e-Gizmo/ArdMPL115A1.ino

## ArdMPL115A1.ino
/*
 MPL115A1 SPI Digital Barometer Test Code
 Created on: September 30, 2010
       By: Jeremiah McConnell - miah at miah.com
 Portions: Jim Lindblom - jim at sparkfun.com

 This is a simple test program for the MPL115A1 Pressure Sensor (SPI version).

 Hardware: ATmega168, ATmega328
 Powered at 3.3V or 5V, running at 8MHz or 16MHz.

 MPL115A1 Breakout ------------- Arduino
 -----------------               -------
         SDN ------------------- D9
         CSN ------------------- D10
         SDO ------------------- D12 *
         SDI ------------------- D11 *
         SCK ------------------- D13 *
         GND ------------------- GND
         VDD ------------------- VCC +

         * These pins physically connect to the SPI device so you can't change them
         + 5V board use 5V VDD, 3.3V board use 3.3V VDD

 License: CCAv3.0 Attribution-ShareAlike (http://creativecommons.org/licenses/by-sa/3.0/)
 You're free to use this code for any venture, but I'd love to hear about what you do with it,
 and any awesome changes you make to it. Attribution is greatly appreciated.
 */

// Includes
#include <SPI.h>

// Get your current altimiter setting from from the National
// Weather Service - http://www.weather.gov
// This value is often labeled "Barometer" or "Barometric Pressure" or just "Pressure"
#define NWS_BARO 29.92

// Pin definitions
#define MPL115A1_ENABLE_PIN 9
#define MPL115A1_SELECT_PIN 10

// Masks for MPL115A1 SPI i/o
#define MPL115A1_READ_MASK  0x80
#define MPL115A1_WRITE_MASK 0x7F

// MPL115A1 register address map
#define PRESH   0x00    // 80
#define PRESL   0x02    // 82
#define TEMPH   0x04    // 84
#define TEMPL   0x06    // 86

#define A0MSB   0x08    // 88
#define A0LSB   0x0A    // 8A
#define B1MSB   0x0C    // 8C
#define B1LSB   0x0E    // 8E
#define B2MSB   0x10    // 90
#define B2LSB   0x12    // 92
#define C12MSB  0x14    // 94
#define C12LSB  0x16    // 96
#define C11MSB  0x18    // 98
#define C11LSB  0x1A    // 9A
#define C22MSB  0x1C    // 9C
#define C22LSB  0x1E    // 9E

// Unit conversion macros
#define FT_TO_M(x) ((long)((x)*(0.3048)))
#define KPA_TO_INHG(x) ((x)*(0.295333727))
#define KPA_TO_MMHG(x) ((x)*(7.50061683))
#define KPA_TO_PSIA(x) ((x)*(0.145037738))
#define KPA_TO_KGCM2(x) ((x)*(0.0102))
#define INHG_TO_PSIA(x) ((x)*(0.49109778))
#define DEGC_TO_DEGF(x) ((x)*(9.0/5.0)+32)


void setup() {

    // initialize serial i/o
    Serial.begin(9600);

    // initialize SPI interface
    SPI.begin();

    // these are the defaults
    //SPI.setDataMode(SPI_MODE0);
    //SPI.setClockDivider(SPI_CLOCK_DIV4);  // MPL115A1 supports up to 8MHz
    //SPI.setBitOrder(MSBFIRST);

    // initialize the chip select and enable pins
    pinMode(MPL115A1_SELECT_PIN, OUTPUT);
    pinMode(MPL115A1_ENABLE_PIN, OUTPUT);

    // sleep the MPL115A1
    digitalWrite(MPL115A1_ENABLE_PIN, LOW);

    // set the chip select inactive, select signal is CS LOW
    digitalWrite(MPL115A1_SELECT_PIN, HIGH);

