fix #100, negative representation (#101)

- fix #100, different negative algorithm support
- bump version

Author: RobTillaart

CHANGELOG.md

@@ -6,6 +6,12 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 
+## [0.5.0] - 2024-11-22
+- fix #100, different negative algorithm support
+- time to bump version
+
+----
+
 ## [0.4.21] - 2024-10-09
 - add dhtnew_pulse_diag_ext.ino for extended diagnosis.
 

README.md

@@ -57,13 +57,23 @@ Feedback (both positive and negative) about the AM232X sensors is welcome.
 **Note: check the datasheet how to connect!**
 
 
+### 0.5.0 Negative temperature
+
+Apparently there are DHT22's which use another representation for negative temperatures.
+Since 0.5.0 the library automatically detects which representation is used by the sensor
+and chooses the correct algorithm to decode the negative temperature.
+
+See issue #100 (solution) and #57 and #52 before.  
+See also https://arduino.stackexchange.com/questions/86448/dht22-sensor-reading-code-interprets-negative-values-weirdly
+
+
 ### Related
 
 - https://github.com/RobTillaart/DHTNew
 - https://github.com/RobTillaart/DHTStable
 - https://github.com/RobTillaart/DHT_Simulator
 - https://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_many.html (interesting)
-- https://github.com/RobTillaart/Temperature (conversions, dewPoint, heatindex etc)
+- https://github.com/RobTillaart/Temperature (conversions, dewPoint, heat index etc.)
 
 
 ## DHT PIN layout from left to right

dhtnew.cpp

@@ -1,7 +1,7 @@
 //
 //    FILE: dhtnew.cpp
 //  AUTHOR: Rob Tillaart
-// VERSION: 0.4.21
+// VERSION: 0.5.0
 // PURPOSE: DHT Temperature & Humidity Sensor library for Arduino
 //     URL: https://github.com/RobTillaart/DHTNEW
 //
@@ -193,17 +193,29 @@ int DHTNEW::_read()
   else               //  DHT22, DHT33, DHT44, compatible + Si7021
   {
     _humidity = (_bits[0] * 256 + _bits[1]) * 0.1;
-    int16_t t = ((_bits[2] & 0x7F) * 256 + _bits[3]);
-    if (t == 0)
+
+    //  positive temperature?
+    if ((_bits[2] & 0x80) != 0x80 )
     {
-      _temperature = 0.0;     //  prevent -0.0;
+      int16_t t = ((_bits[2] & 0x7F) * 256 + _bits[3]);
+      if (t == 0)
+      {
+        _temperature = 0.0;     //  prevent -0.0;
+      }
+      _temperature = t * 0.1;
     }
-    else
+    else  //  negative temperature
     {
-      _temperature = t * 0.1;
-      if((_bits[2] & 0x80) == 0x80 )
+      //  See issue #100 - 2 different representations
+      if ((_bits[2] & 0x40) != 0x40 )
+      {
+        int16_t t = ((_bits[2] & 0x7F) * 256 + _bits[3]);
+        _temperature = t * -0.1;
+      }
+      else
       {
-        _temperature = -_temperature;
+        int16_t t = (_bits[2] << 8) + _bits[3];  //  16 bits, as raw int
+        _temperature = t * 0.1;
       }
     }
   }

dhtnew.h

@@ -2,7 +2,7 @@
 //
 //    FILE: dhtnew.h
 //  AUTHOR: Rob Tillaart
-// VERSION: 0.4.21
+// VERSION: 0.5.0
 // PURPOSE: DHT Temperature & Humidity Sensor library for Arduino
 //     URL: https://github.com/RobTillaart/DHTNEW
 //
@@ -18,7 +18,7 @@
 #include "Arduino.h"
 
 
-#define DHTNEW_LIB_VERSION                (F("0.4.21"))
+#define DHTNEW_LIB_VERSION                (F("0.5.0"))
 
 
 #define DHTLIB_OK                         0

library.json

@@ -13,7 +13,7 @@
     "type": "git",
     "url": "https://github.com/RobTillaart/DHTNEW.git"
   },
-  "version": "0.4.21",
+  "version": "0.5.0",
   "license": "MIT",
   "frameworks": "*",
   "platforms": "*",

library.properties

@@ -1,5 +1,5 @@
 name=DHTNEW
-version=0.4.21
+version=0.5.0
 author=Rob Tillaart <[email protected]>
 maintainer=Rob Tillaart <[email protected]>
 sentence=Arduino library for DHT temperature and humidity sensor, with automatic sensortype recognition.