Philip Smith / DTH22

DTH22, RHT03 and DTH11 sensors can be read with this code!

This DHT22 sensor reading class works with the mbed LPC1768. The code is time dependent but has been tested with mbed LPC1768 currently. I may add timing defines for other platforms if needed but i currently only use the mbed LPC1768 platform. Please feel free to import the code and modify it for other platforms if needed. BITREADTIME define and STARTTRANSBITSIZE define would be the main items to change for any other platform to operate properly. BITREADTIME is the us time value used in a basic look for a wait value to get next reading. This may work simply on other platforms at other system speeds but it may not. A more general solution maybe to add another calculation that generates these defines based on some platform speed value. At this writing the code performs very well with little to no read failures(in fact i have not seen a read failure yet in testing). The issues that i have seen with other classes and this sensor is the fact that the sensor always produces the correct Temperature and Humidity output values on the io pin but the class reading these values miss many reading causing errors. This class avoids this because it reads the output from the DTH22 sensor completely and then processes the values from a run length bit measurement perspective that i feel is far more accurate. Anyways the results speak for them self.

I have now added a member function for reading the DTH11 sensor as it is read the same way as the DTH22 sensor. The only difference is the final use of the retrieved bytes from the sensor for calculating the temperature and humidity. Note the DTH11 sensor has less range and less accuracy but it also can be found for less money!

Diff: DTH22.h

Revision:
4:85dd45e5ab32
Parent:
3:57c9abcd9646
Child:
5:3a5bdf8bd962
--- a/DTH22.h	Tue Dec 03 09:14:49 2013 +0000
+++ b/DTH22.h	Tue Dec 03 09:42:35 2013 +0000
@@ -57,15 +57,15 @@
  * @endcode
  */
 
-#define MAXRAWDATA 1300
-#define DATABITS 40
-#define BITREADTIME 3
-#define MAXBITCOUNT 30
-#define STARTTRANSBITSIZE 14
-#define DATANOISE 2000
-#define STIMINGFAIL 2001
-#define DATANOISE2 2002
-#define CHECKSUMFAIL 2003
+#define MAXRAWDATA 1300         // the total times the DTH22 io pin is read via the mbed pin
+#define DATABITS 40             // DTH22 sensor only produces 40 data bits to be read 
+#define BITREADTIME 3           // the wait time on lpc1768 mbed device in us via wait_us() function
+#define MAXBITCOUNT 30          // the total read cycles that when read as 1 indicate the DTH22 sensor is not sending anymore
+#define STARTTRANSBITSIZE 14    // the median cycle counts of the start transmit bit from the DTH22 sensor note this is based on the BITREADTIME value
+#define DATANOISE 2000          // Error indicating the DTH22 io line must be noisy and data is not valid
+#define STIMINGFAIL 2001        // Error indicating the start bit timing was exceeded on some bit so data could not be properly read
+#define DATANOISE2 2002         // Error indicating the DTH22 io line must be noisy and data is not valid because there was too much data from device that passed earlier error checks
+#define CHECKSUMFAIL 2003       // Error indicating the DTH22 data did not pass the checksum test 
 
 class DTH22
 {
@@ -73,7 +73,7 @@
     /** Create object connected to DTH22 pin ( remember io pin needs pull up resister)
         *
         * Ensure the pull up resistors are used on this pin.
-        * Any pin can be used but the pin can only be used on one DTH22 sensor and nothing else.
+        * Any pin can be used but the pin can only be used for one DTH22 sensor and nothing else.
         *
         * @param sda pin that BMP180 connected to (p9 or p28 as defined on LPC1768)
         * @param slc pin that BMP180 connected to (p10 or p27 ad defined on LPC1768)
@@ -84,13 +84,14 @@
 
     /** Read Temperature and Humidity
     *
-    * This function will return when it has readings.
+    * This function will return when it has readings.  Note the DTH22 documents say that the device should not be read more then once every 2 seconds.
+    * This code should work with DTH11 and RHT03 sensors.  The RHT03 sensor is the same as the DTH22 i believe and the DTH11 sensor is a less percise version of the DTH22.
     *
-    * @param temp the temperature returned in this variable. Degrees celsius with one decimal so 253 is 25.3 C.
-    * @param humidity the humidity is returned in this variable. Humidity is in rH with one decimal so 288 is 28.8 rH.
+    * @param temp the temperature returned in this variable. Degrees celsius with one decimal so 253 is 25.3 C. (-40C to +80C)
+    * @param humidity the humidity is returned in this variable. Humidity is in % rH with one decimal so 288 is 28.8 % rH. (0% to 100% rH)
     * @param returns 0 for no errors.  Any other number is some type of communication error and data might not be valid!
     */
-    int getTH(int *temp,int *humidity);    // get both temperature and pressure fully compensated values! Note this only returns when measurements are complete
+    int getTH(int *temp,int *humidity);    // get both temperature and humidity with an error indicator.  Note only do this read no more then once every 2 seconds as per DTH22 documents
 
 protected:
     DigitalInOut DTH22pin;