MPL3115A2.cpp

/*
 This library allows an Arduino to read from the MPL3115A2 low-cost high-precision pressure sensor.

 Sensor calibration functions added by https://github.com/mariocannistra
 Note: I've put together this code to obtain more precise values and a stable
 behavior from the sensor when using it both for altitude and pressure measurements
 
 List of code merges and improvements:
 - slightly customized the code by http://www.henrylahr.com/?p=99
 - used the offset routines from Michael Lange on mbed.org
 - completed with elevation_offset calculation by digitalmisery comment at https://www.sparkfun.com/products/11084
 - then packaged the calibration function within the original 
   library code by Nathan Seidle - SparkFun Electronics
 - merged with latest Sparkfun fixes by https://github.com/ToniCorinne
 - added some () in a couple of IFs per compiler warnings/suggestions
 - added the proper register config calls sequences using setModeStandby() and setModeActive()
 - fixed the pressure reading routine using the correct bit mask for PDR flag. Thanks to @lectroidmarc and @rabelux for listing the issue here: https://github.com/sparkfun/MPL3115A2_Breakout/issues/12  The chip datasheet https://cdn-shop.adafruit.com/datasheets/1893_datasheet.pdf at page 20 actually shows PDR as 0x04
 
 License: please see below and refer to the original sources for the license stuff. I guess it's all public domain but please check. And if you meet with Nathan for a beer let me know and consider as beerware my code as well.

 Original library by: Nathan Seidle
 SparkFun Electronics
 Date: September 22nd, 2013
 License: This code is public domain but you buy me a beer if you use this and we meet someday (Beerware license).

 */

#include <Wire.h>

#include "MPL3115A2.h"

MPL3115A2::MPL3115A2()
{
  //Set initial values for private vars
}

//Begin
/*******************************************************************************************/
//Start I2C communication
void MPL3115A2::begin(void)
{
	Wire.begin();
}


//Returns the number of meters above sea level
//Returns -1 if no new data is available
float MPL3115A2::readAltitude()
{
	toggleOneShot(); //Toggle the OST bit causing the sensor to immediately take another reading

	//Wait for PDR bit, indicates we have new pressure data
	int counter = 0;
	
	while( (IIC_Read(STATUS) & MPL3115A2_REGISTER_STATUS_PDR ) == 0)
	{
		if(++counter > 512) return(-999); //Error out after max of 512ms for a read
		delay(1);
	}

	// Read pressure registers
	Wire.beginTransmission(MPL3115A2_ADDRESS);
	Wire.write(OUT_P_MSB);  // Address of data to get
	Wire.endTransmission(false); // Send data to I2C dev with option for a repeated start. THIS IS NECESSARY and not supported before Arduino V1.0.1!
	if (Wire.requestFrom(MPL3115A2_ADDRESS, 3) != 3) { // Request three bytes
		return -999;
	}

	byte msb, csb, lsb;
	msb = Wire.read();
	csb = Wire.read();
	lsb = Wire.read();

	// The least significant bytes l_altitude and l_temp are 4-bit,
	// fractional values, so you must cast the calulation in (float),
	// shift the value over 4 spots to the right and divide by 16 (since 
	// there are 16 values in 4-bits). 
	float tempcsb = (lsb>>4)/16.0;

	float altitude = (float)( (msb << 8) | csb) + tempcsb;

	return(altitude);
}

//Returns the number of feet above sea level
float MPL3115A2::readAltitudeFt()
{
  return(readAltitude() * 3.28084);
}

//Reads the current pressure in Pa
//Unit must be set in barometric pressure mode
//Returns -1 if no new data is available
float MPL3115A2::readPressure()
{
	//Check PDR bit, if it's not set then toggle OST
	if((IIC_Read(STATUS) & MPL3115A2_REGISTER_STATUS_PDR ) == 0) toggleOneShot(); //Toggle the OST bit causing the sensor to immediately take another reading

	//Wait for PDR bit, indicates we have new pressure data
	int counter = 0;
	while( (IIC_Read(STATUS) & MPL3115A2_REGISTER_STATUS_PDR ) == 0)
	{
		if(++counter > 512) return(-999); //Error out after max of 512ms for a read
		delay(1);
	}

	// Read pressure registers
	Wire.beginTransmission(MPL3115A2_ADDRESS);
	Wire.write(OUT_P_MSB);  // Address of data to get
	Wire.endTransmission(false); // Send data to I2C dev with option for a repeated start. THIS IS NECESSARY and not supported before Arduino V1.0.1!
	if (Wire.requestFrom(MPL3115A2_ADDRESS, 3) != 3) { // Request three bytes
		return -999;
	}

	byte msb, csb, lsb;
	msb = Wire.read();
	csb = Wire.read();
	lsb = Wire.read();
	
	toggleOneShot(); //Toggle the OST bit causing the sensor to immediately take another reading

