Update – Bosch BMP085 Source (Raspberry Pi)

I have updated my code to Read data from a Bosch BMP085 with a Raspberry Pi to correct some bugs reported back to me.

The main bug was that I’d forgotten to close the i2c file at the end of bmp085_ReadUP() – This shouldn’t have caused any problems if you were calling the function once per execution, but if calling it multiple times, it may crash. On the same note, if you are calling the functions multiple times, you may want to move the opening and closing of the i2c file outside of the functions so the files aren’t opened and closed multiple times. Thanks to Radu P for reporting these issues.

It looks like lm-sensors.org is back up now, but if not, you can find a locally hosted copy of smbus.c and smbus.h in this earlier blog post.

Note that I’ve written a number of posts on using this sensor. Here is a link to all posts on the topic.

Click here for the updated source (testBMP085.c).


Raspberry Pi Bosch BMP085 communication code.
By:      John Burns (www.john.geek.nz)
Date:    13 February 2013
License: CC BY-SA v3.0 - http://creativecommons.org/licenses/by-sa/3.0/

This is a derivative work based on:
	BMP085 Extended Example Code
	by: Jim Lindblom
	SparkFun Electronics
	date: 1/18/11
	license: CC BY-SA v3.0 - http://creativecommons.org/licenses/by-sa/3.0/
	Source: http://www.sparkfun.com/tutorial/Barometric/BMP085_Example_Code.pde

Compile with: gcc -Wall -o testBMP085 ./smbus.c ./testBMP085.c

Circuit detail:
	Using a Spark Fun Barometric Pressure Sensor - BMP085 breakout board
	link: https://www.sparkfun.com/products/9694
	This comes with pull up resistors already on the i2c lines.
	BMP085 pins below are as marked on the Sparkfun BMP085 Breakout board

	SDA	- 	P1-03 / IC20-SDA
	SCL	- 	P1-05 / IC20_SCL
	XCLR	- 	Not Connected
	EOC	-	Not Connected
	GND	-	P1-06 / GND
	VCC	- 	P1-01 / 3.3V
	Note: Make sure you use P1-01 / 3.3V NOT the 5V pin.

#include "smbus.h" 

#define BMP085_I2C_ADDRESS 0x77

const unsigned char BMP085_OVERSAMPLING_SETTING = 3;

// Calibration values - These are stored in the BMP085
short int ac1;
short int ac2; 
short int ac3; 
unsigned short int ac4;
unsigned short int ac5;
unsigned short int ac6;
short int b1; 
short int b2;
short int mb;
short int mc;
short int md;

int b5; 

unsigned int temperature, pressure;

// Open a connection to the bmp085
// Returns a file id
int bmp085_i2c_Begin()
	int fd;
	char *fileName = "/dev/i2c-0";
	// Open port for reading and writing
	if ((fd = open(fileName, O_RDWR)) < 0)
	// Set the port options and set the address of the device
	if (ioctl(fd, I2C_SLAVE, BMP085_I2C_ADDRESS) < 0) {					

	return fd;

// Read two words from the BMP085 and supply it as a 16 bit integer
__s32 bmp085_i2c_Read_Int(int fd, __u8 address)
	__s32 res = i2c_smbus_read_word_data(fd, address);
	if (res < 0) {

	// Convert result to 16 bits and swap bytes
	res = ((res<<8) & 0xFF00) | ((res>>8) & 0xFF);

	return res;

//Write a byte to the BMP085
void bmp085_i2c_Write_Byte(int fd, __u8 address, __u8 value)
	if (i2c_smbus_write_byte_data(fd, address, value) < 0) {

// Read a block of data BMP085
void bmp085_i2c_Read_Block(int fd, __u8 address, __u8 length, __u8 *values)
	if(i2c_smbus_read_i2c_block_data(fd, address,length,values)<0) {

void bmp085_Calibration()
	int fd = bmp085_i2c_Begin();
	ac1 = bmp085_i2c_Read_Int(fd,0xAA);
	ac2 = bmp085_i2c_Read_Int(fd,0xAC);
	ac3 = bmp085_i2c_Read_Int(fd,0xAE);
	ac4 = bmp085_i2c_Read_Int(fd,0xB0);
	ac5 = bmp085_i2c_Read_Int(fd,0xB2);
	ac6 = bmp085_i2c_Read_Int(fd,0xB4);
	b1 = bmp085_i2c_Read_Int(fd,0xB6);
	b2 = bmp085_i2c_Read_Int(fd,0xB8);
	mb = bmp085_i2c_Read_Int(fd,0xBA);
	mc = bmp085_i2c_Read_Int(fd,0xBC);
	md = bmp085_i2c_Read_Int(fd,0xBE);

