sensor/dht: wait DHT_MIN_INTERVAL after initialization

code/espurna/sensors/DHTSensor.h

@@ -20,6 +20,8 @@
 #define DHT_CHIP_DHT21              21
 #define DHT_CHIP_AM2301             21
 
+#define DHT_DUMMY_VALUE             -255
+
 class DHTSensor : public BaseSensor {
 
     public:
@@ -75,6 +77,9 @@ class DHTSensor : public BaseSensor {
             }
             _previous = _gpio;
 
+            // Set now to fail the check in _read at least once
+            _last_ok = millis();
+
             _count = 2;
             _ready = true;
 
@@ -126,7 +131,11 @@ class DHTSensor : public BaseSensor {
         void _read() {
 
             if ((_last_ok > 0) && (millis() - _last_ok < DHT_MIN_INTERVAL)) {
-                _error = SENSOR_ERROR_OK;
+                if ((_temperature == DHT_DUMMY_VALUE) && (_humidity == DHT_DUMMY_VALUE)) {
+                    _error = SENSOR_ERROR_WARM_UP;
+                } else {
+                    _error = SENSOR_ERROR_OK;
+                }
                 return;
             }
 
@@ -246,8 +255,8 @@ class DHTSensor : public BaseSensor {
         unsigned long _last_ok = 0;
         unsigned char _errors = 0;
 
-        double _temperature = 0;
-        double _humidity = 0;
+        double _temperature = DHT_DUMMY_VALUE;
+        double _humidity = DHT_DUMMY_VALUE;
 
 };