The BMP085 is a neat little sensor that measures pressure and temperature. It is pretty precise and you can buy a breakout board from SparkFun for $20.
I will not even try to summarize the BMP085 Datasheet - it is long and pretty boring. The chip returns uncalibrated data and it also returns the calibration factors. It is up to the user to do calibration math and it is really picky about 8/16/32 signed/unsigned ints. I tried to implement it and it did not work, I tried other people's code and it did not work until I found this great article by mitat.tuu.fi. I only had to change one int to long and it worked right away.
I will not show how to use the Bus Pirate with this sensor, because it is pretty useless here. This chip returns nothing of interest directly - you have to do the calibration.
All the code be downloaded from here.
// BMP08 with Arduino
// DANGER: The BMP08 accepts 1.8 to 3.6 Volts - so no chance to connect it directly to 5 Volts.
// Connect VCC to VCC and GND to GND, SCL goes to analogue pin 5, SDA to analogue pin4.
// Notice! Sparkfun breakoutboard contains already 4.7K pull ups,
// If not using pre-built pull-ups:
// --> Add some pull up resistors (1K to 20K, most often something like 4.7K) between SDA, SCL and VCC finishes the setup.
// References: http://interactive-matter.org/2009/12/arduino-barometric-pressure-sensor-bmp085/
// and http://jeelabs.org/2009/02/19/hooking-up-a-bmp085-sensor/
// Specification: http://www.bosch-sensortec.com/content/language1/downloads/BST-BMP085-DS000-05.pdf
// SparkFun breakout board: https://www.sparkfun.com/products/9694
#include "Wire.h"
#define I2C_ADDRESS 0x77
const unsigned char oversampling_setting = 3; //oversamplig for measurement
const unsigned char pressure_waittime[4] = { 5, 8, 14, 26 };
//just taken from the BMP085 datasheet
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;
void setup()
{
Serial.begin(9600); // start serial for output
Serial.println("Setting up BMP085");
Wire.begin();
bmp085_get_cal_data();
}
void bmp085_read_temperature_and_pressure(int& temperature, long& pressure);
void loop()
{
int temperature = 0;
long pressure = 0;
bmp085_read_temperature_and_pressure(&temperature,&pressure);
Serial.print(temperature / 10, DEC);
Serial.print(".");
Serial.print(temperature % 10, DEC);
Serial.print(",");
Serial.print(pressure / 100, DEC);
Serial.print(".");
Serial.print(pressure % 100, DEC);
delay(1000);
}
void bmp085_read_temperature_and_pressure(int* temperature, long* pressure) {
long ut = bmp085_read_ut();
long up = bmp085_read_up();
long x1, x2, x3, b3, b5, b6, p;
unsigned long b4, b7;
//calculate the temperature
x1 = ((long)ut - ac6) * ac5 >> 15;
x2 = ((long) mc << 11) / (x1 + md);
b5 = x1 + x2;
*temperature = (b5 + 8) >> 4;
//calculate the pressure
b6 = b5 - 4000;
x1 = (b2 * (b6 * b6 >> 12)) >> 11;
x2 = ac2 * b6 >> 11;
x3 = x1 + x2;
//b3 = (((int32_t) ac1 * 4 + x3)<> 2;
if (oversampling_setting == 3) b3 = ((int32_t) ac1 * 4 + x3 + 2) << 1;
if (oversampling_setting == 2) b3 = ((int32_t) ac1 * 4 + x3 + 2);
if (oversampling_setting == 1) b3 = ((int32_t) ac1 * 4 + x3 + 2) >> 1;
if (oversampling_setting == 0) b3 = ((int32_t) ac1 * 4 + x3 + 2) >> 2;
x1 = ac3 * b6 >> 13;
x2 = (b1 * (b6 * b6 >> 12)) >> 16;
x3 = ((x1 + x2) + 2) >> 2;
b4 = (ac4 * (uint32_t) (x3 + 32768)) >> 15;
b7 = ((uint32_t) up - b3) * (50000 >> oversampling_setting);
p = b7 < 0x80000000 ? (b7 * 2) / b4 : (b7 / b4) * 2;
x1 = (p >> 8) * (p >> 8);
x1 = (x1 * 3038) >> 16;
x2 = (-7357 * p) >> 16;
*pressure = p + ((x1 + x2 + 3791) >> 4);
}
unsigned int bmp085_read_ut() {
write_register(0xf4,0x2e);
delay(5); //longer than 4.5 ms
return read_int_register(0xf6);
}
void bmp085_get_cal_data() {
Serial.println("Reading Calibration Data");
ac1 = read_int_register(0xAA);
Serial.print("AC1: ");
Serial.println(ac1,DEC);
ac2 = read_int_register(0xAC);
Serial.print("AC2: ");
Serial.println(ac2,DEC);
ac3 = read_int_register(0xAE);
Serial.print("AC3: ");
Serial.println(ac3,DEC);
ac4 = read_int_register(0xB0);
Serial.print("AC4: ");
Serial.println(ac4,DEC);
ac5 = read_int_register(0xB2);
Serial.print("AC5: ");
Serial.println(ac5,DEC);
ac6 = read_int_register(0xB4);
Serial.print("AC6: ");
Serial.println(ac6,DEC);
b1 = read_int_register(0xB6);
Serial.print("B1: ");
Serial.println(b1,DEC);
b2 = read_int_register(0xB8);
Serial.print("B2: ");
Serial.println(b1,DEC);
mb = read_int_register(0xBA);
Serial.print("MB: ");
Serial.println(mb,DEC);
mc = read_int_register(0xBC);
Serial.print("MC: ");
Serial.println(mc,DEC);
md = read_int_register(0xBE);
Serial.print("MD: ");
Serial.println(md,DEC);
}
long bmp085_read_up() {
write_register(0xf4,0x34+(oversampling_setting<<6));
delay(pressure_waittime[oversampling_setting]);
unsigned char msb, lsb, xlsb;
Wire.beginTransmission(I2C_ADDRESS);
Wire.send(0xf6); // register to read
Wire.endTransmission();
Wire.requestFrom(I2C_ADDRESS, 3); // read a byte
while(!Wire.available()) {
// waiting
}
msb = Wire.receive();
while(!Wire.available()) {
// waiting
}
lsb |= Wire.receive();
while(!Wire.available()) {
// waiting
}
xlsb |= Wire.receive();
return (((long)msb<<16) | ((long)lsb<<8) | ((long)xlsb)) >>(8-oversampling_setting);
}
void write_register(unsigned char r, unsigned char v)
{
Wire.beginTransmission(I2C_ADDRESS);
Wire.send(r);
Wire.send(v);
Wire.endTransmission();
}
char read_register(unsigned char r)
{
unsigned char v;
Wire.beginTransmission(I2C_ADDRESS);
Wire.send(r); // register to read
Wire.endTransmission();
Wire.requestFrom(I2C_ADDRESS, 1); // read a byte
while(!Wire.available()) {
// waiting
}
v = Wire.receive();
return v;
}
int read_int_register(unsigned char r)
{
unsigned char msb, lsb;
Wire.beginTransmission(I2C_ADDRESS);
Wire.send(r); // register to read
Wire.endTransmission();
Wire.requestFrom(I2C_ADDRESS, 2); // read a byte
while(!Wire.available()) {
// waiting
}
msb = Wire.receive();
while(!Wire.available()) {
// waiting
}
lsb = Wire.receive();
return (((int)msb<<8) | ((int)lsb));
}