Reading data from a Bosch BMP085 with a Raspberry Pi

Update(2): I’ve fixed some reported bugs in this code. Please use the newer version here.

Update: If the LM Sensors website is still down, you can get smbus.c and smbus.h from here.

I needed a way to measure air pressure as part of my Raspberry Pi controlled weather station.

I decided to use the Bosch BMP085 as it is very sensitive (down to 0.03hPa, or 3Pa) and SparkFun Electronics offer it already soldered to a break out board making it relatively easy to interface.

The breakout board includes pull up resistors on the Data and Clock lines, so it’s a simple four wire connection to the Raspberry Pi.

Breakout Board pin Raspberry Po GPIO Pin
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

I had real trouble talking to the sensor using the standard file write and file read commands. I was having to do multiple reads to get usable data, but I noticed that the i2cget and i2cset commands worked perfectly every time.

I took a look at the i2cget and i2cset source code and noticed it was using smbus to talk to the sensor. A little further delving and I had a working solution.

Click here for the source (testBMP085.c).

This code is a derivative of the Arduino code by Jim Lindblom and is released under the same “CC BY-SA v3.0” license.

You can find smbus.c and smbus.h at and – You will probably want to edit smbus.c to change the path to smbus.h.

Raspberry Pi Bosch BMP085 communication code.
By:      John Burns (
Date:    01 August 2012
License: CC BY-SA v3.0 -

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 -

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

Circuit detail:
	Using a Spark Fun Barometric Pressure Sensor - BMP085 breakout board
	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);

	return up;

// Calculate pressure given uncalibrated pressure
// Value returned will be in units of XXXXX
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. Hi! Your links for smbus.c & smbus.h are dead, ive search for libraries on inet but no luck. Can U pls share them somewhere or just send them to my mail…? I need them bcouse i would like use bmp085 with php web…

  2. Hello. Do you know where I can find smbus.c/smbus.h now? Lm-sensors site is unfortunately completely empty :/ Or maybe You still have those files and You will be so kind do send me them?

  3. Hi,
    I have version 2 of rpi (512MB) and I needed to change I2C bus in code from
    char *fileName = “/dev/i2c-0”;
    char *fileName = “/dev/i2c-1”;
    to make it work. Beside that it works great.


  4. Hey, thanks for the code, works far better than the python code I got from Adafruit, not sure why such a difference, but your C code produces temperature results far better.

    Any code to calculate altitude based on the pressure readings? I’ve tried a couple of things, but get spurious results 🙂

  5. Thanks for the code. I spotted a problem after modifying the main routine to take a sample a minute and output the results to a log file. The process was mysteriously dying after just over 17 hours (fully repeatable). After some investigation and noting that my log files contained exactly 1022 samples, I found the reason. The process was running out of file descriptors. The bmp085_ReadUP function omits to “close (fd);” just before returning from the function.

    This doesn’t matter if you compile the code as supplied, as you call each function once. However, if you modify it to sit in a loop, each reading opens another channel to /dev/i2c-0 and you gradually use up file descriptors until you run out and fall over.

    Once I’d fixed that issue, the code worked great.

  6. Thanks for the code. Works perfectly.

    I tried the Adafruit python code and for some reason it kept giving a error. Your code works first time.

  7. I get

    gcc -Wall -o testBMP085 ./smbus.c ./testBMP085.c
    ./smbus.c: In function ‘i2c_smbus_access’:
    ./smbus.c:47:30: error: storage size of ‘args’ isn’t known
    ./smbus.c:55:20: error: ‘I2C_SMBUS’ undeclared (first use in this function)
    ./smbus.c:55:20: note: each undeclared identifier is reported only once for each function it appears in
    ./smbus.c:47:30: warning: unused variable ‘args’ [-Wunused-variable]
    ./testBMP085.c: In function ‘bmp085_i2c_Begin’:
    ./testBMP085.c:78:16: error: ‘I2C_SLAVE’ undeclared (first use in this function)
    ./testBMP085.c:78:16: note: each undeclared identifier is reported only once for each function it appears in
    distcc[6315] ERROR: compile ./smbus.c on localhost failed
    make: *** [all] Fehler 1

  8. Hi, I started of using your code but kept getting corrupted results, about 5-15 per 6 hour period while sampling every 5 minutes. I think it was due to overclocking messing with ‘usleep’. So I re-wrote the code to do away with SMBUS and added a polling feature to check if the conversion is complete.
    code can be found here: and HTU21D-F
    I started testing this morning with the pi running on a 12v battery and lighter adapter USB PSU and ‘turbo’ overclock enabled, so far so good.


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