	// Pressure comes back as a left shifted 20 bit number
	long pressure_whole = (long)msb<<16 | (long)csb<<8 | (long)lsb;
	pressure_whole >>= 6; //Pressure is an 18 bit number with 2 bits of decimal. Get rid of decimal portion.

	lsb &= B00110000; //Bits 5/4 represent the fractional component
	lsb >>= 4; //Get it right aligned
	float pressure_decimal = (float)lsb/4.0; //Turn it into fraction

	float pressure = (float)pressure_whole + pressure_decimal;

	return(pressure);
}

float MPL3115A2::readTemp()
{
	if((IIC_Read(STATUS) & (1<<1)) == 0) toggleOneShot(); //Toggle the OST bit causing the sensor to immediately take another reading

	//Wait for TDR bit, indicates we have new temp data
	int counter = 0;
	while( (IIC_Read(STATUS) & MPL3115A2_REGISTER_STATUS_TDR ) == 0)
	{
		if(++counter > 512) return(-999); //Error out after max of 512ms for a read
		delay(1);
	}

	// Read temperature registers
	Wire.beginTransmission(MPL3115A2_ADDRESS);
	Wire.write(OUT_T_MSB);  // Address of data to get
	Wire.endTransmission(false); // Send data to I2C dev with option for a repeated start. THIS IS NECESSARY and not supported before Arduino V1.0.1!
	if (Wire.requestFrom(MPL3115A2_ADDRESS, 2) != 2) { // Request two bytes
		return -999;
	}

	byte msb, lsb;
	msb = Wire.read();
	lsb = Wire.read();

	toggleOneShot(); //Toggle the OST bit causing the sensor to immediately take another reading

    //Negative temperature fix by D.D.G.
	word foo = 0;
    bool negSign = false;

    //Check for 2s compliment
	if(msb > 0x7F)
	{
        foo = ~((msb << 8) + lsb) + 1;  //2’s complement
        msb = foo >> 8;
        lsb = foo & 0x00F0; 
        negSign = true;
	}

	// The least significant bytes l_altitude and l_temp are 4-bit,
	// fractional values, so you must cast the calulation in (float),
	// shift the value over 4 spots to the right and divide by 16 (since 
	// there are 16 values in 4-bits). 
	float templsb = (lsb>>4)/16.0; //temp, fraction of a degree

	float temperature = (float)(msb + templsb);

	if (negSign) temperature = 0 - temperature;
	
	return(temperature);
}

//Give me temperature in fahrenheit!
float MPL3115A2::readTempF()
{
  return((readTemp() * 9.0)/ 5.0 + 32.0); // Convert celsius to fahrenheit
}

//Sets the mode to Barometer
//CTRL_REG1, ALT bit
void MPL3115A2::setModeBarometer()
{
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting &= ~(1<<7); //Clear ALT bit
  IIC_Write(CTRL_REG1, tempSetting);
}

//Sets the mode to Altimeter
//CTRL_REG1, ALT bit
void MPL3115A2::setModeAltimeter()
{
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting |= (1<<7); //Set ALT bit
  IIC_Write(CTRL_REG1, tempSetting);
}

//Puts the sensor in standby mode
//This is needed so that we can modify the major control registers
void MPL3115A2::setModeStandby()
{
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting &= ~(1<<0); //Clear SBYB bit for Standby mode
  IIC_Write(CTRL_REG1, tempSetting);
}

//Puts the sensor in active mode
//This is needed so that we can modify the major control registers
void MPL3115A2::setModeActive()
{
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting |= (1<<0); //Set SBYB bit for Active mode
  IIC_Write(CTRL_REG1, tempSetting);
}

//Call with a rate from 0 to 7. See page 33 for table of ratios.
//Sets the over sample rate. Datasheet calls for 128 but you can set it 
//from 1 to 128 samples. The higher the oversample rate the greater
//the time between data samples.
void MPL3115A2::setOversampleRate(byte sampleRate)
{
  if(sampleRate > 7) sampleRate = 7; //OS cannot be larger than 0b.0111
  sampleRate <<= 3; //Align it for the CTRL_REG1 register
  
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting &= B11000111; //Clear out old OS bits
  tempSetting |= sampleRate; //Mask in new OS bits
  IIC_Write(CTRL_REG1, tempSetting);
}

//Enables the pressure and temp measurement event flags so that we can
//test against them. This is recommended in datasheet during setup.
void MPL3115A2::enableEventFlags()
{
  IIC_Write(PT_DATA_CFG, 0x07); // Enable all three pressure and temp event flags 
}