    // spam welcome banner
    Serial.print("\n************************************************************************************\n");
    Serial.print("* Using Altimiter Setting: ");
    Serial.print(NWS_BARO, 2);
    Serial.print(" in Hg                                             *\n");
    Serial.print("* Visit http://www.weather.gov for your current local value.                       *\n");
    Serial.print("************************************************************************************\n");
}


void loop() {

    float pressure_pKa = 0;
    float temperature_c= 0;
    long altitude_ft = 0;

    // wake the MPL115A1
    digitalWrite(MPL115A1_ENABLE_PIN, HIGH);
    delay(20);  // give the chip a few ms to wake up

    pressure_pKa = calculatePressurekPa();
    temperature_c = calculateTemperatureC();
    altitude_ft = calculateAltitudeFt(pressure_pKa);

    // put the MPL115A1 to sleep, it has this feature why not use it
    // while in shutdown the part draws ~1uA
    digitalWrite(MPL115A1_ENABLE_PIN, LOW);

    // print table of altitude, pressures, and temperatures to console
    Serial.print(altitude_ft);
    Serial.print(" ft | ");
    Serial.print(FT_TO_M(altitude_ft));
    Serial.print(" m | ");
    Serial.print(KPA_TO_INHG(pressure_pKa), 2);
    Serial.print(" in Hg | ");
    Serial.print(KPA_TO_MMHG(pressure_pKa), 0);
    Serial.print(" mm Hg | ");
    Serial.print(KPA_TO_PSIA(pressure_pKa), 2);
    Serial.print(" psia | ");
    //Serial.print(KPA_TO_KGCM2(pressure_pKa), 3);
    ///Serial.print(" kg/cm2 | ");
    Serial.print(pressure_pKa, 1);
    Serial.print(" kPa | ");

    // At a res of -5.35 counts/°C, digits lower than 0.1°C are not significant
    Serial.print(temperature_c, 1);
    Serial.print(" C | ");
    Serial.print(DEGC_TO_DEGF(temperature_c), 1);
    Serial.print(" F\n");

    // wait a few seconds before looping
    delay(5000);
}


long calculateAltitudeFt(float pressure_kPa) {

    float delta;
    long altitude_ft;


    // See http://en.wikipedia.org/wiki/Barometric_formula
    // If you're a pilot you need to know what's going on here,
    // otherwise just know these steps calculate the barometric altitude (ft)
    // based on the ratio of absolute barometric pressure (psia) to the
    // altimiter setting (psia).
    delta = KPA_TO_PSIA(pressure_kPa) / INHG_TO_PSIA( NWS_BARO );
    altitude_ft = (1 - pow(delta, (1 / 5.25587611))) / 0.0000068756;

    return altitude_ft;
}


float calculateTemperatureC() {

    unsigned int uiTadc;
    unsigned char uiTH, uiTL;

    unsigned int temperature_counts = 0;

    writeRegister(0x22, 0x00);  // Start temperature conversion
    delay(2);                   // Max wait time is 0.7ms, typ 0.6ms

    // Read pressure
    uiTH = readRegister(TEMPH);
    uiTL = readRegister(TEMPL);

    uiTadc = (unsigned int) uiTH << 8;
    uiTadc += (unsigned int) uiTL & 0x00FF;

    // Temperature is a 10bit value
    uiTadc = uiTadc >> 6;

    // -5.35 counts per °C, 472 counts is 25°C
    return 25 + (uiTadc - 472) / -5.35;
}


float calculatePressurekPa() {

    // See Freescale document AN3785 for detailed explanation
    // of this implementation.

    signed char sia0MSB, sia0LSB;
    signed char sib1MSB, sib1LSB;
    signed char sib2MSB, sib2LSB;
    signed char sic12MSB, sic12LSB;
    signed char sic11MSB, sic11LSB;
    signed char sic22MSB, sic22LSB;
    signed int sia0, sib1, sib2, sic12, sic11, sic22, siPcomp;
    float decPcomp;
    signed long lt1, lt2, lt3, si_c11x1, si_a11, si_c12x2;
    signed long si_a1, si_c22x2, si_a2, si_a1x1, si_y1, si_a2x2;
    unsigned int uiPadc, uiTadc;
    unsigned char uiPH, uiPL, uiTH, uiTL;

    writeRegister(0x24, 0x00);      // Start Both Conversions
    //writeRegister(0x20, 0x00);    // Start Pressure Conversion
    //writeRegister(0x22, 0x00);    // Start temperature conversion
    delay(2);                       // Max wait time is 1ms, typ 0.8ms

