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Fix for DHT_ERROR_CHECKSUM with negative temperatures

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  DHT22.cpp - DHT22 sensor library
  Developed by Ben Adams - 2011

Humidity and Temperature Sensor DHT22 info found at
The same sensor has also been spotted going by the names RHT22, AM2302
and RHT03 from a variety of vendors. It uses a non-standard 1-wire digital
signalling protocol.

To install this library for use with the Arduino IDE, copy it
to the `libraries' folder and restart the IDE. For an example of
how to use it, see File->Examples->DHT22->Serial .

Version 0.5: 15-Jan-2012 by Craig Ringer
Update to support Arduino 1.0
Make accessors inlineable so they can be optimised away
Preserve original integer values from sensor and offer access to them
Permit floating point support to be disalbed by defining DHT22_NO_FLOAT before including DHT22.h

Version 0.4: 24-Jan-2011 by Ben Adams
Added return code constants to keywords.txt
Returns 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!

Version 0.1: 10-Jan-2011 by Ben Adams nethoncho AT
First Version is a skeleton. This version will not read data!
Code adapted from the following sources:
The Arduino OneWire lib


Arduino library for the DHT22 humidity and temperature sensor



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