Paul Adams
Library for DHT22/RHT03 Temperature and Humidity Sensor
Diff: DHT22.cpp
- Revision:
- 0:42ffca439fcc
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/DHT22.cpp Thu Mar 29 06:14:07 2012 +0000
@@ -0,0 +1,217 @@
+/*
+ DHT22.cpp - DHT22 sensor library
+ Developed by HO WING KIT
+ Modifications by Paul Adams
+
+ 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
+ same as RHT03 http://www.humiditycn.com
+
+ Version 0.1: 8-Jan-2011 by Ho Wing Kit
+ Version 0.2: 29-Mar-2012 by Paul Adams
+
+ Additional Credit: All the commenters on http://www.sparkfun.com/products/10167
+
+*/
+
+#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
+ _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) {
+ printf("DHT22 Error Access Time < 2s");
+ err = DHT_ERROR_TOO_QUICK;
+ }
+ retryCount = 0;
+ // Pin needs to start HIGH, wait unit it is HIGH with a timeout
+ 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;
+ } else {
+ printf("DHT22 Checksum error!\n");
+ printf("Calculate check sum is %d\n",(csPart1 + csPart2 + csPart3 + csPart4) & 0xFF);
+ printf("Reading check sum is %d",checkSum);
+ err = DHT_ERROR_CHECKSUM;
+ }
+ return err;
+}
+
+float DHT22::getTemperatureC() {
+ return _lastTemperature;
+}
+
+float DHT22::getHumidity() {
+ return _lastHumidity;
+}
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