BMP085 sensor with BabyO 328

This is my second project using a BabyO (first one went very well, love the little guys), and I’m trying to use the BMP085 barometric sensor as an altimeter for a rocketry project. However, I’m having some difficulty with the I2C communication. I’ve found the “I2C master routines for the BabyO” forum post, but am unsure how to adapt it for reading from the BMP085. Any help would be greatly appreciated!


On the resources page for the BMP085, there is a datasheet which has information about the I2C protocol. There is also sample ATmega328 (the microcontroller on your Baby Orangutan) code written by SparkFun. It looks like the code defines printf (via put_char) to print to the serial port.

- Ryan

Thanks for the reply. I’ve now more thoroughly looked through the datasheet and example code, but am still running into some problems. I’m using a very lightly edited version of the BMP085_example_code from sparkfun’s website–I changed the value of FOSC to 2000000, since my babyO runs at 20MHz (also changed an instance of F_CPU to 20000000 in i2c.h, which was previously giving a “first use of” error), and commented out all of the printf calls, since I’m not hooking the babyo up to a serial port, I’m simply trying to read the pressure and do some on-board calculations with it.
A couple of things:
First, the red LED of the babyO turns on and stays on, despite the fact that I have a red_led(LOW) call at the beginning of my main(), and no red_led(HIGH) anywhere.

Second, when I put in delay_ms() calls they seem to pass much faster than they should (around 20x faster). This is not the case with simple test codes I’ve tried on the same babyO.

Third, and most confusingly, although the bmp085convert() function returns a reasonable-sounding value for pressure (about 10% higher than reports), it is always exactly the same (110406), regardless of start-up location or condition. The same is true for temperature–386, or 38.6 degrees Celsius, is always the reading. This again is a bit high (but frustratingly in the correct range, at least), as its currently about 22 degrees Celsius here.

I’m stumped so far.

(also) Ryan

here’s the code I’m using (more or less straight from the website you suggested):

    BMP085 Test Code
	April 7, 2010
	by: Jim Lindblom (with extremely minor modifications for testing)
	Test code for the BMP085 Barometric Pressure Sensor.
	We'll first read all the calibration values from the sensor.
	Then the pressure and temperature readings will be read and calculated.
	Also attempts to calculate altitude (remove comments)
	The sensor is run in ultra low power mode.
	Tested on a 3.3V 8MHz Arduino Pro
	A4 (PC4) -> SDA
	A5 (PC5) -> SCL
	No Connection to EOC or XCLR pins

#include <stdlib.h>
#include <stdio.h>
#include <avr/io.h>
#include <avr/pgmspace.h>
#include <avr/interrupt.h>
#include "types.h"
#include "defs.h"
#include "math.h"	// To calculate altitude
#include "i2c.h"
#include <pololu/orangutan.h>

#define FOSC 20000000 //changed this value from original 8000000
#define BAUD 9600
#define BMP085_R 0xEF
#define BMP085_W 0xEE
#define OSS 0	// Oversampling Setting (note: code is not set up to use other OSS values)

#define sbi(var, mask)   ((var) |= (uint8_t)(1 << mask))
#define cbi(var, mask)   ((var) &= (uint8_t)~(1 << mask))

///============Function Prototypes=========/////////////////
void BMP085_Calibration(void);

///============I2C Prototypes=============//////////////////
short bmp085ReadShort(unsigned char address);
long bmp085ReadTemp(void);
long bmp085ReadPressure(void);
void bmp085Convert(long * temperature, long * pressure);

///============Initialize Prototypes=====//////////////////
void ioinit(void);
void UART_Init(unsigned int ubrr);
static int uart_putchar(char c, FILE *stream);
void put_cahar(unsigned char byte);
static FILE mystdout = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);
void delay_ms(uint16_t x);

/////=========Global Variables======////////////////////
short ac1;
short ac2; 
short ac3; 
unsigned short ac4;
unsigned short ac5;
unsigned short ac6;
short b1; 
short b2;
short mb;
short mc;
short md;

int run_num = 1;
int first_temp;
int first_pressure;

int main(void)
	long temperature = 0;
	long pressure = 0;
	//long altitude = 0;
	//double temp = 0;

	long test_value = 110406;


		bmp085Convert(&temperature, &pressure);

		//if(run_num == 1) {
			//first_temp = temperature;
			//first_pressure = pressure;

		if (pressure == test_value) {  //this case, which blinks a connected LED, is always satisfied.
			set_digital_output(IO_B0, HIGH);
			delay_ms(20000); //this lasts about 1 second
			set_digital_output(IO_B0, LOW);
			set_digital_output(IO_B0, HIGH);

		else set_digital_output(IO_B0, LOW);

		if (pressure > test_value) {
			set_digital_output(IO_B1, HIGH);

		else set_digital_output(IO_B1, LOW);

