Added barometer calibration functions and example code

library/MPL3115A2_Pressure/MPL3115A2.cpp

@@ -287,3 +287,57 @@ void MPL3115A2::IIC_Write(byte regAddr, byte 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;
+
+	setModeBarometer(); // Measure pressure in Pascals
+	setOversampleRate(7); // Set Oversample to the recommended 128 --> 512ms
+	enableEventFlags(); // Enable all three pressure and temp event flags 
+	for (byte i=0;i<6;i++){
+		delay(550); //wait for sensor to read pressure (512ms in datasheet)
+		pressureAccum = pressureAccum + readPressure();
+	}
+	float currpress = pressureAccum / 6; //average pressure over (6/550) seconds
+
+	//Serial.print("Current 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);
+}

library/MPL3115A2_Pressure/MPL3115A2.h

@@ -86,7 +86,22 @@ class MPL3115A2 {
   void setOversampleRate(byte); // Sets the # of samples from 1 to 128. See datasheet.
   void enableEventFlags(); // Sets the fundamental event flags. Required during setup.
 
+  // added by https://github.com/mariocannistra
+  // to declare the functions by Michael Lange on mbed.org
+  int8_t offsetAltitude();
+  void setOffsetAltitude(int8_t offset);
+  float offsetPressure();
+  void setOffsetPressure(const char offset);
+  float offsetTemperature();
+  void setOffsetTemperature(const char offset);
+  // and the functions by http://www.henrylahr.com/?p=99
+  void setBarometricInput(float pressSeaLevel);
+  void runCalibration(float currentElevation);
+
   //Public Variables
+  float elevation_offset;
+  float calculated_sea_level_press;
+
 
 private:
   //Private Functions

library/MPL3115A2_Pressure/examples/barometer_calibration/barometer_calibration.ino

