SparkFun Forums 

Where electronics enthusiasts find answers.

For the discussion of Arduino related topics.
I have some older code that I am trying to get the correct temperature from. I have the sensor hooked up correctly and seems to be a calibration issue. I run other code and it seems ok. The reason why I would like this code and calibrations to read correctly is that I have some other code that I am using that has been extended for reading Air Density and would like to fix the issue in both sets of code.

Once again Temperature is in Celcius and is way high.
Code: Select all
/* BMP085 & BMP180 Extended Example Code
  by: Jim Lindblom
  SparkFun Electronics
  date: 1/18/11
  license: CC BY-SA v3.0 -
  Get pressure and temperature from the BMP085 and calculate altitude.
  Serial.print it out at 9600 baud to serial monitor.
  Update (7/19/11): I've heard folks may be encountering issues
  with this code, who're running an Arduino at 8MHz. If you're 
  using an Arduino Pro 3.3V/8MHz, or the like, you may need to 
  increase some of the delays in the bmp085ReadUP and 
  bmp085ReadUT functions.

#include <Wire.h>

#define BMP085_ADDRESS 0x77  // I2C address of BMP085

const unsigned char OSS = 0;  // Oversampling Setting

// Calibration values
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;

// b5 is calculated in bmp085GetTemperature(...), this variable is also used in bmp085GetPressure(...)
// so ...Temperature(...) must be called before ...Pressure(...).
long b5; 

short temp;
short temperature;
long pressure;

void setup()

void loop()
  temp = bmp085GetTemperature(bmp085ReadUT()); // sopposed to be temperature
  pressure = bmp085GetPressure(bmp085ReadUP());
  Serial.print("Temperature: ");
  Serial.print(temp, DEC); // sopposed to be temperature
  Serial.println(" *0.1 deg C");
  Serial.print("Pressure: ");
  Serial.print(pressure, DEC);
  Serial.println(" Pa");

// Stores all of the bmp085's calibration values into global variables
// Calibration values are required to calculate temp and pressure
// This function should be called at the beginning of the program
void bmp085Calibration()
  ac1 = bmp085ReadInt(0xAA);
  ac2 = bmp085ReadInt(0xAC);
  ac3 = bmp085ReadInt(0xAE);
  ac4 = bmp085ReadInt(0xB0);
  ac5 = bmp085ReadInt(0xB2);
  ac6 = bmp085ReadInt(0xB4);
  b1 = bmp085ReadInt(0xB6);
  b2 = bmp085ReadInt(0xB8);
  mb = bmp085ReadInt(0xBA);
  mc = bmp085ReadInt(0xBC);
  md = bmp085ReadInt(0xBE);

// Calculate temperature given ut.
// Value returned will be in units of 0.1 deg C
short bmp085GetTemperature(unsigned int ut)
  long x1, x2;
  x1 = (((long)ut - (long)ac6)*(long)ac5) >> 15;
  x2 = ((long)mc << 11)/(x1 + md);
  b5 = x1 + x2;

  return ((b5 + 8)>>4);  

// Calculate pressure given up
// calibration values must be known
// b5 is also required so bmp085GetTemperature(...) must be called first.
// Value returned will be pressure in units of Pa.
long bmp085GetPressure(unsigned long up)
  long x1, x2, x3, b3, b6, p;
  unsigned long b4, b7;
  b6 = b5 - 4000;
  // Calculate B3
  x1 = (b2 * (b6 * b6)>>12)>>11;
  x2 = (ac2 * b6)>>11;
  x3 = x1 + x2;
  b3 = (((((long)ac1)*4 + x3)<<OSS) + 2)>>2;
  // Calculate B4
  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>>OSS));
  if (b7 < 0x80000000)
    p = (b7<<1)/b4;
    p = (b7/b4)<<1;
  x1 = (p>>8) * (p>>8);
  x1 = (x1 * 3038)>>16;
  x2 = (-7357 * p)>>16;
  p += (x1 + x2 + 3791)>>4;
  return p;

// Read 1 byte from the BMP085 at 'address'
char bmp085Read(unsigned char address)
  unsigned char data;
  Wire.requestFrom(BMP085_ADDRESS, 1);

// Read 2 bytes from the BMP085
// First byte will be from 'address'
// Second byte will be from 'address'+1
int bmp085ReadInt(unsigned char address)
  unsigned char msb, lsb;
  Wire.requestFrom(BMP085_ADDRESS, 2);
  msb =;
  lsb =;
  return (int) msb<<8 | lsb;

// Read the uncompensated temperature value
unsigned int bmp085ReadUT()
  unsigned int ut;
  // Write 0x2E into Register 0xF4
  // This requests a temperature reading
  // Wait at least 4.5ms
  // Read two bytes from registers 0xF6 and 0xF7
  ut = bmp085ReadInt(0xF6);
  return ut;

// Read the uncompensated pressure value
unsigned long bmp085ReadUP()
  unsigned char msb, lsb, xlsb;
  unsigned long up = 0;
  // Write 0x34+(OSS<<6) into register 0xF4
  // Request a pressure reading w/ oversampling setting
  Wire.write(0x34 + (OSS<<6));
  // Wait for conversion, delay time dependent on OSS
  delay(2 + (3<<OSS));
  // Read register 0xF6 (MSB), 0xF7 (LSB), and 0xF8 (XLSB)
  Wire.requestFrom(BMP085_ADDRESS, 3);
  // Wait for data to become available
  while(Wire.available() < 3)
  msb =;
  lsb =;
  xlsb =;
  up = (((unsigned long) msb << 16) | ((unsigned long) lsb << 8) | (unsigned long) xlsb) >> (8-OSS);
  return up;

Hi Kevin,

I do not have an BMP085/BMP180, but looking at your code, there a couple if things that I would change and could help. A computer program will do what you ask, and thus you have to be very precise and leave NO room for expected / assumed behavior :

1. Get the brackets clear to make sure you get the correct calculation. Like change : X1 = ((long)ut - (long)ac6)*(long)ac5) >> 15 to X1 = (long) ((ut - c6)* (ac5 >> 15)) Now it MIGHT result in the same, but I am speaking from experience that I have lost a LOT of time on that kind of assumption. :-)

2. I am surprised about the return in bmp085GetTemperature(). The fact that the routine is defined as returning a short, does not make the calculation of B5 'short'. Not sure why you have defined temp as short instead of long, as you lose precision. Still if you want to do that, then you need to do type casting : return ( (short) (b5 + 8)>>4); even then I am not convinced if would provide the correct results (depending on the number of bits used in B5).

3. in the bmp085GetPressure() apart from the brackets (see point 1), I sometimes see a type casting ( b3 = (((((long)ac1)*4 + x3)<<OSS) + 2)>>2;) and sometimes not ( x2 = (ac2 * b6)>>11;). In the later it could read 'X2 = ((long) ac2 * (b6 >> 11) '.

To debug your code, use the "fake-number" as in the data sheet for DMPP085, pag 13 as input/result. It will help to make sure your calculation method is correct.