fully tested working DHT11/21/22 with built in '2 second' timer

DHT.cpp

@@ -3,3 +3,148 @@
 MIT license
 written by Adafruit Industries
 */
+
+#include "DHT.h"
+
+DHT::DHT(uint8_t pin, uint8_t type) {
+  _pin = pin;
+  _type = type;
+  firstreading = true;
+}
+
+void DHT::begin(void) {
+  // set up the pins!
+  pinMode(_pin, INPUT);
+  digitalWrite(_pin, HIGH);
+  _lastreadtime = 0;
+}
+
+float DHT::readTemperature(void) {
+  float f;
+
+  if (read()) {
+    switch (_type) {
+    case DHT11:
+      f = data[2];
+      return f;
+    case DHT22:
+    case DHT21:
+      f = data[2] & 0x7F;
+      f *= 256;
+      f += data[3];
+      f /= 10;
+      if (data[2] & 0x80)
+	f *= -1;
+
+      return f;
+    }
+  }
+  Serial.print("Read fail");
+  return NAN;
+}
+
+float DHT::readHumidity(void) {
+  float f;
+  if (read()) {
+    switch (_type) {
+    case DHT11:
+      f = data[0];
+      return f;
+    case DHT22:
+    case DHT21:
+      f = data[0];
+      f *= 256;
+      f += data[1];
+      f /= 10;
+      return f;
+    }
+  }
+  Serial.print("Read fail");
+  return NAN;
+}
+
+
+boolean DHT::read(void) {
+  uint8_t laststate = HIGH;
+  uint8_t counter = 0;
+  uint8_t j = 0, i;
+  unsigned long currenttime;
+
+  // pull the pin high and wait 250 milliseconds
+  digitalWrite(_pin, HIGH);
+  delay(250);
+
+  currenttime = millis();
+  if (currenttime < _lastreadtime) {
+    // ie there was a rollover
+    _lastreadtime = 0;
+  }
+  if (!firstreading && ((currenttime - _lastreadtime) < 2000)) {
+    return true; // return last correct measurement
+    //delay(2000 - (currenttime - _lastreadtime));
+  }
+  firstreading = false;
+  /*
+    Serial.print("Currtime: "); Serial.print(currenttime);
+    Serial.print(" Lasttime: "); Serial.print(_lastreadtime);
+  */
+  _lastreadtime = millis();
+
+  data[0] = data[1] = data[2] = data[3] = data[4] = 0;
+  
+  // now pull it low for ~20 milliseconds
+  pinMode(_pin, OUTPUT);
+  digitalWrite(_pin, LOW);
+  delay(20);
+  cli();
+  digitalWrite(_pin, HIGH);
+  delayMicroseconds(40);
+  pinMode(_pin, INPUT);
+
+  // read in timings
+  for ( i=0; i< MAXTIMINGS; i++) {
+    counter = 0;
+    while (digitalRead(_pin) == laststate) {
+      counter++;
+      delayMicroseconds(1);
+      if (counter == 255) {
+        break;
+      }
+    }
+    laststate = digitalRead(_pin);
+
+    if (counter == 255) break;
+
+    // ignore first 3 transitions
+    if ((i >= 4) && (i%2 == 0)) {
+      // shove each bit into the storage bytes
+      data[j/8] <<= 1;
+      if (counter > 6)
+        data[j/8] |= 1;
+      j++;
+    }
+
+  }
+
+  sei();
+  
+  /*
+  Serial.println(j, DEC);
+  Serial.print(data[0], HEX); Serial.print(", ");
+  Serial.print(data[1], HEX); Serial.print(", ");
+  Serial.print(data[2], HEX); Serial.print(", ");
+  Serial.print(data[3], HEX); Serial.print(", ");
+  Serial.print(data[4], HEX); Serial.print(" =? ");
+  Serial.println(data[0] + data[1] + data[2] + data[3], HEX);
+  */
+
+  // check we read 40 bits and that the checksum matches
+  if ((j >= 40) && 
+      (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) ) {
+    return true;
+  }
+  
+
+  return false;
+
+}

DHT.h

@@ -1,5 +1,32 @@
+
+#include <WProgram.h>
+
 /* DHT library 
 
 MIT license
 written by Adafruit Industries
 */
+
+// how many timing transitions we need to keep track of. 2 * number bits + extra
+#define MAXTIMINGS 85
+
+#define DHT11 11
+#define DHT22 22
+#define DHT21 21
+#define AM2301 21
+
+class DHT {
+ private:
+  uint8_t data[6];
+  uint8_t _pin, _type;
+  boolean read(void);
+  unsigned long _lastreadtime;
+  boolean firstreading;
+
+ public:
+  DHT(uint8_t pin, uint8_t type);
+  void begin(void);
+  float readTemperature(void);
+  float readHumidity(void);
+
+};