Arduino code to use with the BMP180 pressure and temperature sensor. SCL -> A5. SDA -> A4. Code worked out from the official datasheet: http://www.mouser.com/ds/2/783/BST-BMP180-DS000-12~1-786481.pdf

BMP180.ino

#include <Wire.h>
#define BMP180_I2C_Adress 0x77 //Usually 0x77

int p0 = 1013.25; //Pressure at sea level; used to calculate altitude
int i = 2; //Pressure precision setting: 0, 1, 2 or 3 (3 is broken right now. will fix.)

int AC1;
int AC2; 
int AC3; 
unsigned int AC4;
unsigned int AC5;
unsigned int AC6;
int b1; 
int B2;
int MB;
int MC;
int MD;
long B5;

float T;
float P;

void setup(){
  Wire.begin();
  BMP180Calibration();
}

void loop(){
  //your code here
}


int BMP180ReadInt(unsigned char adress){
  unsigned char MSB, LSB;
  Wire.beginTransmission(BMP180_I2C_Adress);
  Wire.write(adress);
  Wire.endTransmission();
  Wire.requestFrom(BMP180_I2C_Adress, 2);
  while (Wire.available() < 2){
    Serial.print(Wire.available());
    delay(100);
  }
  MSB = Wire.read();
  LSB = Wire.read();
  return (int) MSB<<8 | LSB;
}

void BMP180Calibration(){
  AC1 = BMP180ReadInt(0xAA);
  AC2 = BMP180ReadInt(0xAC);
  AC3 = BMP180ReadInt(0xAE);
  AC4 = BMP180ReadInt(0xB0);
  AC5 = BMP180ReadInt(0xB2);
  AC6 = BMP180ReadInt(0xB4);
  b1 = BMP180ReadInt(0xB6);
  B2 = BMP180ReadInt(0xB8);
  MB = BMP180ReadInt(0xBA);
  MC = BMP180ReadInt(0xBC);
  MD = BMP180ReadInt(0xBE);
}

float BMP180Temperature(){
  long X1, X2, UT;
  Wire.beginTransmission(BMP180_I2C_Adress);
  Wire.write(0xF4);
  Wire.write(0x2E);
  Wire.endTransmission();
  delayMicroseconds(4500);
  UT = BMP180ReadInt(0xF6);
  X1 = (((long)UT - (long)AC6)*(long)AC5) >> 15;
  X2 = ((long)MC << 11)/(X1 + MD);
  B5 = X1 + X2;
  T = ((B5 + 8)>>4);
  T *= 0.1;
  return T;
}

float BMP180Pressure(unsigned int precision){
  long X1, X2, X3, B3, B6, p;
  unsigned long B4, B7, UP;
  unsigned char MSB, LSB, XLSB;
  Wire.beginTransmission(BMP180_I2C_Adress);
  Wire.write(0xF4);
  Wire.write(0x34 + (precision<<6));
  Wire.endTransmission();
  if (precision == 0){
    delayMicroseconds(4500);
  }else if (precision == 1){
    delayMicroseconds(7500);
  }else if (precision == 2){
    delayMicroseconds(13500);
  }else if (precision == 3){
    delayMicroseconds(30000);
  }else{
    return 0;
  }
  Wire.beginTransmission(BMP180_I2C_Adress);
  Wire.write(0xF6);
  Wire.endTransmission();
  Wire.requestFrom(BMP180_I2C_Adress, 3);
  while(Wire.available() < 3){
    ;
  }
  MSB = Wire.read();
  LSB = Wire.read();
  XLSB = Wire.read();
  UP = (((unsigned long) MSB << 16) | ((unsigned long) LSB << 8) | (unsigned long) XLSB) >> (8-precision);
  B6 = B5 - 4000;
  X1 = (B2 * (B6 * B6)>>12)>>11;
  X2 = (AC2 * B6)>>11;
  X3 = X1 + X2;
  B3 = (((((long)AC1)*4 + X3)<<precision) + 2)>>2;
  X1 = (AC3 * B6)>>13;
  X2 = (B1 * ((B6 * B6)>>12))>>16;
  X3 = ((X1 + X2) + 2)>>2;
  B4 = (AC4 * (unsigned long)(X3 + 32768))>>15;
  B7 = ((unsigned long)(UP - B3) * (50000>>precision));
  if (B7 < 0x80000000){
    p = (B7<<1)/B4;
  }else{
    p = (B7/B4)<<1;
  }
  X1 = (p>>8)*(p>>8);
  X1 = (X1 * 3038)>>16;
  X2 = (-7357 * p)>>16;
  p += (X1 + X2 + 3791)>>4;
  return 0.01*p;
}

int BMP180Altitude(float p0){
  return 44330*(1-pow(BMP180Pressure(i)/p0, 1/5.255));
}