DHT sensor for measuring temperature

Files at this revision

API Documentation at this revision

Comitter:
sam_grove
Date:
Fri Aug 15 20:55:43 2014 +0000
Parent:
1:25c96950b6ed
Child:
3:6937e130feca
Child:
4:171c12efa794
Commit message:
Update reading sensor to eliminate the checksum error (error 6). Many other changes for good coding practices

Changed in this revision

DHT.cpp Show annotated file Show diff for this revision Revisions of this file
DHT.h Show annotated file Show diff for this revision Revisions of this file
--- a/DHT.cpp	Wed Jul 24 06:59:28 2013 +0000
+++ b/DHT.cpp	Fri Aug 15 20:55:43 2014 +0000
@@ -9,7 +9,7 @@
  *
  *  Copyright (C) Wim De Roeve
  *                based on DHT22 sensor library by HO WING KIT
- *                Arduino DHT11 library 
+ *                Arduino DHT11 library
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documnetation files (the "Software"), to deal
@@ -32,113 +32,99 @@
 
 #include "DHT.h"
 
-#define DHT_DATA_BIT_COUNT 41
+#define DHT_DATA_BIT_COUNT 40
 
-DHT::DHT(PinName pin,int DHTtype) {
+DHT::DHT(PinName pin, eType DHTtype)
+{
     _pin = pin;
     _DHTtype = DHTtype;
-    _firsttime=true;
+    _firsttime = true;
+}
+
+DHT::~DHT()
+{
+    
 }
 
-DHT::~DHT() {
+eError DHT::stall(DigitalInOut &io, int const level, int const max_time)
+{
+    int cnt = 0;
+    while (level == io) {
+        if (cnt > max_time) {
+            return ERROR_NO_PATIENCE;
+        }
+        cnt++;
+        wait_us(1);
+    }
+    return ERROR_NONE;
 }
 
-int DHT::readData() {
-    int i, j, retryCount,b;
-    unsigned int bitTimes[DHT_DATA_BIT_COUNT];
+eError DHT::readData()
+{
+    uint8_t i = 0, j = 0, b = 0, data_valid = 0;
+    uint32_t bit_value[DHT_DATA_BIT_COUNT] = {0};
 
     eError err = ERROR_NONE;
     time_t currentTime = time(NULL);
 
     DigitalInOut DHT_io(_pin);
 
-    for (i = 0; i < DHT_DATA_BIT_COUNT; i++) {
-        bitTimes[i] = 0;
-    }
-#if 0
-    if (!_firsttime) {
-        if (int(currentTime - _lastReadTime) < 2) {
-            err = ERROR_NO_PATIENCE;
-            return err;
-        }
-    } else {
-        _firsttime=false;
-        _lastReadTime=currentTime;
+    // IO must be in hi state to start
+    if (ERROR_NONE != stall(DHT_io, 0, 250)) {
+        return BUS_BUSY;
     }
-#endif
-    retryCount = 0;
 