    // Read pressure
    uiPH = readRegister(PRESH);
    uiPL = readRegister(PRESL);
    uiTH = readRegister(TEMPH);
    uiTL = readRegister(TEMPL);

    uiPadc = (unsigned int) uiPH << 8;
    uiPadc += (unsigned int) uiPL & 0x00FF;
    uiTadc = (unsigned int) uiTH << 8;
    uiTadc += (unsigned int) uiTL & 0x00FF;

    // Placing Coefficients into 16-bit Variables
    // a0
    sia0MSB = readRegister(A0MSB);
    sia0LSB = readRegister(A0LSB);
    sia0 = (signed int) sia0MSB << 8;
    sia0 += (signed int) sia0LSB & 0x00FF;

    // b1
    sib1MSB = readRegister(B1MSB);
    sib1LSB = readRegister(B1LSB);
    sib1 = (signed int) sib1MSB << 8;
    sib1 += (signed int) sib1LSB & 0x00FF;

    // b2
    sib2MSB = readRegister(B2MSB);
    sib2LSB = readRegister(B2LSB);
    sib2 = (signed int) sib2MSB << 8;
    sib2 += (signed int) sib2LSB & 0x00FF;

    // c12
    sic12MSB = readRegister(C12MSB);
    sic12LSB = readRegister(C12LSB);
    sic12 = (signed int) sic12MSB << 8;
    sic12 += (signed int) sic12LSB & 0x00FF;

    // c11
    sic11MSB = readRegister(C11MSB);
    sic11LSB = readRegister(C11LSB);
    sic11 = (signed int) sic11MSB << 8;
    sic11 += (signed int) sic11LSB & 0x00FF;

    // c22
    sic22MSB = readRegister(C22MSB);
    sic22LSB = readRegister(C22LSB);
    sic22 = (signed int) sic22MSB << 8;
    sic22 += (signed int) sic22LSB & 0x00FF;

    // Coefficient 9 equation compensation
    uiPadc = uiPadc >> 6;
    uiTadc = uiTadc >> 6;

    // Step 1 c11x1 = c11 * Padc
    lt1 = (signed long) sic11;
    lt2 = (signed long) uiPadc;
    lt3 = lt1*lt2;
    si_c11x1 = (signed long) lt3;

    // Step 2 a11 = b1 + c11x1
    lt1 = ((signed long)sib1)<<14;
    lt2 = (signed long) si_c11x1;
    lt3 = lt1 + lt2;
    si_a11 = (signed long)(lt3>>14);

    // Step 3 c12x2 = c12 * Tadc
    lt1 = (signed long) sic12;
    lt2 = (signed long) uiTadc;
    lt3 = lt1*lt2;
    si_c12x2 = (signed long)lt3;

    // Step 4 a1 = a11 + c12x2
    lt1 = ((signed long)si_a11<<11);
    lt2 = (signed long)si_c12x2;
    lt3 = lt1 + lt2;
    si_a1 = (signed long) lt3>>11;

    // Step 5 c22x2 = c22*Tadc
    lt1 = (signed long)sic22;
    lt2 = (signed long)uiTadc;
    lt3 = lt1 * lt2;
    si_c22x2 = (signed long)(lt3);

    // Step 6 a2 = b2 + c22x2
    lt1 = ((signed long)sib2<<15);
    lt2 = ((signed long)si_c22x2>1);
    lt3 = lt1+lt2;
    si_a2 = ((signed long)lt3>>16);

