MAX31855.cpp

/*******************************************************************************
* MAX31855 Library
* Version: 1.10
* Date: 24-07-2012
* Company: Rocket Scream Electronics
* Website: www.rocketscream.com
*
* This is a MAX31855 library for Arduino. Please check our wiki 
* (www.rocketscream.com/wiki) for more information on using this piece of 
* library.
*
* This library is licensed under Creative Commons Attribution-ShareAlike 3.0 
* Unported License.
*
* Revision  Description
* ========  ===========
* 1.10			Added negative temperature support for both junction & thermocouple.
* 1.00      Initial public release.
*
*******************************************************************************/
#include	"MAX31855.h"

MAX31855::MAX31855(unsigned char SO, unsigned	char CS, unsigned char SCK)
{
	so = SO;
	cs = CS;
	sck = SCK;
	
	// MAX31855 data output pin
	pinMode(so, INPUT);
	// MAX31855 chip select input pin
	pinMode(cs, OUTPUT);
	// MAX31855 clock input pin
	pinMode(sck, OUTPUT);
	
	// Default output pins state
	digitalWrite(cs, HIGH);
	digitalWrite(sck, LOW);
}

/*******************************************************************************
* Name: readThermocouple
* Description: Read the thermocouple temperature either in Degree Celsius or
*							 Fahrenheit. Internally, the conversion takes place in the
*							 background within 100 ms. Values are updated only when the CS
*							 line is high.
*
* Argument  	Description
* =========  	===========
* 1. unit   	Unit of temperature required: CELSIUS or FAHRENHEIT
*
* Return			Description
* =========		===========
*	temperature	Temperature of the thermocouple either in Degree Celsius or 
*							Fahrenheit. If fault is detected, FAULT_OPEN, FAULT_SHORT_GND or
*							FAULT_SHORT_VCC will be returned. These fault values are outside
*							of the temperature range the MAX31855 is capable of.
*******************************************************************************/	
double	MAX31855::readThermocouple(unit_t	unit)
{
	unsigned long data;
	double temperature;
	
	// Initialize temperature
	temperature = 0;
	
	// Shift in 32-bit of data from MAX31855
	data = readData();
	
	// If fault is detected
	if (data & 0x00010000)
	{
		// Check for fault type (3 LSB)
		switch (data & 0x00000007)
		{
			// Open circuit 
			case 0x01:
				temperature = FAULT_OPEN;
				break;
			
			// Thermocouple short to GND
			case 0x02:
				temperature = FAULT_SHORT_GND;
				break;
			
			// Thermocouple short to VCC	
			case 0x04:
				temperature = FAULT_SHORT_VCC;
				break;
		}
	}
	// No fault detected
	else
	{
		// Retrieve thermocouple temperature data and strip redundant data
		data = data >> 18;
		// Bit-14 is the sign
		temperature = (data & 0x00001FFF);

		// Check for negative temperature		
		if (data & 0x00002000)
		{
			// 2's complement operation
			// Invert
			data = ~data; 
			// Ensure operation involves lower 13-bit only
			temperature = data & 0x00001FFF;
			// Add 1 to obtain the positive number
			temperature += 1;
			// Make temperature negative
			temperature *= -1; 
		}
		
		// Convert to Degree Celsius
		temperature *= 0.25;
		
		// If temperature unit in Fahrenheit is desired
		if (unit == FAHRENHEIT)
		{
			// Convert Degree Celsius to Fahrenheit
			temperature = (temperature * 9.0/5.0)+ 32; 
		}
	}
	return (temperature);
}

/*******************************************************************************
* Name: readJunction
* Description: Read the thermocouple temperature either in Degree Celsius or
*							 Fahrenheit. Internally, the conversion takes place in the
*							 background within 100 ms. Values are updated only when the CS
*							 line is high.
*
* Argument  	Description
* =========  	===========
* 1. unit   	Unit of temperature required: CELSIUS or FAHRENHEIT
*
* Return			Description
* =========		===========
*	temperature	Temperature of the cold junction either in Degree Celsius or 
*							Fahrenheit. 
*
*******************************************************************************/
double	MAX31855::readJunction(unit_t	unit)
{
	double	temperature;
	unsigned long data;
	
	// Shift in 32-bit of data from MAX31855
	data = readData();
	
	// Strip fault data bits & reserved bit
	data = data >> 4;
	// Bit-12 is the sign
	temperature = (data & 0x000007FF);
	
	// Check for negative temperature
	if (data & 0x00000800)
	{
		// 2's complement operation
		// Invert
		data = ~data; 
		// Ensure operation involves lower 11-bit only
		temperature = data & 0x000007FF;
		// Add 1 to obtain the positive number
		temperature += 1;	
		// Make temperature negative
		temperature *= -1; 
	}
	
	// Convert to Degree Celsius
	temperature *= 0.0625;
	
	// If temperature unit in Fahrenheit is desired
	if (unit == FAHRENHEIT)
	{
		// Convert Degree Celsius to Fahrenheit
		temperature = (temperature * 9.0/5.0)+ 32; 	
	}
	
	// Return the temperature
	return (temperature);
}

/*******************************************************************************
* Name: readData
* Description: Shift in 32-bit of data from MAX31855 chip. Minimum clock pulse
*							 width is 100 ns. No delay is required in this case.
*
* Argument  	Description
* =========  	===========
* 1. NIL
*
* Return			Description
* =========		===========
*	data				32-bit of data acquired from the MAX31855 chip.
*				
*******************************************************************************/
unsigned long MAX31855::readData()
{
	int bitCount;
	unsigned long data;
	
	// Clear data 
	data = 0;

	// Select the MAX31855 chip
	digitalWrite(cs, LOW);
	
	// Shift in 32-bit of data
	for (bitCount = 31; bitCount >= 0; bitCount--)
	{
		digitalWrite(sck, HIGH);
		
		// If data bit is high
		if (digitalRead(so))
		{
			// Need to type cast data type to unsigned long, else compiler will 
			// truncate to 16-bit
			data |= ((unsigned long)1 << bitCount);
		}	
		
		digitalWrite(sck, LOW);
	}
	
	// Deselect MAX31855 chip
	digitalWrite(cs, HIGH);
	
	return(data);
}