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libraries/DHT/DHT.cpp

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+/******************************************************************
+  DHT Temperature & Humidity Sensor library for Arduino.
+
+  Features:
+  - Support for DHT11 and DHT22/AM2302/RHT03
+  - Auto detect sensor model
+  - Very low memory footprint
+  - Very small code
+
+  http://www.github.com/markruys/arduino-DHT
+
+  Written by Mark Ruys, mark@paracas.nl.
+
+  BSD license, check license.txt for more information.
+  All text above must be included in any redistribution.
+
+  Datasheets:
+  - http://www.micro4you.com/files/sensor/DHT11.pdf
+  - http://www.adafruit.com/datasheets/DHT22.pdf
+  - http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Sensors/Weather/RHT03.pdf
+  - http://meteobox.tk/files/AM2302.pdf
+
+  Changelog:
+   2013-06-10: Initial version
+   2013-06-12: Refactored code
+   2013-07-01: Add a resetTimer method
+ ******************************************************************/
+
+#include "DHT.h"
+
+void DHT::setup(uint8_t pin, DHT_MODEL_t model)
+{
+  DHT::pin = pin;
+  DHT::model = model;
+  DHT::resetTimer(); // Make sure we do read the sensor in the next readSensor()
+
+  if ( model == AUTO_DETECT) {
+    DHT::model = DHT22;
+    readSensor();
+    if ( error == ERROR_TIMEOUT ) {
+      DHT::model = DHT11;
+      // Warning: in case we auto detect a DHT11, you should wait at least 1000 msec
+      // before your first read request. Otherwise you will get a time out error.
+    }
+  }
+}
+
+void DHT::resetTimer()
+{
+  DHT::lastReadTime = millis() - 3000;
+}
+
+float DHT::getHumidity()
+{
+  readSensor();
+  return humidity;
+}
+
+float DHT::getTemperature()
+{
+  readSensor();
+  return temperature;
+}
+
+#ifndef OPTIMIZE_SRAM_SIZE
+
+const char* DHT::getStatusString()
+{
+  switch ( error ) {
+    case DHT::ERROR_TIMEOUT:
+      return "TIMEOUT";
+
+    case DHT::ERROR_CHECKSUM:
+      return "CHECKSUM";
+
+    default:
+      return "OK";
+  }
+}
+
+#else
+
+// At the expense of 26 bytes of extra PROGMEM, we save 11 bytes of
+// SRAM by using the following method:
+
+prog_char P_OK[]       PROGMEM = "OK";
+prog_char P_TIMEOUT[]  PROGMEM = "TIMEOUT";
+prog_char P_CHECKSUM[] PROGMEM = "CHECKSUM";
+
+const char *DHT::getStatusString() {
+  prog_char *c;
+  switch ( error ) {
+    case DHT::ERROR_CHECKSUM:
+      c = P_CHECKSUM; break;
+
+    case DHT::ERROR_TIMEOUT:
+      c = P_TIMEOUT; break;
+
+    default:
+      c = P_OK; break;
+  }
+
+  static char buffer[9];
+  strcpy_P(buffer, c);
+
+  return buffer;
+}
+
+#endif
+
+void DHT::readSensor()
+{
+  // Make sure we don't poll the sensor too often
+  // - Max sample rate DHT11 is 1 Hz   (duty cicle 1000 ms)
+  // - Max sample rate DHT22 is 0.5 Hz (duty cicle 2000 ms)
+  unsigned long startTime = millis();
+  if ( (unsigned long)(startTime - lastReadTime) < (model == DHT11 ? 999L : 1999L) ) {
+    return;
+  }
+  lastReadTime = startTime;
+
+  temperature = NAN;
+  humidity = NAN;
+
+  // Request sample
+
+  digitalWrite(pin, LOW); // Send start signal
+  pinMode(pin, OUTPUT);
+  if ( model == DHT11 ) {
+    delay(18);
+  }
+  else {
+    // This will fail for a DHT11 - that's how we can detect such a device
+    delayMicroseconds(800);
+  }
+
+  pinMode(pin, INPUT);
+  digitalWrite(pin, HIGH); // Switch bus to receive data
+
+  // We're going to read 83 edges:
+  // - First a FALLING, RISING, and FALLING edge for the start bit
+  // - Then 40 bits: RISING and then a FALLING edge per bit
+  // To keep our code simple, we accept any HIGH or LOW reading if it's max 85 usecs long
+
+  word rawHumidity;
+  word rawTemperature;
+  word data;
+
+  for ( int8_t i = -3 ; i < 2 * 40; i++ ) {
+    byte age;
+    startTime = micros();
+
+    do {
+      age = (unsigned long)(micros() - startTime);
+      if ( age > 90 ) {
+        error = ERROR_TIMEOUT;
+        return;
+      }
+    }
+    while ( digitalRead(pin) == (i & 1) ? HIGH : LOW );
+
+    if ( i >= 0 && (i & 1) ) {
+      // Now we are being fed our 40 bits
+      data <<= 1;
+
+      // A zero max 30 usecs, a one at least 68 usecs.
+      if ( age > 30 ) {
+        data |= 1; // we got a one
+      }
+    }
+
+    switch ( i ) {
+      case 31:
+        rawHumidity = data;
+        break;
+      case 63:
+        rawTemperature = data;
+        data = 0;
+        break;
+    }
+  }
+
+  // Verify checksum
+
+  if ( (byte)(((byte)rawHumidity) + (rawHumidity >> 8) + ((byte)rawTemperature) + (rawTemperature >> 8)) != data ) {
+    error = ERROR_CHECKSUM;
+    return;
+  }
+
+  // Store readings
+
+  if ( model == DHT11 ) {
+    humidity = rawHumidity >> 8;
+    temperature = rawTemperature >> 8;
+  }
+  else {
+    humidity = rawHumidity * 0.1;
+
+    if ( rawTemperature & 0x8000 ) {
+      rawTemperature = -(int16_t)(rawTemperature & 0x7FFF);
+    }
+    temperature = ((int16_t)rawTemperature) * 0.1;
+  }
+
+  error = ERROR_NONE;
+}