    // Step 7 a1x1 = a1 * Padc
    lt1 = (signed long)si_a1;
    lt2 = (signed long)uiPadc;
    lt3 = lt1*lt2;
    si_a1x1 = (signed long)(lt3);

    // Step 8 y1 = a0 + a1x1
    lt1 = ((signed long)sia0<<10);
    lt2 = (signed long)si_a1x1;
    lt3 = lt1+lt2;
    si_y1 = ((signed long)lt3>>10);

    // Step 9 a2x2 = a2 * Tadc
    lt1 = (signed long)si_a2;
    lt2 = (signed long)uiTadc;
    lt3 = lt1*lt2;
    si_a2x2 = (signed long)(lt3);

    // Step 10 pComp = y1 + a2x2
    lt1 = ((signed long)si_y1<<10);
    lt2 = (signed long)si_a2x2;
    lt3 = lt1+lt2;

    // Fixed point result with rounding
    //siPcomp = ((signed int)lt3>>13);
    siPcomp = lt3/8192;

    // decPcomp is defined as a floating point number
    // Conversion to decimal value from 1023 ADC count value
    // ADC counts are 0 to 1023, pressure is 50 to 115kPa respectively
    decPcomp = ((65.0/1023.0)*siPcomp)+50;

    return decPcomp;
}


unsigned int readRegister(byte thisRegister) {

    byte result = 0;

    // select the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, LOW);

    // send the request
    SPI.transfer(thisRegister | MPL115A1_READ_MASK);
    result = SPI.transfer(0x00);

    // deselect the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, HIGH);

    return result;
}


void writeRegister(byte thisRegister, byte thisValue) {

    // select the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, LOW);

    // send the request
    SPI.transfer(thisRegister & MPL115A1_WRITE_MASK);
    SPI.transfer(thisValue);

    // deselect the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, HIGH);
}
	/*
	MPL115A1 SPI Digital Barometer Test Code
	Created on: September 30, 2010
	By: Jeremiah McConnell - miah at miah.com
	Portions: Jim Lindblom - jim at sparkfun.com

	This is a simple test program for the MPL115A1 Pressure Sensor (SPI version).

	Hardware: ATmega168, ATmega328
	Powered at 3.3V or 5V, running at 8MHz or 16MHz.

	MPL115A1 Breakout ------------- Arduino
	----------------- -------
	SDN ------------------- D9
	CSN ------------------- D10
	SDO ------------------- D12 *
	SDI ------------------- D11 *
	SCK ------------------- D13 *
	GND ------------------- GND
	VDD ------------------- VCC +

	* These pins physically connect to the SPI device so you can't change them
	+ 5V board use 5V VDD, 3.3V board use 3.3V VDD

	License: CCAv3.0 Attribution-ShareAlike (http://creativecommons.org/licenses/by-sa/3.0/)
	You're free to use this code for any venture, but I'd love to hear about what you do with it,
	and any awesome changes you make to it. Attribution is greatly appreciated.
	*/

	// Includes
	#include <SPI.h>

	// Get your current altimiter setting from from the National
	// Weather Service - http://www.weather.gov
	// This value is often labeled "Barometer" or "Barometric Pressure" or just "Pressure"
	#define NWS_BARO 29.92

	// Pin definitions
	#define MPL115A1_ENABLE_PIN 9
	#define MPL115A1_SELECT_PIN 10

	// Masks for MPL115A1 SPI i/o
	#define MPL115A1_READ_MASK 0x80
	#define MPL115A1_WRITE_MASK 0x7F

	// MPL115A1 register address map
	#define PRESH 0x00 // 80
	#define PRESL 0x02 // 82
	#define TEMPH 0x04 // 84
	#define TEMPL 0x06 // 86

