Merge branch 'dynamic_timing'

Author: tdicola

DHT.cpp

@@ -1,4 +1,4 @@
-/* DHT library 
+/* DHT library
 
 MIT license
 written by Adafruit Industries
@@ -9,8 +9,9 @@ written by Adafruit Industries
 DHT::DHT(uint8_t pin, uint8_t type, uint8_t count) {
   _pin = pin;
   _type = type;
-  _count = count;
-  firstreading = true;
+  _firstreading = true;
+  // Note that count is now ignored as the DHT reading algorithm adjusts itself
+  // basd on the speed of the processor.
 }
 
 void DHT::begin(void) {
@@ -22,158 +23,198 @@ void DHT::begin(void) {
 
 //boolean S == Scale.  True == Farenheit; False == Celcius
 float DHT::readTemperature(bool S) {
-  float f;
+  float f = NAN;
 
   if (read()) {
     switch (_type) {
     case DHT11:
       f = data[2];
-      if(S)
-      	f = convertCtoF(f);
-      	
-      return f;
+      if(S) {
+        f = convertCtoF(f);
+      }
+      break;
     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;
+      if (data[2] & 0x80) {
+        f *= -1;
+      }
+      if(S) {
+        f = convertCtoF(f);
+      }
+      break;
     }
   }
-  return NAN;
+  return f;
 }
 
 float DHT::convertCtoF(float c) {
-	return c * 9 / 5 + 32;
+  return c * 9 / 5 + 32;
 }
 
 float DHT::convertFtoC(float f) {
-  return (f - 32) * 5 / 9; 
+  return (f - 32) * 5 / 9;
 }
 
 float DHT::readHumidity(void) {
-  float f;
+  float f = NAN;
   if (read()) {
     switch (_type) {
     case DHT11:
       f = data[0];
-      return f;
+      break;
     case DHT22:
     case DHT21:
       f = data[0];
       f *= 256;
       f += data[1];
       f /= 10;
-      return f;
+      break;
     }
   }
-  return NAN;
+  return f;
 }
 
 float DHT::computeHeatIndex(float tempFahrenheit, float percentHumidity) {
   // Adapted from equation at: https://github.com/adafruit/DHT-sensor-library/issues/9 and
   // Wikipedia: http://en.wikipedia.org/wiki/Heat_index
-  return -42.379 + 
-           2.04901523 * tempFahrenheit + 
+  return -42.379 +
+           2.04901523 * tempFahrenheit +
           10.14333127 * percentHumidity +
           -0.22475541 * tempFahrenheit*percentHumidity +
           -0.00683783 * pow(tempFahrenheit, 2) +
-          -0.05481717 * pow(percentHumidity, 2) + 
-           0.00122874 * pow(tempFahrenheit, 2) * percentHumidity + 
+          -0.05481717 * pow(percentHumidity, 2) +
+           0.00122874 * pow(tempFahrenheit, 2) * percentHumidity +
            0.00085282 * tempFahrenheit*pow(percentHumidity, 2) +
           -0.00000199 * pow(tempFahrenheit, 2) * pow(percentHumidity, 2);
 }
 
-
 boolean DHT::read(void) {
-  uint8_t laststate = HIGH;
-  uint8_t counter = 0;
-  uint8_t j = 0, i;
-  unsigned long currenttime;
-
   // Check if sensor was read less than two seconds ago and return early
   // to use last reading.
-  currenttime = millis();
+  uint32_t 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));
+  if (!_firstreading && ((currenttime - _lastreadtime) < 2000)) {
+    return _lastresult; // return last correct measurement
   }
-  firstreading = false;
-  /*
-    Serial.print("Currtime: "); Serial.print(currenttime);
-    Serial.print(" Lasttime: "); Serial.print(_lastreadtime);
-  */
+  _firstreading = false;
   _lastreadtime = millis();
 
+  // Reset 40 bits of received data to zero.
   data[0] = data[1] = data[2] = data[3] = data[4] = 0;
-  
-  // pull the pin high and wait 250 milliseconds
+
+  // 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);
 
-  // now pull it low for ~20 milliseconds
+  // First set data line low for 20 milliseconds.
   pinMode(_pin, OUTPUT);
   digitalWrite(_pin, LOW);
   delay(20);
+
+  // Turn off interrupts temporarily because the next sections are timing critical
+  // and we don't want any interruptions.
   noInterrupts();
+
+  // End the start signal by setting data line high for 40 microseconds.
   digitalWrite(_pin, HIGH);
   delayMicroseconds(40);
+
+  // Now start reading the data line to get the value from the DHT sensor.
   pinMode(_pin, INPUT);
+  delayMicroseconds(10);  // Delay a bit to let sensor pull data line low.
+
+  // First expect a low signal for ~80 microseconds followed by a high signal
+  // for ~80 microseconds again.
+  if (expectPulse(LOW) == 0) {
+    DEBUG_PRINTLN(F("Timeout waiting for start signal low pulse."));
+    _lastresult = false;
+    return _lastresult;
+  }
+  if (expectPulse(HIGH) == 0) {
+    DEBUG_PRINTLN(F("Timeout waiting for start signal high pulse."));
+    _lastresult = false;
+    return _lastresult;
+  }
 
