PDA

View Full Version : I2C barometer BMP085



freto
01-26-2013, 03:18 AM
Hello,
I am trying the I2C barometer Bosch BMP085. For a start I am using the breakout sold here (https://www.sparkfun.com/products/11282?) with pullups enabled. The code is so far an example which give correct values with an Arduino but different results with the Teensy3.0.

The values read for ac1, ac2, ac3, ac4, ac5, ac6, b1, b2, mb, mc, md are respectively 7238, 64390, 51024, 34479, 24530, 21740, 5498, 61, 32768, 54461, 2432 on the Teensy3.0 but 7238 -1146 -14512 34479 24530 21740 5498 61 -32768 -11075 2432 on the Arduino. I tried different combinations of int, unsigned char, etc... and also used Beta10 and Beta12 but no success so far.

Any idea is appreciated.

Here is the code I tried:

#include <Wire.h>

#define BMP085_ADDRESS 0x77 // I2C address of BMP085

const unsigned char OSS = 3; // 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;

void setup(){
Serial.begin(9600);

Wire.begin();
while (!Serial) {} // wait for Leonardo
bmp085Calibration();
}

void loop()
{
float temperature = bmp085GetTemperature(bmp085ReadUT()); //MUST be called first
float pressure = bmp085GetPressure(bmp085ReadUP());
float atm = pressure / 101325; // "standard atmosphere"
float altitude = calcAltitude(pressure); //Uncompensated caculation - in Meters

Serial.print("Temperature: ");
Serial.print(temperature, 2); //display 2 decimal places
Serial.println("deg C");

Serial.print("Pressure: ");
Serial.print(pressure, 0); //whole number only.
Serial.println(" Pa");

Serial.print("Standard Atmosphere: ");
Serial.println(atm, 4); //display 4 decimal places

Serial.print("Altitude: ");
Serial.print(altitude, 2); //display 2 decimal places
Serial.println(" M");

Serial.println();//line break

delay(1000); //wait a second and get values again.
}

// 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);
Serial.print(ac1);Serial.print(" ");
ac2 = bmp085ReadInt(0xAC);
Serial.print(ac2);Serial.print(" ");
ac3 = bmp085ReadInt(0xAE);
Serial.print(ac3);Serial.print(" ");
ac4 = bmp085ReadInt(0xB0);
Serial.print(ac4);Serial.print(" ");
ac5 = bmp085ReadInt(0xB2);
Serial.print(ac5);Serial.print(" ");
ac6 = bmp085ReadInt(0xB4);
Serial.print(ac6);Serial.print(" ");
b1 = bmp085ReadInt(0xB6);
Serial.print(b1);Serial.print(" ");
b2 = bmp085ReadInt(0xB8);
Serial.print(b2);Serial.print(" ");
mb = bmp085ReadInt(0xBA);
Serial.print(mb);Serial.print(" ");
mc = bmp085ReadInt(0xBC);
Serial.print(mc);Serial.print(" ");
md = bmp085ReadInt(0xBE);
Serial.print(md);Serial.println();
}

// Calculate temperature in deg C
float bmp085GetTemperature(unsigned int ut){
long x1, x2;

x1 = (((long)ut - (long)ac6)*(long)ac5) >> 15;
x2 = ((long)mc << 11)/(x1 + md);
b5 = x1 + x2;

float temp = ((b5 + 8)>>4);
temp = temp /10;

return temp;
}

// 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;
else
p = (b7/b4)<<1;

x1 = (p>>8) * (p>>8);
x1 = (x1 * 3038)>>16;
x2 = (-7357 * p)>>16;
p += (x1 + x2 + 3791)>>4;

long temp = p;
return temp;
}

// Read 1 byte from the BMP085 at 'address'
char bmp085Read(unsigned char address)
{
unsigned char data;

Wire.beginTransmission(BMP085_ADDRESS);
Wire.write(address);
Wire.endTransmission();

Wire.requestFrom(BMP085_ADDRESS, 1);
while(!Wire.available())
;

return Wire.read();
}

// 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.beginTransmission(BMP085_ADDRESS);
Wire.write(address);
Wire.endTransmission();

Wire.requestFrom(BMP085_ADDRESS, 2);
while(Wire.available()<2)
;
msb = Wire.read();
lsb = Wire.read();

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
Wire.beginTransmission(BMP085_ADDRESS);
Wire.write(0xF4);
Wire.write(0x2E);
Wire.endTransmission();

// Wait at least 4.5ms
delay(5);

// 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.beginTransmission(BMP085_ADDRESS);
Wire.write(0xF4);
Wire.write(0x34 + (OSS<<6));
Wire.endTransmission();

// Wait for conversion, delay time dependent on OSS
delay(2 + (3<<OSS));

// Read register 0xF6 (MSB), 0xF7 (LSB), and 0xF8 (XLSB)
msb = bmp085Read(0xF6);
lsb = bmp085Read(0xF7);
xlsb = bmp085Read(0xF8);

up = (((unsigned long) msb << 16) | ((unsigned long) lsb << 8) | (unsigned long) xlsb) >> (8-OSS);

return up;
}

void writeRegister(int deviceAddress, byte address, byte val) {
Wire.beginTransmission(deviceAddress); // start transmission to device
Wire.write(address); // send register address
Wire.write(val); // send value to write
Wire.endTransmission(); // end transmission
}

int readRegister(int deviceAddress, byte address){

int v;
Wire.beginTransmission(deviceAddress);
Wire.write(address); // register to read
Wire.endTransmission();

Wire.requestFrom(deviceAddress, 1); // read a byte

while(!Wire.available()) {
// waiting
}

v = Wire.read();
return v;
}

float calcAltitude(float pressure){

float A = pressure/101325;
float B = 1/5.25588;
float C = pow(A,B);
C = 1 - C;
C = C /0.0000225577;

return C;
}

Nantonos
01-26-2013, 07:58 PM
Looks like 16-bit two's complement (http://en.wikipedia.org/wiki/Two%27s_complement) values being misinterpreted as 32-bit values. So the last bit is treated as a sign bit on Arduino, but not on Teensy. Change your code to use int16_t instead of int, to get the same values on Teensy 3.0 and AVR-based Arduino.

freto
01-27-2013, 12:40 AM
Thanks a lot Nantonos, the problem is solved with your advice.
Merci.

Nantonos
01-27-2013, 01:14 AM
And change unsigned int to uint16_t.

PaulStoffregen
01-27-2013, 06:19 PM
Has anyone tried to send these fixes back to Sparkfun?

I'm sure these same problems with happen on any other 32 bit boards, so they really should have some incentive to fix their code.