	#define A0MSB 0x08 // 88
	#define A0LSB 0x0A // 8A
	#define B1MSB 0x0C // 8C
	#define B1LSB 0x0E // 8E
	#define B2MSB 0x10 // 90
	#define B2LSB 0x12 // 92
	#define C12MSB 0x14 // 94
	#define C12LSB 0x16 // 96
	#define C11MSB 0x18 // 98
	#define C11LSB 0x1A // 9A
	#define C22MSB 0x1C // 9C
	#define C22LSB 0x1E // 9E

	// Unit conversion macros
	#define FT_TO_M(x) ((long)((x)*(0.3048)))
	#define KPA_TO_INHG(x) ((x)*(0.295333727))
	#define KPA_TO_MMHG(x) ((x)*(7.50061683))
	#define KPA_TO_PSIA(x) ((x)*(0.145037738))
	#define KPA_TO_KGCM2(x) ((x)*(0.0102))
	#define INHG_TO_PSIA(x) ((x)*(0.49109778))
	#define DEGC_TO_DEGF(x) ((x)*(9.0/5.0)+32)


	void setup() {

	// initialize serial i/o
	Serial.begin(9600);

	// initialize SPI interface
	SPI.begin();

	// these are the defaults
	//SPI.setDataMode(SPI_MODE0);
	//SPI.setClockDivider(SPI_CLOCK_DIV4); // MPL115A1 supports up to 8MHz
	//SPI.setBitOrder(MSBFIRST);

	// initialize the chip select and enable pins
	pinMode(MPL115A1_SELECT_PIN, OUTPUT);
	pinMode(MPL115A1_ENABLE_PIN, OUTPUT);

	// sleep the MPL115A1
	digitalWrite(MPL115A1_ENABLE_PIN, LOW);

	// set the chip select inactive, select signal is CS LOW
	digitalWrite(MPL115A1_SELECT_PIN, HIGH);

	// spam welcome banner
	Serial.print("\n************************************************************************************\n");
	Serial.print("* Using Altimiter Setting: ");
	Serial.print(NWS_BARO, 2);
	Serial.print(" in Hg *\n");
	Serial.print("* Visit http://www.weather.gov for your current local value. *\n");
	Serial.print("************************************************************************************\n");
	}


	void loop() {

	float pressure_pKa = 0;
	float temperature_c= 0;
	long altitude_ft = 0;

	// wake the MPL115A1
	digitalWrite(MPL115A1_ENABLE_PIN, HIGH);
	delay(20); // give the chip a few ms to wake up

	pressure_pKa = calculatePressurekPa();
	temperature_c = calculateTemperatureC();
	altitude_ft = calculateAltitudeFt(pressure_pKa);

	// put the MPL115A1 to sleep, it has this feature why not use it
	// while in shutdown the part draws ~1uA
	digitalWrite(MPL115A1_ENABLE_PIN, LOW);

	// print table of altitude, pressures, and temperatures to console
	Serial.print(altitude_ft);
	Serial.print(" ft \| ");
	Serial.print(FT_TO_M(altitude_ft));
	Serial.print(" m \| ");
	Serial.print(KPA_TO_INHG(pressure_pKa), 2);
	Serial.print(" in Hg \| ");
	Serial.print(KPA_TO_MMHG(pressure_pKa), 0);
	Serial.print(" mm Hg \| ");
	Serial.print(KPA_TO_PSIA(pressure_pKa), 2);
	Serial.print(" psia \| ");
	//Serial.print(KPA_TO_KGCM2(pressure_pKa), 3);
	///Serial.print(" kg/cm2 \| ");
	Serial.print(pressure_pKa, 1);
	Serial.print(" kPa \| ");

	// At a res of -5.35 counts/°C, digits lower than 0.1°C are not significant
	Serial.print(temperature_c, 1);
	Serial.print(" C \| ");
	Serial.print(DEGC_TO_DEGF(temperature_c), 1);
	Serial.print(" F\n");

	// wait a few seconds before looping
	delay(5000);
	}


	long calculateAltitudeFt(float pressure_kPa) {

	float delta;
	long altitude_ft;