		//printf("Temperature: %ld (in 0.1 deg C)\n", temperature);
		//printf("Pressure: %ld Pa\n\n", pressure);
		// For fun, lets convert to altitude
		/*temp = (double) pressure/101325;
		temp = 1-pow(temp, 0.19029);
		altitude = round(44330*temp);
		printf("Altitude: %ldm\n\n", altitude);*/
		run_num = run_num+1;


void BMP085_Calibration(void)
//	printf("\nCalibration Information:\n");
//	printf("------------------------\n");
	ac1 = bmp085ReadShort(0xAA);
	ac2 = bmp085ReadShort(0xAC);
	ac3 = bmp085ReadShort(0xAE);
	ac4 = bmp085ReadShort(0xB0);
	ac5 = bmp085ReadShort(0xB2);
	ac6 = bmp085ReadShort(0xB4);
	b1 = bmp085ReadShort(0xB6);
	b2 = bmp085ReadShort(0xB8);
	mb = bmp085ReadShort(0xBA);
	mc = bmp085ReadShort(0xBC);
	md = bmp085ReadShort(0xBE);
/*	printf("\tAC1 = %d\n", ac1);
	printf("\tAC2 = %d\n", ac2);
	printf("\tAC3 = %d\n", ac3);
	printf("\tAC4 = %d\n", ac4);
	printf("\tAC5 = %d\n", ac5);
	printf("\tAC6 = %d\n", ac6);
	printf("\tB1 = %d\n", b1);
	printf("\tB2 = %d\n", b2);
	printf("\tMB = %d\n", mb);
	printf("\tMC = %d\n", mc);
	printf("\tMD = %d\n", md);

// bmp085ReadShort will read two sequential 8-bit registers, and return a 16-bit value
// the MSB register is read first
// Input: First register to read
// Output: 16-bit value of (first register value << 8) | (sequential register value)
short bmp085ReadShort(unsigned char address)
	char msb, lsb;
	short data;
	i2cSendByte(BMP085_W);	// write 0xEE
	i2cSendByte(address);	// write register address
	i2cSendByte(BMP085_R);	// write 0xEF
	msb = i2cGetReceivedByte();	// Get MSB result
	lsb = i2cGetReceivedByte();	// Get LSB result
	data = msb << 8;
	data |= lsb;
	return data;

long bmp085ReadTemp(void)
	i2cSendByte(BMP085_W);	// write 0xEE
	i2cSendByte(0xF4);	// write register address
	i2cSendByte(0x2E);	// write register data for temp
	delay_ms(10);	// max time is 4.5ms
	return (long) bmp085ReadShort(0xF6);

long bmp085ReadPressure(void)
	long pressure = 0;
	i2cSendByte(BMP085_W);	// write 0xEE
	i2cSendByte(0xF4);	// write register address
	i2cSendByte(0x34);	// write register data for temp
	delay_ms(10);	// max time is 4.5ms
	pressure = bmp085ReadShort(0xF6);
	pressure &= 0x0000FFFF;
	return pressure;
	//return (long) bmp085ReadShort(0xF6);

void bmp085Convert(long* temperature, long* pressure)
	long ut;
	long up;
	long x1, x2, b5, b6, x3, b3, p;
	unsigned long b4, b7;
	ut = bmp085ReadTemp();
	ut = bmp085ReadTemp();	// some bug here, have to read twice to get good data
	up = bmp085ReadPressure();
	up = bmp085ReadPressure();
	x1 = ((long)ut - ac6) * ac5 >> 15;
	x2 = ((long) mc << 11) / (x1 + md);
	b5 = x1 + x2;
	*temperature = (b5 + 8) >> 4;
	b6 = b5 - 4000;
	x1 = (b2 * (b6 * b6 >> 12)) >> 11;
	x2 = ac2 * b6 >> 11;
	x3 = x1 + x2;
	b3 = (((int32_t) ac1 * 4 + x3) + 2)/4;
	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);
	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);

void ioinit (void)
    //1 = output, 0 = input
	DDRB = 0b01100000; //PORTB4, B5 output
    DDRC = 0b00010000; //PORTC4 (SDA), PORTC5 (SCL), PORTC all others are inputs
    DDRD = 0b11111110; //PORTD (RX on PD0), PD2 is status output
	PORTC = 0b00110000; //pullups on the I2C bus
	UART_Init((unsigned int)(FOSC/16/BAUD-1));		// ocillator fq/16/baud rate -1	

void UART_Init( unsigned int ubrr)
	// Set baud rate 
	UBRR0H = ubrr>>8;
	UBRR0L = ubrr;
	// Enable receiver and transmitter 
	UCSR0A = (0<<U2X0);
	UCSR0B = (1<<RXEN0)|(1<<TXEN0);
	// Set frame format: 8 bit, no parity, 1 stop bit,   
	UCSR0C = (1<<UCSZ00)|(1<<UCSZ01);
	stdout = &mystdout; //Required for printf init

static int uart_putchar(char c, FILE *stream)
    if (c == '\n') uart_putchar('\r', stream);
    loop_until_bit_is_set(UCSR0A, UDRE0);
    UDR0 = c;
    return 0;

void put_char(unsigned char byte)
	/* Wait for empty transmit buffer */
	while ( !( UCSR0A & (1<<UDRE0)) );
	/* Put data into buffer, sends the data */
	UDR0 = byte;

Success!! Sorry for the double post, but I figured (almost) everything out.

  1. the red led is still stubbornly on (not in other test programs). I can live with that.

  2. the i2c.h file was defining its own delay_ms routine, which was being used instead of the one in orangutan.h, so I simply commented out that function definition

  3. I had somehow managed to wire the sensor wrong, flipping SCL and SDA. So I guess I was always reading the last buffered data value, or something. No Idea.

But the BabyO now responds appropriately to changes in altitude and temperature, so I call that a victory.



I’m glad to hear that you got things working. I only briefly scanned your code, but it looks like you are enabling the Baby Orangutan’s UART module. If so, this would explain why the red LED is on: the red LED is on pin PD1, which is also the UART TX pin, and when the UART is enabled, the TX pin is an output with an idle state of high.

- Ben