Diff: DHT22.cpp

Revision:

0:547e68daeb1b

Child:

1:5b20ff4fd227

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/DHT22.cpp	Tue Jun 21 03:44:05 2011 +0000
@@ -0,0 +1,176 @@
+/*
+    DHT22.cpp - DHT22 sensor library
+    Developed by Ben Adams = 2011
+
+    This library is free software; you can redistribute it and / or
+    modify it under the terms of the GNU Leser General Public
+    License as published by the Free Software Foundation; either
+    version 2.1 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRENTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PATRICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+
+
+    Humidity and Temperature Sensor DHT22 info found at
+    http://www.sparkfun.com/products/10167
+
+    Version 0.4: 24-Jan-2011 by Ben Adams
+    Added return code constants to keywords.txt
+    Return DHT_ERROR_CHECKSUM on check sum mismatch
+
+    Version 0.3: 17-Jan-2011 by Ben Adams
+    This version reads data
+    Needs check sum code added at the end of readData
+
+    Version 0.2: 16-Jan-2011 by Ben Adams
+    Changed coding style to match other Arduino Libraries.
+    This version will not read data either!
+
+*/
+
+#include "DHT22.h"
+
+// This should be 40, but the sensor is adding an extra bit at the start
+#define DHT22_DATA_BIT_COUNT 41
+
+DHT22::DHT22(PinName Data) {
+    _data = Data;                // Set Data Pin
+    DigitalInOut      VCC(_vcc);
+    _lastReadTime = time(NULL)-2;
+    _lastHumidity = 0;
+    _lastTemperature = DHT22_ERROR_VALUE;
+}
+
+DHT22::~DHT22() {
+}
+
+DHT22_ERROR DHT22::readData() {
+    int i, retryCount;
+    int currentTemperature;
+    int currentHumidity;
+    unsigned int checkSum, csPart1, csPart2, csPart3, csPart4;
+    unsigned int bitTimes[DHT22_DATA_BIT_COUNT];
+    time_t currentTime;
+
+    DigitalInOut  DATA(_data);
+
+    currentHumidity = 0;
+    currentTemperature = 0;
+    checkSum = 0;
+    currentTime = time(NULL);
+    for (i = 0; i < DHT22_DATA_BIT_COUNT; i++) {
+        bitTimes[i] = 0;
+    }
+    if (currentTime - _lastReadTime < 2) {
+        // Caller needs to wait 2 seconds between each call to read Data
+        return DHT_ERROR_TOOQUICK;
+    }
+    _lastReadTime = currentTime;
+    retryCount = 0;
+    // Pin needs to start HIGH, wait unit it is HIGH with a timeout
+    do {
+        if (retryCount > 125) {
+            return DHT_BUS_HUNG;
+        }
+        retryCount ++;
+        wait_us(2);
+    } while (DATA==1);
+    // Send the activate pulse
+    DATA = 0;        // MCU send out start signal to DHT22
+    wait_us(1100);   // 1.1 ms
+    // Find the start of the ACK Pulse
+    retryCount = 0;
+    do {
+        if (retryCount > 25)  {// (Spec is 20 to 40 us, 25*2 == 50us
+            return DHT_ERROR_NOT_PRESENT;
+        }
+        retryCount ++;
+        wait_us(2);
+    } while (DATA==1);
+    // Find the end of the ACK Pulse
+    retryCount = 0;
+    do {
+        if (retryCount > 50) {// (Spec is 80 us, 50 * 2 == 100us)
+            return DHT_ERROR_ACK_TOO_LONG;
+        }
+        retryCount++;
+        wait_us(2);
+    } while (DATA==0);
+    // Read the 40 bit data stream
+    for (i=0; i < DHT22_DATA_BIT_COUNT; i++) {
+        retryCount = 0;
+        do {
+            if (retryCount > 35) { // spec is 50 u, 35*2 = 70 us
+                return DHT_ERROR_SYNC_TIMEOUT;
+            }
+            retryCount ++;
+            wait_us(2);
+        } while (DATA==1);
+        // Measure the width of the data pulse
+        retryCount = 0;
+        do {
+            if (retryCount > 50) { // spec is 80us, 50*2 == 100us
+                return DHT_ERROR_DATA_TIMEOUT;
+            }
+            retryCount++;
+            wait_us(2);
+        } while (DATA==0);
+        bitTimes[i] = retryCount;
+    }
+
+    // Now bitTimes have the number of retries (us *2)
+    // that were needed to find the end of each data bit
+    // Spec: 0 is 26 to 28 us
+    // Spec: 1 is 70 us
+    // bitTimes[x] <= 11 is a 0
+    // bitTimes[x] > 11 is a 1
+    // Note: the bits are offset by one from the data sheet, not sure why
+    for (i=0; i<16; i++) {
+        if (bitTimes[i+1] > 11) {
+            currentHumidity |= ( 1 << (15-i));
+        }
+    }
+
+    for (i=0; i<16; i ++) {
+        if (bitTimes[i+17] > 11) {
+            currentTemperature |= (1 <<(15-i));
+        }
+    }
+
+    for (i=0; i<8; i++) {
+        if (bitTimes[i+33]>11) {
+            checkSum |= (1 << (7-i));
+        }
+    }
+    _lastHumidity = (float(currentHumidity & 0x7FFF) / 10.0);
+    if (currentTemperature &= 0x8000) {
+        currentTemperature &= 0x7FFF;
+        _lastTemperature = (float(currentTemperature) / 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)) {
+        return DHT_ERROR_NONE;
+    }
+    return DHT_ERROR_CHECKSUM;
+}
+
+float DHT22::getTemperatureC() {
+    return _lastTemperature;
+}
+
+float DHT22::getHumidity() {
+    return _lastHumidity;
+}
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