	// See http://en.wikipedia.org/wiki/Barometric_formula
	// If you're a pilot you need to know what's going on here,
	// otherwise just know these steps calculate the barometric altitude (ft)
	// based on the ratio of absolute barometric pressure (psia) to the
	// altimiter setting (psia).
	delta = KPA_TO_PSIA(pressure_kPa) / INHG_TO_PSIA( NWS_BARO );
	altitude_ft = (1 - pow(delta, (1 / 5.25587611))) / 0.0000068756;

	return altitude_ft;
	}


	float calculateTemperatureC() {

	unsigned int uiTadc;
	unsigned char uiTH, uiTL;

	unsigned int temperature_counts = 0;

	writeRegister(0x22, 0x00); // Start temperature conversion
	delay(2); // Max wait time is 0.7ms, typ 0.6ms

	// Read pressure
	uiTH = readRegister(TEMPH);
	uiTL = readRegister(TEMPL);

	uiTadc = (unsigned int) uiTH << 8;
	uiTadc += (unsigned int) uiTL & 0x00FF;

	// Temperature is a 10bit value
	uiTadc = uiTadc >> 6;

	// -5.35 counts per °C, 472 counts is 25°C
	return 25 + (uiTadc - 472) / -5.35;
	}


	float calculatePressurekPa() {

	// See Freescale document AN3785 for detailed explanation
	// of this implementation.

	signed char sia0MSB, sia0LSB;
	signed char sib1MSB, sib1LSB;
	signed char sib2MSB, sib2LSB;
	signed char sic12MSB, sic12LSB;
	signed char sic11MSB, sic11LSB;
	signed char sic22MSB, sic22LSB;
	signed int sia0, sib1, sib2, sic12, sic11, sic22, siPcomp;
	float decPcomp;
	signed long lt1, lt2, lt3, si_c11x1, si_a11, si_c12x2;
	signed long si_a1, si_c22x2, si_a2, si_a1x1, si_y1, si_a2x2;
	unsigned int uiPadc, uiTadc;
	unsigned char uiPH, uiPL, uiTH, uiTL;

	writeRegister(0x24, 0x00); // Start Both Conversions
	//writeRegister(0x20, 0x00); // Start Pressure Conversion
	//writeRegister(0x22, 0x00); // Start temperature conversion
	delay(2); // Max wait time is 1ms, typ 0.8ms

	// Read pressure
	uiPH = readRegister(PRESH);
	uiPL = readRegister(PRESL);
	uiTH = readRegister(TEMPH);
	uiTL = readRegister(TEMPL);

	uiPadc = (unsigned int) uiPH << 8;
	uiPadc += (unsigned int) uiPL & 0x00FF;
	uiTadc = (unsigned int) uiTH << 8;
	uiTadc += (unsigned int) uiTL & 0x00FF;

	// Placing Coefficients into 16-bit Variables
	// a0
	sia0MSB = readRegister(A0MSB);
	sia0LSB = readRegister(A0LSB);
	sia0 = (signed int) sia0MSB << 8;
	sia0 += (signed int) sia0LSB & 0x00FF;

	// b1
	sib1MSB = readRegister(B1MSB);
	sib1LSB = readRegister(B1LSB);
	sib1 = (signed int) sib1MSB << 8;
	sib1 += (signed int) sib1LSB & 0x00FF;

	// b2
	sib2MSB = readRegister(B2MSB);
	sib2LSB = readRegister(B2LSB);
	sib2 = (signed int) sib2MSB << 8;
	sib2 += (signed int) sib2LSB & 0x00FF;

	// c12
	sic12MSB = readRegister(C12MSB);
	sic12LSB = readRegister(C12LSB);
	sic12 = (signed int) sic12MSB << 8;
	sic12 += (signed int) sic12LSB & 0x00FF;

	// c11
	sic11MSB = readRegister(C11MSB);
	sic11LSB = readRegister(C11LSB);
	sic11 = (signed int) sic11MSB << 8;
	sic11 += (signed int) sic11LSB & 0x00FF;