//Clears then sets the OST bit which causes the sensor to immediately take another reading
//Needed to sample faster than 1Hz
void MPL3115A2::toggleOneShot(void)
{
  byte tempSetting = IIC_Read(CTRL_REG1); //Read current settings
  tempSetting &= ~(1<<1); //Clear OST bit
  IIC_Write(CTRL_REG1, tempSetting);

  tempSetting = IIC_Read(CTRL_REG1); //Read current settings to be safe
  tempSetting |= (1<<1); //Set OST bit
  IIC_Write(CTRL_REG1, tempSetting);
}


// These are the two I2C functions in this sketch.
byte MPL3115A2::IIC_Read(byte regAddr)
{
  // This function reads one byte over IIC
  Wire.beginTransmission(MPL3115A2_ADDRESS);
  Wire.write(regAddr);  // Address of CTRL_REG1
  Wire.endTransmission(false); // Send data to I2C dev with option for a repeated start. THIS IS NECESSARY and not supported before Arduino V1.0.1!
  Wire.requestFrom(MPL3115A2_ADDRESS, 1); // Request the data...
  return Wire.read();
}

void MPL3115A2::IIC_Write(byte regAddr, byte value)
{
  // This function writes one byto over IIC
  Wire.beginTransmission(MPL3115A2_ADDRESS);
  Wire.write(regAddr);
  Wire.write(value);
  Wire.endTransmission(true);
}

// added by https://github.com/mariocannistra
// the following offset routines are from Michael Lange on mbed.org
// modified for some data types and to pre-calculate the offset in °C 
// when reading the temperature offset

//! Returns the altitude offset stored in the sensor.
int8_t MPL3115A2::offsetAltitude() { return (int8_t) IIC_Read(OFF_H); }
//! Sets the altitude offset stored in the sensor. The allowed offset range is from -128 to 127 meters.
void MPL3115A2::setOffsetAltitude(int8_t offset) { IIC_Write(OFF_H, offset); } 
//! Returns the pressure offset stored in the sensor.
float MPL3115A2::offsetPressure() { return (float) IIC_Read(OFF_P); }
//! Sets the pressure offset stored in the sensor. The allowed offset range is from -128 to 127 where each LSB represents 2 Pa.
void MPL3115A2::setOffsetPressure(const char offset) { IIC_Write(OFF_P, offset); }
//! Returns the temperature offset stored in the sensor.
float MPL3115A2::offsetTemperature() { return (float) IIC_Read(OFF_T) * 0.0625 ; }
//! Sets the temperature offset stored in the sensor. The allowed offset range is from -128 to 127 where each LSB represents 0.0625ºC.
void MPL3115A2::setOffsetTemperature(const char offset) { IIC_Write(OFF_T, offset); } 

// the 2 following functions comes from http://www.henrylahr.com/?p=99
// edited to merge them with the Sparkfun library

// sets the "Barometric input for Altitude calculation" (see datasheet)
void MPL3115A2::setBarometricInput(float pressSeaLevel)
{
	IIC_Write(BAR_IN_MSB, (unsigned int)(pressSeaLevel / 2)>>8);
	IIC_Write(BAR_IN_LSB, (unsigned int)(pressSeaLevel / 2)&0xFF);
}

void MPL3115A2::runCalibration(float currentElevation)
{
	float pressureAccum = 0.0;
	float pressure = 0.0;
	
    setModeStandby();		// this is needed to change control registers
	setModeBarometer();		// Measure pressure in Pascals
    setOversampleRate(7);	// Set Oversample to the recommended 128 --> 512ms
    enableEventFlags();		// Enable all three pressure and temp event flags 
    setModeActive();		// switch back to active measurement mode
	
	for (byte i=0;i<6;i++){
		pressure = readPressure();
		//Serial.print("Pressure reading: "); 
		//Serial.print(pressure); 
		//Serial.println(" Pa");
		pressureAccum = pressureAccum + pressure;
	}
	float currpress = pressureAccum / 6; //average pressure over 6 samples

	//Serial.print("Average pressure: "); 
	//Serial.print(currpress); 
	//Serial.println(" Pa");

	//calculate pressure at mean sea level based on the known altitude
	float powElement = pow(1.0-(currentElevation*0.0000225577), 5.255877);
	calculated_sea_level_press = currpress / powElement;
	//Serial.print("Calculated sea level pressure: "); 
	//Serial.print(calculated_sea_level_press); 
	//Serial.println(" Pa");

	elevation_offset = 101325.0 - (101325.0 * powElement);
}