@@ -0,0 +1,153 @@
+/*
+ MPL3115A2 Barometric Pressure Sensor - Calibration Code
+ by https://github.com/mariocannistra
+
+ i've put together this code to obtain more precise values 
+ from the sensor   --> when using it for altitude measurements <--
+ and to get more precise barometer indications
+
+ How I glued things together:
+ - slightly customize 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
+
+ About the following line, hope to meet Nathan someday for the beer :-) If I will meet all of them.. then also food :-)
+ 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"
+
+//Create an instance of the object
+MPL3115A2 baro3115;
+
+#define ALTBASIS 332.0  // start altitude to calculate mean sea level pressure in meters
+					    // got this averaging readouts from 3 different GPS
+
+void setup()
+{
+	// give me time to open serial monitor:
+	delay(3000);
+
+	Wire.begin();        // Join i2c bus
+	Serial.begin(57600);  // Start serial for output
+
+	baro3115.begin(); // Get sensor online
+
+	//Configure the sensor
+	// Set the # of samples from 1 to 128. See datasheet.
+	// Integer values between 0 < n < 7 give oversample ratios 2^n and 
+	// sampling intervals of 0=6 ms , 1=10, 2=18, 3=34, 4=66, 5=130, 6=258, and 7=512
+	baro3115.setOversampleRate(7); // Set Oversample to the recommended 128 --> 512ms
+	baro3115.enableEventFlags(); // Enable all three pressure and temp event flags 
+
+	Serial.println("BEFORE calibration...");
+	// show sample data readouts before starting the calibration:
+	outData();
+
+	// set to zero all the sensor offsets and input values:
+	baro3115.setOffsetTemperature(0);
+	baro3115.setOffsetPressure(0);
+	baro3115.setOffsetAltitude(0);
+	baro3115.setBarometricInput(0.0);
+
+	// let's have a look, just in case:
+	Serial.println("OFFSETS:");
+	Serial.print("  pressure: ");
+	Serial.println(baro3115.offsetPressure() ,2);
+	Serial.print("  altitude: ");
+	Serial.println((float)baro3115.offsetAltitude() ,2);
+	Serial.print("  temperature(C): ");
+	Serial.println(baro3115.offsetTemperature() ,2);
+
+	// now perform the calibration
+
+	//calculate pressure for current ALTBASIS altitude by averaging a few readings
+	Serial.println("Pressure calibration...");
+
+	// this function calculates a new sea level pressure ref value
+	// it will NOT change the sensor registers
+	// see below setBarometricInput() where that value is actually set
+	// in the registers. the sensor will start using it just after.
+	baro3115.runCalibration(ALTBASIS);
+
+
+	Serial.print("calculated sea level pressure: "); 
+	Serial.print(baro3115.calculated_sea_level_press ,2); 
+	Serial.println(" Pa");
+
+	Serial.print("calculated elevation_offset: "); 
+	Serial.print(baro3115.elevation_offset ,2); 
+	Serial.println(" Pa");
+
+	// i originally had 35 hPa error in pressure readouts
+	// once i added the elevation_offset calculation, the error
+	// decreased and is now within 4 hPa (ie: 1015.18 instead of official 1011.5)
+	// (official = pro measurement at same elevation at 500 m from my laptop)
+
+	// This configuration option calibrates the sensor according to
+	// the sea level pressure for the measurement location (2 Pa per LSB)
+	// The default value for "Barometric input for Altitude calculation" is 101,326 Pa
+
+	baro3115.setBarometricInput( baro3115.calculated_sea_level_press );
+
+	// calibration performed
+
+	// add temperature offset for my tested sensor
+	// seems the temperature probe is within the ADC and should not be used
+	// to measure the environment. Will try adding the offset i got by comparison
+	// with another thermometer
+	// you can enable this if you need it:
+	// baro3115.setOffsetTemperature((char) (0.65 / 0.0625) );
+}
+
+void outData()
+{
+	baro3115.setModeBarometer(); // Measure pressure in Pascals
+	delay(520);
+	// since we are using the calibration then we also have to add the
+	// calculated elevation related pressure offset:
+	float pressure = baro3115.readPressure() + baro3115.elevation_offset;
+	Serial.print("Pressure(Pa): ");
+	Serial.print(pressure, 2);
+
+	// output is in Pa
+	// 1 kPa = 10 hPa = 1000 Pa
+	// 1 hPa = 100 Pascals = 1 mb
+	pressure = (pressure / 100) ;		//  ... / 1000 * 10 ;
+	Serial.print(" Pressure(hPa): ");
+	Serial.print(pressure, 2);
+
+	float temperature = baro3115.readTemp();
+	Serial.print(" Temperature(C): ");
+	Serial.print(temperature, 2);
+
+	baro3115.setModeAltimeter(); // Measure altitude above sea level in meters
+	delay(520);
+	float altitude = baro3115.readAltitude();
+	Serial.print(" Altitude above sea(m): ");
+	Serial.println(altitude, 2);
+}
+
+void loop()
+{
+	outData();
+}
+
+// sample output:
+//BEFORE calibration...
+//Pressure(Pa): 1327.00 Pressure(hPa): 13.27 Temperature(C): 21.25 Altitude above sea(m): 332.25
+//OFFSETS:
+//  pressure: 0.00
+//  altitude: 0.00
+//  temperature(C): 0.00
+//Pressure calibration...
+//calculated sea level pressure: 101552.34 Pa
+//calculated elevation_offset: 3925.32 Pa
+//Pressure(Pa): 101547.56 Pressure(hPa): 1015.48 Temperature(C): 21.25 Altitude above sea(m): 331.94
+//Pressure(Pa): 101543.31 Pressure(hPa): 1015.43 Temperature(C): 21.25 Altitude above sea(m): 331.62
+//Pressure(Pa): 101544.31 Pressure(hPa): 1015.44 Temperature(C): 21.25 Altitude above sea(m): 331.94
+//Pressure(Pa): 101546.06 Pressure(hPa): 1015.46 Temperature(C): 21.25 Altitude above sea(m): 331.75
+//Pressure(Pa): 101543.56 Pressure(hPa): 1015.44 Temperature(C): 21.25 Altitude above sea(m): 331.94