-  // read in timings
-  for ( i=0; i< MAXTIMINGS; i++) {
-    counter = 0;
-    while (digitalRead(_pin) == laststate) {
-      counter++;
-      delayMicroseconds(1);
-      if (counter == 255) {
-        break;
-      }
+  // Now read the 40 bits sent by the sensor.  Each bit is sent as a 50
+  // microsecond low pulse followed by a variable length high pulse.  If the
+  // high pulse is ~28 microseconds then it's a 0 and if it's ~70 microseconds
+  // then it's a 1.  We measure the cycle count of the initial 50us low pulse
+  // and use that to compare to the cycle count of the high pulse to determine
+  // if the bit is a 0 (high state cycle count < low state cycle count), or a
+  // 1 (high state cycle count > low state cycle count).
+  for (int i=0; i<40; ++i) {
+    uint32_t lowCycles = expectPulse(LOW);
+    if (lowCycles == 0) {
+      DEBUG_PRINTLN(F("Timeout waiting for bit low pulse."));
+      _lastresult = false;
+      return _lastresult;
     }
-    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++;
+    uint32_t highCycles = expectPulse(HIGH);
+    if (highCycles == 0) {
+      DEBUG_PRINTLN(F("Timeout waiting for bit high pulse."));
+      _lastresult = false;
+      return _lastresult;
     }
-
+    data[i/8] <<= 1;
+    // Now compare the low and high cycle times to see if the bit is a 0 or 1.
+    if (highCycles > lowCycles) {
+      // High cycles are greater than 50us low cycle count, must be a 1.
+      data[i/8] |= 1;
+    }
+    // Else high cycles are less than (or equal to, a weird case) the 50us low
+    // cycle count so this must be a zero.  Nothing needs to be changed in the
+    // stored data.
   }
 
+  // Re-enable interrupts, timing critical code is complete.
   interrupts();
-  
-  /*
-  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;
+  DEBUG_PRINTLN(F("Received:"));
+  DEBUG_PRINT(data[0], HEX); DEBUG_PRINT(F(", "));
+  DEBUG_PRINT(data[1], HEX); DEBUG_PRINT(F(", "));
+  DEBUG_PRINT(data[2], HEX); DEBUG_PRINT(F(", "));
+  DEBUG_PRINT(data[3], HEX); DEBUG_PRINT(F(", "));
+  DEBUG_PRINT(data[4], HEX); DEBUG_PRINT(F(" =? "));
+  DEBUG_PRINTLN(data[0] + data[1] + data[2] + data[3], HEX);
+
+  // Check we read 40 bits and that the checksum matches.
+  if (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) {
+    _lastresult = true;
+    return _lastresult;
+  }
+  else {
+    DEBUG_PRINTLN(F("Checksum failure!"));
+    _lastresult = false;
+    return _lastresult;
+  }
+}
 
+// Expect the signal line to be at the specified level for a period of time and
+// return a count of loop cycles spent at that level (this cycle count can be
+// used to compare the relative time of two pulses).  If more than a millisecond
+// ellapses without the level changing then the call fails with a 0 response.
+uint32_t DHT::expectPulse(bool level) {
+  uint32_t count = 0;
+  uint32_t end = micros() + 1000;
+  // Loop while counting cycles until the level changes.
+  while (digitalRead(_pin) == level) {
+    count++;
+    if (micros() >= end) {
+      // Exceeded timeout waiting for level to change, fail.
+      return 0;
+    }
+  }
+  return count;
 }

DHT.h

@@ -1,41 +1,60 @@
-#ifndef DHT_H
-#define DHT_H
-#if ARDUINO >= 100
- #include "Arduino.h"
-#else
- #include "WProgram.h"
-#endif
-
-/* 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, _count;
-  unsigned long _lastreadtime;
-  boolean firstreading;
-
- public:
-  DHT(uint8_t pin, uint8_t type, uint8_t count=6);
-  void begin(void);
-  float readTemperature(bool S=false);
-  float convertCtoF(float);
-  float convertFtoC(float);
-  float computeHeatIndex(float tempFahrenheit, float percentHumidity);
-  float readHumidity(void);
-  boolean read(void);
-
-};
-#endif
+/* DHT library
+
+MIT license
+written by Adafruit Industries
+*/
+#ifndef DHT_H
+#define DHT_H
+
+#if ARDUINO >= 100
+ #include "Arduino.h"
+#else
+ #include "WProgram.h"
+#endif
+
+
+// Uncomment to enable printing out nice debug messages.
+//#define DHT_DEBUG
+
+// Define where debug output will be printed.
+#define DEBUG_PRINTER Serial
+
+// Setup debug printing macros.
+#ifdef DHT_DEBUG
+  #define DEBUG_PRINT(...) { DEBUG_PRINTER.print(__VA_ARGS__); }
+  #define DEBUG_PRINTLN(...) { DEBUG_PRINTER.println(__VA_ARGS__); }
+#else
+  #define DEBUG_PRINT(...) {}
+  #define DEBUG_PRINTLN(...) {}
+#endif
+
+// Define types of sensors.
+#define DHT11 11
+#define DHT22 22
+#define DHT21 21
+#define AM2301 21
+
+
+class DHT {
+  public:
+   DHT(uint8_t pin, uint8_t type, uint8_t count=6);
+   void begin(void);
+   float readTemperature(bool S=false);
+   float convertCtoF(float);
+   float convertFtoC(float);
+   float computeHeatIndex(float tempFahrenheit, float percentHumidity);
+   float readHumidity(void);
+   boolean read(void);
+
+ private:
+  uint8_t data[6];
+  uint8_t _pin, _type;
+  uint32_t _lastreadtime;
+  bool _firstreading;
+  bool _lastresult;
+
+  uint32_t expectPulse(bool level);
+
+};
+
+#endif