// Read the uncompensated temperature value
unsigned int bmp085_ReadUT()
	unsigned int ut = 0;
	int fd = bmp085_i2c_Begin();

	// Write 0x2E into Register 0xF4
	// This requests a temperature reading
	// Wait at least 4.5ms

	// Read the two byte result from address 0xF6
	ut = bmp085_i2c_Read_Int(fd,0xF6);

	// Close the i2c file
	close (fd);
	return ut;

// Read the uncompensated pressure value
unsigned int bmp085_ReadUP()
	unsigned int up = 0;
	int fd = bmp085_i2c_Begin();

	// Write 0x34+(BMP085_OVERSAMPLING_SETTING<<6) into register 0xF4
	// Request a pressure reading w/ oversampling setting
	bmp085_i2c_Write_Byte(fd,0xF4,0x34 + (BMP085_OVERSAMPLING_SETTING<<6));

	// Wait for conversion, delay time dependent on oversampling setting
	usleep((2 + (3<> (8-BMP085_OVERSAMPLING_SETTING);

	// Close the i2c file
	close (fd);
	return up;

// Calculate pressure given uncalibrated pressure
// Value returned will be in units of Pa
unsigned int bmp085_GetPressure(unsigned int up)
	int x1, x2, x3, b3, b6, p;
	unsigned int b4, b7;
	b6 = b5 - 4000;
	// Calculate B3
	x1 = (b2 * (b6 * b6)>>12)>>11;
	x2 = (ac2 * b6)>>11;
	x3 = x1 + x2;
	b3 = (((((int)ac1)*4 + x3)<>2;
	// Calculate B4
	x1 = (ac3 * b6)>>13;
	x2 = (b1 * ((b6 * b6)>>12))>>16;
	x3 = ((x1 + x2) + 2)>>2;
	b4 = (ac4 * (unsigned int)(x3 + 32768))>>15;
	b7 = ((unsigned int)(up - b3) * (50000>>BMP085_OVERSAMPLING_SETTING));
	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;

// Calculate temperature given uncalibrated temperature
// Value returned will be in units of 0.1 deg C
unsigned int bmp085_GetTemperature(unsigned int ut)
	int x1, x2;
	x1 = (((int)ut - (int)ac6)*(int)ac5) >> 15;
	x2 = ((int)mc << 11)/(x1 + md);
	b5 = x1 + x2;

	unsigned int result = ((b5 + 8)>>4);  

	return result;

int main(int argc, char **argv)
	temperature = bmp085_GetTemperature(bmp085_ReadUT());
	pressure = bmp085_GetPressure(bmp085_ReadUP());
	printf("Temperature\t%0.1f%cC\n", ((double)temperature)/10,0x00B0);
	printf("Pressure\t%0.2fhPa\n", ((double)pressure)/100);
	return 0;

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About the Author: John


  1. Hello,
    Thank you for your good work!
    I just want to make single note for Raspberry Pi Model B users:
    – Change the bus number from 0 (zero) to 1 (one) in line number 71!

    for model A users: char *fileName = “/dev/i2c-0”;
    for model B users: char *fileName = “/dev/i2c-1”;

    Best regards,

  2. Hello John-san
    Thank you for a good sample code!!

    Tokyo became below the freezing point this morning.
    And I wrote a patch for below the freezing point.

    — readBMP085.c.org 2013-09-16 15:20:10.000000000 +0900
    +++ readBMP085.c 2014-01-07 11:06:50.877884260 +0900
    @@ -62,7 +62,8 @@

    int b5;

    -unsigned int temperature, pressure, altitude;
    +int temperature;
    +unsigned int pressure, altitude;

    // Open a connection to the bmp085
    @@ -137,9 +138,9 @@

    // Read the uncompensated temperature value
    -unsigned int bmp085_ReadUT()
    +int bmp085_ReadUT()
    – unsigned int ut = 0;
    + int ut = 0;
    int fd = bmp085_i2c_Begin();

    // Write 0x2E into Register 0xF4
    @@ -217,7 +218,7 @@

    // Calculate temperature given uncalibrated temperature
    // Value returned will be in units of 0.1 deg C
    -unsigned int bmp085_GetTemperature(unsigned int ut)
    +int bmp085_GetTemperature(int ut)
    int x1, x2;

  3. Code is working perfectly, I am using Raspberry Pi Model B and as other users I have the bus number from 0 (zero) to 1 (one) in line number 71!

    Will you release software with minus temperature corrected ?

    Thanks for this good job


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