Add force parameter to read methods

This allows forcing a read, even if the previous read was less than 2
seconds ago. This is useful in cases where the millis() timer is not
reliable, such as when sleeping. In this case, it is up to the caller to
ensure that at least 2 seconds elapse between calls with force set to
true.

Author: matthijskooijman

DHT.cpp

@@ -27,10 +27,10 @@ void DHT::begin(void) {
 }
 
 //boolean S == Scale.  True == Fahrenheit; False == Celcius
-float DHT::readTemperature(bool S) {
+float DHT::readTemperature(bool S, bool force) {
   float f = NAN;
 
-  if (read()) {
+  if (read(force)) {
     switch (_type) {
     case DHT11:
       f = data[2];
@@ -64,7 +64,7 @@ float DHT::convertFtoC(float f) {
   return (f - 32) * 5 / 9;
 }
 
-float DHT::readHumidity(void) {
+float DHT::readHumidity(bool force) {
   float f = NAN;
   if (read()) {
     switch (_type) {
@@ -113,15 +113,15 @@ float DHT::computeHeatIndex(float temperature, float percentHumidity, bool isFah
   }
 }
 
-boolean DHT::read(void) {
+boolean DHT::read(bool force) {
   // Check if sensor was read less than two seconds ago and return early
   // to use last reading.
   uint32_t currenttime = millis();
   if (currenttime < _lastreadtime) {
     // ie there was a rollover
     _lastreadtime = 0;
   }
-  if (!_firstreading && ((currenttime - _lastreadtime) < 2000)) {
+  if (!force && !_firstreading && ((currenttime - _lastreadtime) < 2000)) {
     return _lastresult; // return last correct measurement
   }
   _firstreading = false;

DHT.h

@@ -39,12 +39,12 @@ class DHT {
   public:
    DHT(uint8_t pin, uint8_t type, uint8_t count=6);
    void begin(void);
-   float readTemperature(bool S=false);
+   float readTemperature(bool S=false, bool force=false);
    float convertCtoF(float);
    float convertFtoC(float);
    float computeHeatIndex(float temperature, float percentHumidity, bool isFahrenheit=true);
-   float readHumidity(void);
-   boolean read(void);
+   float readHumidity(bool force=false);
+   boolean read(bool force=false);
 
  private:
   uint8_t data[6];