Diff: DHT22.cpp

Revision:

0:ede9cc7e508b

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/DHT22.cpp	Tue Dec 08 19:25:47 2015 +0000
@@ -0,0 +1,190 @@
+
+#include "DHT22.h"
+#define DHT22_DATA_BIT_COUNT 41
+
+DHT22::DHT22(PinName Data) {
+
+    _data = Data;                // Set Data Pin
+    _lastReadTime = time(NULL);
+    _lastHumidity = 0;
+    _lastTemperature = DHT22_ERROR_VALUE;
+}
+
+DHT22::~DHT22() {
+}
+
+DHT22_ERROR DHT22::readData() {
+    int i, j, retryCount;
+    int currentTemperature=0;
+    int currentHumidity=0;
+    unsigned int checkSum = 0, csPart1, csPart2, csPart3, csPart4;
+    unsigned int bitTimes[DHT22_DATA_BIT_COUNT];
+    DHT22_ERROR err = DHT_ERROR_NONE;
+    time_t currentTime = time(NULL);
+
+    DigitalInOut  DATA(_data);
+       
+    for (i = 0; i < DHT22_DATA_BIT_COUNT; i++) {
+        bitTimes[i] = 0;
+    }
+    
+    if (int(currentTime - _lastReadTime) < 2) {        
+        err = DHT_ERROR_TOO_QUICK;
+    }   
+    retryCount = 0;
+    
+    do {
+        if (retryCount > 125) {
+            printf("DHT22 Bus busy!");
+            err = DHT_BUS_HUNG;
+        }
+        retryCount ++;
+        wait_us(2);
+    } while (DATA==0);   // exit on DHT22 return 'High' Signal within 250us
+    
+    // Send the activate pulse
+    // Step 1: MCU send out start signal to DHT22 and DHT22 send
+    //         response signal to MCU.
+    // If always signal high-voltage-level, it means DHT22 is not 
+    // working properly, please check the electrical connection status.
+    //
+    DATA.output(); // set pin to output data
+    DATA = 0;      // MCU send out start signal to DHT22
+    wait_ms(18);   // 18 ms wait (spec: at least 1ms)
+    DATA = 1;      // MCU pull up 
+    wait_us(40);
+    DATA.input();  // set pin to receive data
+    // Find the start of the ACK Pulse
+    retryCount = 0;
+    do {
+        if (retryCount > 40)  { // (Spec is 20-40 us high)
+            printf("DHT22 not responding!");
+            err = DHT_ERROR_NOT_PRESENT;
+        }
+        retryCount++;
+        wait_us(1);
+    } while (DATA==1);   // Exit on DHT22 pull low within 40us
+    if (err != DHT_ERROR_NONE) {
+        // initialisation failed
+        return err;    
+    }
+    wait_us(80); // DHT pull up ready to transmit data
+    
+    /*
+    if (DATA == 0) {
+        printf("DHT22 not ready!");
+        err = DHT_ERROR_ACK_TOO_LONG;
+        return err;
+    }
+    */
+    
+    // Reading the 5 byte data stream
+    // Step 2: DHT22 send data to MCU
+    //         Start bit -> low volage within 50us (actually could be anything from 35-75us)
+    //         0         -> high volage within 26-28us (actually could be 10-40us)
+    //         1         -> high volage within 70us (actually could be 60-85us)
+    //         See http://www.sparkfun.com/products/10167#comment-4f118d6e757b7f536e000000
+    
+    
+    for (i = 0; i < 5; i++) {
+        for (j = 0; j < 8; j++) {
+            // Instead of relying on the data sheet, just wait while the RHT03 pin is low
+            retryCount = 0;
+            do {
+                if (retryCount > 75)  {
+                    printf("DHT22 timeout waiting for data!");
+                    err = DHT_ERROR_DATA_TIMEOUT;
+                }
+                retryCount++;
+                wait_us(1);
+            } while (DATA == 0);
+            // We now wait for 40us
+            wait_us(40);
+            if (DATA == 1) {  
+                // If pin is still high, bit value is a 1
+                bitTimes[i*8+j] = 1;
+            } else {  
+                // The bit value is a 0
+                bitTimes[i*8+j] = 0;
+            }
+            int count = 0;
+            while (DATA == 1 && count < 100) {  
+                wait_us(1); // Delay for 1 microsecond  
+                count++;  
+            }
+        }
+    }
+    // Re-init DHT22 pin  
+    DATA.output();  
+    DATA = 1;
+
+    // Now bitTimes have the actual bits
+    // that were needed to find the end of each data bit
+    // Note: the bits are offset by one from the data sheet, not sure why
+    currentHumidity    = 0;
+    currentTemperature = 0;
+    checkSum           = 0;
+    // First 16 bits is Humidity
+    for (i=0; i<16; i++) {
+        //printf("bit %d: %d  ", i, bitTimes[i+1]);
+        if (bitTimes[i+1] > 0) {
+            currentHumidity |= ( 1 << (15-i));
+        }
+    }
+    
+    // Second 16 bits is Temperature 
+    for (i=0; i<16; i ++) {
+        //printf("bit %d: %d  ", i+16, bitTimes[i+17]);
+        if (bitTimes[i+17] > 0) {
+            currentTemperature |= (1 <<(15-i));
+        }
+    }
+
+    // Last 8 bit is Checksum
+    for (i=0; i<8; i++) {
+        //printf("bit %d: %d  ", i+32, bitTimes[i+33]);
+        if (bitTimes[i+33] > 0) {
+            checkSum |= (1 << (7-i));
+        }
+    }
+   
+    _lastHumidity = (float(currentHumidity) / 10.0);
+    
+    // if first bit of currentTemperature is 1, it is negative value.
+    if ((currentTemperature & 0x8000)==0x8000) {        
+        _lastTemperature = (float(currentTemperature & 0x7FFF) / 10.0) * -1.0;
+    } else {
+        _lastTemperature = float(currentTemperature) / 10.0;
+    }
+
+    // Calculate Check Sum
+    csPart1 = currentHumidity >> 8;
+    csPart2 = currentHumidity & 0xFF;
+    csPart3 = currentTemperature >> 8;
+    csPart4 = currentTemperature & 0xFF;
+    
+    if (checkSum == ((csPart1 + csPart2 + csPart3 + csPart4) & 0xFF)) {
+        _lastReadTime = currentTime;
+        //printf("OK-->Temperature :: %f , Humidity :: %f\r\n", _lastTemperature, _lastHumidity);
+        err = DHT_ERROR_NONE;
+        printf("\n");
+        
+    }
+    else {
+        //printf("DHT22 Checksum error!\n");
+        //printf("Calculate check sum is %d\n",(csPart1 + csPart2 + csPart3 + csPart4) & 0xFF);
+        //printf("Reading check sum is %d\n",checkSum);
+        err = DHT_ERROR_CHECKSUM;
+        printf("\n");
+    }
+    
+    return err;
+}
+
+float DHT22::getTemperatureC() {
+    return _lastTemperature;
+}
+
+float DHT22::getHumidity() {
+    return _lastHumidity;
+}
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