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lib/DHT.cpp

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+/* DHT library 
+
+MIT license
+written by Adafruit Industries
+*/
+
+#include "DHT.h"
+#define NAN 0
+
+DHT::DHT(uint8_t pin, uint8_t type, uint8_t count) {
+  _pin = pin;
+  _type = type;
+  _count = count;
+  firstreading = true;
+}
+
+void DHT::begin(void) {
+  // set up the pins!
+  pinMode(_pin, INPUT);
+  digitalWrite(_pin, HIGH);
+  _lastreadtime = 0;
+}
+
+//boolean S == Scale.  True == Farenheit; False == Celcius
+float DHT::readTemperature(bool S) {
+  float f;
+
+  if (read()) {
+    switch (_type) {
+    case DHT11:
+      f = data[2];
+      if(S)
+      	f = convertCtoF(f);
+
+      return f;
+    case DHT22:
+    case DHT21:
+      f = data[2] & 0x7F;
+      f *= 256;
+      f += data[3];
+      f /= 10;
+      if (data[2] & 0x80)
+	f *= -1;
+      if(S)
+	f = convertCtoF(f);
+
+      return f;
+    }
+  }
+  Serial.print("Read fail");
+  return NAN;
+}
+
+float DHT::convertCtoF(float c) {
+	return c * 9 / 5 + 32;
+}
+
+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 > _count)
+        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;
+
+}