-    do {
-        if (retryCount > 125) {
-            err = BUS_BUSY;
-            return err;
-        }
-        retryCount ++;
-        wait_us(2);
-    } while ((DHT_io==0));
-
-
+    // start the transfer
     DHT_io.output();
     DHT_io = 0;
-    wait_ms(18);
+    // only 500uS for DHT22 but 18ms for DHT11
+    (_DHTtype == 11) ? wait_ms(18) : wait(1);
     DHT_io = 1;
-    wait_us(40);
+    wait_us(30);
     DHT_io.input();
-
-    retryCount = 0;
-    do {
-        if (retryCount > 40)  {
-            err = ERROR_NOT_PRESENT;
-            return err;
-        }
-        retryCount++;
-        wait_us(1);
-    } while ((DHT_io==1));
-
-    if (err != ERROR_NONE) {
-        return err;
+    // wait till the sensor grabs the bus
+    if (ERROR_NONE != stall(DHT_io, 1, 40)) {
+        return ERROR_NOT_PRESENT;
     }
-
-    wait_us(80);
-
+    // sensor should signal low 80us and then hi 80us
+    if (ERROR_NONE != stall(DHT_io, 0, 100)) {
+        return ERROR_SYNC_TIMEOUT;
+    }
+    if (ERROR_NONE != stall(DHT_io, 1, 100)) {
+        return ERROR_NO_PATIENCE;
+    }
+    // capture the data
     for (i = 0; i < 5; i++) {
         for (j = 0; j < 8; j++) {
-
-            retryCount = 0;
-            do {
-                if (retryCount > 75)  {
-                    err = ERROR_DATA_TIMEOUT;
-                    return err;
-                }
-                retryCount++;
-                wait_us(1);
-            } while (DHT_io == 0);
+            if (ERROR_NONE != stall(DHT_io, 0, 75)) {
+                return ERROR_DATA_TIMEOUT;
+            }
+            // logic 0 is 28us max, 1 is 70us
             wait_us(40);
-            bitTimes[i*8+j]=DHT_io;
-
-            int count = 0;
-            while (DHT_io == 1 && count < 100) {
-                wait_us(1);
-                count++;
+            bit_value[i*8+j] = DHT_io;
+            if (ERROR_NONE != stall(DHT_io, 1, 50)) {
+                return ERROR_DATA_TIMEOUT;
             }
         }
     }
-    DHT_io.output();
-    DHT_io = 1;
+    // store the data
     for (i = 0; i < 5; i++) {
         b=0;
         for (j=0; j<8; j++) {
-            if (bitTimes[i*8+j+1] > 0) {
-                b |= ( 1 << (7-j));
+            if (bit_value[i*8+j] == 1) {
+                b |= (1 << (7-j));
             }
         }
         DHT_data[i]=b;
     }
 
-    if (DHT_data[4] == ((DHT_data[0] + DHT_data[1] + DHT_data[2] + DHT_data[3]) & 0xFF)) {
+    // uncomment to see the checksum error if it exists
+    //printf(" 0x%02x + 0x%02x + 0x%02x + 0x%02x = 0x%02x \n", DHT_data[0], DHT_data[1], DHT_data[2], DHT_data[3], DHT_data[4]);
+    data_valid = DHT_data[0] + DHT_data[1] + DHT_data[2] + DHT_data[3];
+    if (DHT_data[4] == data_valid) {
         _lastReadTime = currentTime;
-        _lastTemperature=CalcTemperature();
-        _lastHumidity=CalcHumidity();
+        _lastTemperature = CalcTemperature();
+        _lastHumidity = CalcHumidity();
 
     } else {
         err = ERROR_CHECKSUM;
@@ -148,7 +134,8 @@
 
 }
 
-float DHT::CalcTemperature() {
+float DHT::CalcTemperature()
+{
     int v;
 
     switch (_DHTtype) {
@@ -167,21 +154,25 @@
     return 0;
 }
 
-float DHT::ReadHumidity() {
+float DHT::ReadHumidity()
+{
     return _lastHumidity;
 }
 
-float DHT::ConvertCelciustoFarenheit(float celsius) {
+float DHT::ConvertCelciustoFarenheit(float const celsius)
+{
     return celsius * 9 / 5 + 32;
 }
 
-float DHT::ConvertCelciustoKelvin(float celsius) {
+float DHT::ConvertCelciustoKelvin(float const celsius)
+{
     return celsius + 273.15;
 }
 
 // dewPoint function NOAA
 // reference: http://wahiduddin.net/calc/density_algorithms.htm
-float DHT::CalcdewPoint(float celsius, float humidity) {
+float DHT::CalcdewPoint(float const celsius, float const humidity)
+{
     float A0= 373.15/(273.15 + celsius);
     float SUM = -7.90298 * (A0-1);
     SUM += 5.02808 * log10(A0);
@@ -196,16 +187,17 @@
 // delta max = 0.6544 wrt dewPoint()
 // 5x faster than dewPoint()
 // reference: http://en.wikipedia.org/wiki/Dew_point
-float DHT::CalcdewPointFast(float celsius, float humidity)
+float DHT::CalcdewPointFast(float const celsius, float const humidity)
 {
-        float a = 17.271;
-        float b = 237.7;
-        float temp = (a * celsius) / (b + celsius) + log(humidity/100);
-        float Td = (b * temp) / (a - temp);
-        return Td;
+    float a = 17.271;
+    float b = 237.7;
+    float temp = (a * celsius) / (b + celsius) + log(humidity/100);
+    float Td = (b * temp) / (a - temp);
+    return Td;
 }
 