	// c22
	sic22MSB = readRegister(C22MSB);
	sic22LSB = readRegister(C22LSB);
	sic22 = (signed int) sic22MSB << 8;
	sic22 += (signed int) sic22LSB & 0x00FF;

	// Coefficient 9 equation compensation
	uiPadc = uiPadc >> 6;
	uiTadc = uiTadc >> 6;

	// Step 1 c11x1 = c11 * Padc
	lt1 = (signed long) sic11;
	lt2 = (signed long) uiPadc;
	lt3 = lt1*lt2;
	si_c11x1 = (signed long) lt3;

	// Step 2 a11 = b1 + c11x1
	lt1 = ((signed long)sib1)<<14;
	lt2 = (signed long) si_c11x1;
	lt3 = lt1 + lt2;
	si_a11 = (signed long)(lt3>>14);

	// Step 3 c12x2 = c12 * Tadc
	lt1 = (signed long) sic12;
	lt2 = (signed long) uiTadc;
	lt3 = lt1*lt2;
	si_c12x2 = (signed long)lt3;

	// Step 4 a1 = a11 + c12x2
	lt1 = ((signed long)si_a11<<11);
	lt2 = (signed long)si_c12x2;
	lt3 = lt1 + lt2;
	si_a1 = (signed long) lt3>>11;

	// Step 5 c22x2 = c22*Tadc
	lt1 = (signed long)sic22;
	lt2 = (signed long)uiTadc;
	lt3 = lt1 * lt2;
	si_c22x2 = (signed long)(lt3);

	// Step 6 a2 = b2 + c22x2
	lt1 = ((signed long)sib2<<15);
	lt2 = ((signed long)si_c22x2>1);
	lt3 = lt1+lt2;
	si_a2 = ((signed long)lt3>>16);

	// Step 7 a1x1 = a1 * Padc
	lt1 = (signed long)si_a1;
	lt2 = (signed long)uiPadc;
	lt3 = lt1*lt2;
	si_a1x1 = (signed long)(lt3);

	// Step 8 y1 = a0 + a1x1
	lt1 = ((signed long)sia0<<10);
	lt2 = (signed long)si_a1x1;
	lt3 = lt1+lt2;
	si_y1 = ((signed long)lt3>>10);

	// Step 9 a2x2 = a2 * Tadc
	lt1 = (signed long)si_a2;
	lt2 = (signed long)uiTadc;
	lt3 = lt1*lt2;
	si_a2x2 = (signed long)(lt3);

	// Step 10 pComp = y1 + a2x2
	lt1 = ((signed long)si_y1<<10);
	lt2 = (signed long)si_a2x2;
	lt3 = lt1+lt2;

	// Fixed point result with rounding
	//siPcomp = ((signed int)lt3>>13);
	siPcomp = lt3/8192;

	// decPcomp is defined as a floating point number
	// Conversion to decimal value from 1023 ADC count value
	// ADC counts are 0 to 1023, pressure is 50 to 115kPa respectively
	decPcomp = ((65.0/1023.0)*siPcomp)+50;

	return decPcomp;
	}


	unsigned int readRegister(byte thisRegister) {

	byte result = 0;

	// select the MPL115A1
	digitalWrite(MPL115A1_SELECT_PIN, LOW);

	// send the request
	SPI.transfer(thisRegister \| MPL115A1_READ_MASK);
	result = SPI.transfer(0x00);

	// deselect the MPL115A1
	digitalWrite(MPL115A1_SELECT_PIN, HIGH);

	return result;
	}


	void writeRegister(byte thisRegister, byte thisValue) {

	// select the MPL115A1
	digitalWrite(MPL115A1_SELECT_PIN, LOW);

	// send the request
	SPI.transfer(thisRegister & MPL115A1_WRITE_MASK);
	SPI.transfer(thisValue);

	// deselect the MPL115A1
	digitalWrite(MPL115A1_SELECT_PIN, HIGH);
	}