Applied DHT22 library FIX as per adafruit/DHT-sensor-library#94 (comment)

Author: fontanon

ESP32_Heltec_V1_LoRa_Mote/libraries/DHT_sensor_library/DHT.cpp

@@ -134,38 +134,61 @@ boolean DHT::read(bool force) {
 
   // Send start signal.  See DHT datasheet for full signal diagram:
   //   http://www.adafruit.com/datasheets/Digital%20humidity%20and%20temperature%20sensor%20AM2302.pdf
-
-  // Go into high impedence state to let pull-up raise data line level and
-  // start the reading process.
-  digitalWrite(_pin, HIGH);
-  delay(250);
-
-  // First set data line low for 20 milliseconds.
+  // First set data line low to send start signal.
   pinMode(_pin, OUTPUT);
   digitalWrite(_pin, LOW);
-  delay(20);
+  // set wait time according to sensor type.
+  switch (_type) {
+    case DHT22:
+    case DHT21:
+      delayMicroseconds(1100);  //data sheet says at least 1ms, 1.1ms should be ok
+      break;
+    case DHT11:
+    default:
+      delay(20); //data sheet says at least 18ms, 20ms just to be safe
+      break;
+  }
 
   uint32_t cycles[80];
   {
     // Turn off interrupts temporarily because the next sections are timing critical
     // and we don't want any interruptions.
+    // NOTE: From this point we cannot use function such as delay(), delayMicroseconds(),
+    //       Those functions among others rely on interrupts.
     InterruptLock lock;
 
-    // End the start signal by setting data line high for 40 microseconds.
-    digitalWrite(_pin, HIGH);
-    delayMicroseconds(40);
-
+    // End of the start signal.
     // Now start reading the data line to get the value from the DHT sensor.
     pinMode(_pin, INPUT_PULLUP);
-    delayMicroseconds(10);  // Delay a bit to let sensor pull data line low.
+    // from this point we listen, it's sensor's turn to talk.
+
+
+    // The following step is a busy-wait loop to resume processing only
+    // when the data line has been pulled LOW by the sensor.
+    //
+    // To avoid infinite loops here, we use a counter as a timeout.
+    // 700 iterations is about 4 ms on a 16Mhz processor.  The sensor should
+    // set the pin LOW within 20 to 40 microseconds.
+    // The (F_CPU / 16000000.0) is a ratio to keep the timeout to approximately
+    // the same period if processor used runs at a different frequency.
+    unsigned long timesup = (F_CPU / 16000000.0) * 700;
+    unsigned long count = 0;
+    while (digitalRead(_pin) == HIGH && ++count < timesup)
+        ;
+    if (count == timesup) {
+      DEBUG_PRINTLN(F("Timeout waiting sensor to pull the data line LOW."));
+      _lastresult = false;
+      return _lastresult;
+    }
 
-    // First expect a low signal for ~80 microseconds followed by a high signal
-    // for ~80 microseconds again.
+    // expect a low signal for ~80 microseconds.
     if (expectPulse(LOW) == 0) {
       DEBUG_PRINTLN(F("Timeout waiting for start signal low pulse."));
       _lastresult = false;
       return _lastresult;
     }
+
+    // followed by a high signal for ~80 microseconds.
     if (expectPulse(HIGH) == 0) {
       DEBUG_PRINTLN(F("Timeout waiting for start signal high pulse."));
       _lastresult = false;
@@ -184,6 +207,7 @@ boolean DHT::read(bool force) {
       cycles[i]   = expectPulse(LOW);
       cycles[i+1] = expectPulse(HIGH);
     }
+    // Interrupts will turn back on when exiting this scope.
   } // Timing critical code is now complete.
 
   // Inspect pulses and determine which ones are 0 (high state cycle count < low