-float DHT::ReadTemperature(eScale Scale) {
+float DHT::ReadTemperature(eScale Scale)
+{
     if (Scale == FARENHEIT)
         return ConvertCelciustoFarenheit(_lastTemperature);
     else if (Scale == KELVIN)
@@ -214,7 +206,8 @@
         return _lastTemperature;
 }
 
-float DHT::CalcHumidity() {
+float DHT::CalcHumidity()
+{
     int v;
 
     switch (_DHTtype) {
--- a/DHT.h	Wed Jul 24 06:59:28 2013 +0000
+++ b/DHT.h	Fri Aug 15 20:55:43 2014 +0000
@@ -1,10 +1,10 @@
-/* 
- *  DHT Library for  Digital-output Humidity and Temperature sensors 
- *  
+/*
+ *  DHT Library for  Digital-output Humidity and Temperature sensors
+ *
  *  Works with DHT11, DHT21, DHT22
  *             SEN11301P,  Grove - Temperature&Humidity Sensor     (Seeed Studio)
  *             SEN51035P,  Grove - Temperature&Humidity Sensor Pro (Seeed Studio)
- *             AM2302   ,  temperature-humidity sensor    
+ *             AM2302   ,  temperature-humidity sensor
  *             RHT01,RHT02, RHT03    ,  Humidity and Temperature Sensor         (Sparkfun)
  *
  *  Copyright (C) Wim De Roeve
@@ -35,46 +35,50 @@
 
 #include "mbed.h"
 
-enum eType{
-        DHT11     = 11,
-        SEN11301P = 11,
-        RHT01     = 11,
-        DHT22     = 22,
-        AM2302    = 22,
-        SEN51035P = 22,
-        RHT02     = 22,
-        RHT03     = 22
-    } ;
+typedef enum eType eType;
+enum eType {
+    DHT11     = 11,
+    SEN11301P = 11,
+    RHT01     = 11,
+    DHT22     = 22,
+    AM2302    = 22,
+    SEN51035P = 22,
+    RHT02     = 22,
+    RHT03     = 22
+};
 
+typedef enum eError eError;
 enum eError {
     ERROR_NONE = 0,
-    BUS_BUSY =1,
-    ERROR_NOT_PRESENT =2 ,
-    ERROR_ACK_TOO_LONG =3 ,
-    ERROR_SYNC_TIMEOUT = 4,
-    ERROR_DATA_TIMEOUT =5 ,
-    ERROR_CHECKSUM = 6,
-    ERROR_NO_PATIENCE =7
-} ;
+    BUS_BUSY,
+    ERROR_NOT_PRESENT,
+    ERROR_ACK_TOO_LONG,
+    ERROR_SYNC_TIMEOUT,
+    ERROR_DATA_TIMEOUT,
+    ERROR_CHECKSUM,
+    ERROR_NO_PATIENCE
+};
 
-typedef enum {
-    CELCIUS =0 ,
-    FARENHEIT =1,
-    KELVIN=2
-} eScale;
+typedef enum eScale eScale;
+enum eScale {
+    CELCIUS = 0,
+    FARENHEIT,
+    KELVIN
+};
 
 
-class DHT {
+class DHT
+{
 
 public:
 
-    DHT(PinName pin,int DHTtype);
+    DHT(PinName pin, eType DHTtype);
     ~DHT();
-    int readData(void);
+    eError readData(void);
     float ReadHumidity(void);
-    float ReadTemperature(eScale Scale);
-    float CalcdewPoint(float celsius, float humidity);
-    float CalcdewPointFast(float celsius, float humidity);
+    float ReadTemperature(eScale const Scale);
+    float CalcdewPoint(float const celsius, float const humidity);
+    float CalcdewPointFast(float const celsius, float const humidity);
 
 private:
     time_t  _lastReadTime;
@@ -82,12 +86,13 @@
     float _lastHumidity;
     PinName _pin;
     bool _firsttime;
-    int _DHTtype;
-    int DHT_data[6];
+    eType _DHTtype;
+    uint8_t DHT_data[5];
     float CalcTemperature();
     float CalcHumidity();
-    float ConvertCelciustoFarenheit(float);
-    float ConvertCelciustoKelvin(float);
+    float ConvertCelciustoFarenheit(float const);
+    float ConvertCelciustoKelvin(float const);
+    eError stall(DigitalInOut &io, int const level, int const max_time);
 
 };