Arduino: DS18B20P Library
ds18b20p/ds18b20p.cpp

/**
* DS18B20-PAR (aka. DS18B20P) interface library
*
* Created by Janne Enberg aka. Lietu
* http://lietu.net/
*
* Requires slightly modified version of OneWire class in it's folder, which should be fully
* compatible with original, just adds ability to run empty constructor and initialize with .begin()
*
* Uncomment DS18B20P_LIB_DEBUG to write read data to serial port (if we get that far).
* Return codes -55 to 100 are good, 124-127 are errors.
* - 124: First presence check failed, device not present(?)
* - 125: Presence check after temperature conversion failed, device not compatible(?)
* - 126: CRC check after read failed repeatedly, device broken or not compatible(?)
* - 127: Unknown error
*
* This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
*
* This allows you to share and modify the work, including for commercial uses, as long as:
* - You credit me on the original code
* - Any altered code will be released under the exact same license
*
* http://creativecommons.org/licenses/by-sa/3.0/
*
* This license does not apply to the OneWire libraries distributed with this code,
* its license is mentioned in the OneWire.cpp file.
*/

#include "ds18b20p.h"

// Empty constructor, for "good arduino library style", can initialize with .begin(pin)
DS18B20P::DS18B20P() {}

// Constructor with pin given
DS18B20P::DS18B20P( unsigned int pin )
{
// Just call begin(), no need to duplicate code here
this->begin( pin );
}

// Initialize the object
void DS18B20P::begin( unsigned int pin )
{
// Initialize our onewire instance with the pin
// This line is basically what we needed to modify OneWire for...
this->onewire.begin( pin );
}

// Read temperature in C from sensor
double DS18B20P::read()
{
// Container for the 9 data bytes we will read from the sensor
byte data[9];

// Send reset pulse, check for presence
if( this->onewire.reset() )
{
// Issue Skip ROM command (we don't want to search for a specific sensor)
this->onewire.skip();
// Issue temperature conversion command, enable strong parasite power
this->onewire.write( 0x44, 1 );

// TODO: Return here, rename function to startRead() and move rest to a continueRead() function that checks it's been 800msec since the above line was run
// Implement new function: double DS18B20P::read() { startRead(); return continueRead(); }
// ... we could probably do quite a lot of things during this 800msec ....

// Wait for conversion to complete
delay( 800 ); // 750ms is enough according to datasheet, just in case we give it 800ms

// Issue reset
if( this->onewire.reset() )
{
// Skip ROM and issue read scratchpad command, to read temperature data and CRC
this->onewire.skip();
this->onewire.write( 0xBE );

#ifdef DS18B20P_LIB_DEBUG
Serial.println("DS18B20P .. reading 9 bytes:");
#endif

// Read 9 bytes
for( unsigned int i = 0; i < 9; ++i )
{
data[i] = this->onewire.read();

#ifdef DS18B20P_LIB_DEBUG
Serial.print( data[i], HEX );
Serial.print(" ");
#endif
}

#ifdef DS18B20P_LIB_DEBUG
Serial.println("");

Serial.print( "Calculated CRC: " );
Serial.print( OneWire::crc8( data, 8 ), HEX );
Serial.println( "" );
#endif

// Check CRC, calculate CRC from first 8 bytes, should match against last byte
if( OneWire::crc8( data, 8 )!=data[8] )
return DS18B20P::crcError;

// Convert to celsius with double precision and return
return this->getValue( data );
}
else
{
return DS18B20P::secondPresenceError;
}
}
else
{
return DS18B20P::firstPresenceError;
}
}

double DS18B20P::getValue( byte data[9] )
{
// Define result variable
double value;

// The other variables we will be using..
int reading, signedBit;

// Use them to build the full reading
reading = (data[1] /* high byte */ << 8) + data[0] /* low byte */;

// Check if the signed bit is set by testing most significant bit
signedBit = reading & 0x8000;

// If it is set, the reading is negative and actual numbers need adjusting to be readable
if( signedBit==true )
reading = (reading ^ 0xffff) + 1; // Toggle bits, add 1

// The data is still in a bit odd format, a little multiplication will fix that..
// I assume this is to give a better range with the two bytes of data..
value = (double)( reading ) * 0.0625; // * 6.25 / 100;
// In case you want speed, this might be slightly faster
// value = (double)((6 * reading) + (reading / 4)) / 100;

return value;
}