... The Other Things: 2015

Some quick notes for connecting Honeywell's 3-Axis Digital Compass IC
HMC5883L to the I2C bus on a Raspbrry Pi board.

HMC5883L breakout board from SparkFun:
https://www.sparkfun.com/products/10530

Connections

GND -> GND (pin 6 on RPi I/O header)
VCC -> 3.3 V (pin 1 on RPi )
SDA -> SDA1 I2C (pin 3 on RPi)
SCL -> SCL1 I2C (pin 5 on RPi)

Configuration

Enable I2C using "raspi-config". Add user to i2c group.

pi@raspberrypi ~ $ i2cdetect 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 1e --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Run

See below for example C code.

pi@raspberrypi ~ $ gcc magnetometer.c -o magnetometer -lm

pi@raspberrypi ~ $ ./magnetometer
Identification: 'H43' HMC5883L sensor detected
1434246270.000000, -548, -649, 349, 67030.4 nT
1434246270.100000, -547, -650, 349, 67038.5 nT
1434246270.200000, -548, -648, 348, 66951.1 nT
1434246270.300000, -548, -650, 350, 67109.8 nT
1434246270.400000, -548, -650, 350, 67109.8 nT
1434246270.500000, -547, -650, 350, 67066.3 nT
1434246270.600000, -547, -650, 349, 67038.5 nT
1434246270.700000, -548, -650, 350, 67109.8 nT
1434246270.800000, -547, -649, 348, 66959.2 nT
1434246270.900000, -548, -650, 351, 67137.6 nT

Code

C code to read and display the magnetometer data

	#define _GNU_SOURCE

	#include <stdio.h>
	#include <stdlib.h>
	#define _USE_MATH_DEFINES
	#include <math.h>
	#include <fcntl.h>
	#include <unistd.h>
	#include <string.h>
	#include <sys/ioctl.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <sys/time.h>
	#include <linux/i2c-dev.h>

	const int HMC5883L_I2C_ADDR = 0x1E;

	#define CONFIG_A 0x00
	#define CONFIG_B 0x01
	#define MODE 0x02
	#define DATA 0x03 //read 6 bytes: x msb, x lsb, z msb, z lsb, y msb, y lsb
	#define STATUS 0x09
	#define ID_A 0x0A
	#define ID_B 0x0B
	#define ID_C 0x0C
	#define ID_STRING "H43"

	#define GAIN 1370 //000 setting

	void selectDevice(int fd, int addr, char * name)
	{
	if (ioctl(fd, I2C_SLAVE, addr) < 0)
	{
	fprintf(stderr, "%s not present\n", name);
	//exit(1);
	}
	}

	void writeToDevice(int fd, int reg, int val)
	{
	char buf[2];
	buf[0]=reg;
	buf[1]=val;

	if (write(fd, buf, 2) != 2)
	{
	fprintf(stderr, "Can't write to ADXL345\n");
	//exit(1);
	}
	}



	int main(int argc, char **argv)
	{
	int fd;
	unsigned char buf[16];
	struct timeval tv;
	struct timezone tz; //ignored
	struct timeval data_timestamp; //
	int resolution = 100000; //microseconds
	long next_timestamp;

	if ((fd = open("/dev/i2c-1", O_RDWR)) < 0)
	{
	// Open port for reading and writing
	fprintf(stderr, "Failed to open i2c bus\n");

	return 1;
	}

	/* initialise HMC5883L */

	selectDevice(fd, HMC5883L_I2C_ADDR, "HMC5883L");

	//first read the 3 ID bytes
	buf[0] = ID_A;
	if ((write(fd, buf, 1)) != 1)
	{
	// Send the register to read from
	fprintf(stderr, "Error writing to i2c slave\n");
	}

	if (read(fd, buf, 3) != 3) {
	fprintf(stderr, "Unable to read from HMC5883L\n");
	}
	buf[3]=0;
	printf("Identification: '%s' ",buf);
	if (strncmp(buf, ID_STRING, 3) == 0)
	printf("HMC5883L sensor detected\n");
	else {
	printf("unknown sensor. Exiting.\n");
	exit(1);
	}

	//Configuration

	//writeToDevice(fd, 0x01, 0);
	writeToDevice(fd, CONFIG_A, 0b01101000); //8 sample averaging
	writeToDevice(fd, CONFIG_B, 0b00000000); //max gain
	writeToDevice(fd, MODE, 0b00000011); //idle mode

	//find current time
	gettimeofday(&data_timestamp,&tz);
	data_timestamp.tv_sec += 1; //start loggin at start of next second
	data_timestamp.tv_usec = 0;

	while(1){ //record forever

	//read time & wait until next reading required

	while (1) {
	//get time
	gettimeofday(&tv,&tz);

	//if seconds >= next_secs && usecs >= next_usecs
	if (tv.tv_sec >= data_timestamp.tv_sec && tv.tv_usec >= data_timestamp.tv_usec)
	break;

	usleep(1000);
	}

	//initiate single conversion
	writeToDevice(fd, MODE, 0b00000001); //single measurement

	//wait 7 milliseconds
	usleep(7000);

	buf[0] = DATA;

	if ((write(fd, buf, 1)) != 1)
	{
	// Send the register to read from
	fprintf(stderr, "Error writing to i2c slave\n");
	}

	if (read(fd, buf, 6) != 6) {
	fprintf(stderr, "Unable to read from HMC5883L\n");
	} else {


	short x = (buf[0] << 8) \| buf[1];
	short y = (buf[4] << 8) \| buf[5];
	short z = (buf[2] << 8) \| buf[3];

	float angle = atan2(y, x) * 180 / M_PI;
	float mag = sqrt(xx+yy+z*z);


	//printf("x=%d, y=%d, z=%d\n", x, y, z);
	// printf("angle = %0.1f, mag = %0.1f\n", angle,mag);
	//printf("time: %ld.%06ld\n",tv.tv_sec, tv.tv_usec);
	printf("%ld.%06ld, %d, %d, %d, %0.1f nT\n",data_timestamp.tv_sec, data_timestamp.tv_usec,x,y,z,1e5*mag/GAIN);
	fflush(stdout);
	}

	//advance data timestamp to next required time
	data_timestamp.tv_usec += resolution;
	if (data_timestamp.tv_usec >= 1e6) {
	data_timestamp.tv_sec += 1;
	data_timestamp.tv_usec -= 1e6;
	}

	}

	return 0;
	}