);
      if (millis() - previousMillis >= interval) 
      {
        //  previousMillis = millis();
        previousMillis += interval;
        state++;

        //u8x8.clear();
        Serial.println("case 2");
      }
    break;

    case 3 :                             // Druck
      Serial.print("Millis: ");Serial.println(millis() - previousMillis);
      if (millis() - previousMillis >= interval) 
      {
        //  previousMillis = millis();
        previousMillis += interval;
        state++;

        Serial.println("case 3");
      }
    break;

    default :                             // Datum
      Serial.print("Millis: ");Serial.println(millis() - previousMillis);
      if (millis() - previousMillis >= interval) 
      {
        //  previousMillis = millis();
        previousMillis += interval;
        state = 0;
        Serial.println("case default");
      }
    break;

  }
}