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/* compass.c
 *
 * Software to read from the CMPS03 magnetic compass. 
 *
 * By Josh Marshall, joshua dot marshall at studentmail dot newcastle dot edu dot au
 * October 15, 2004.
 *
 * Adapted from code for the SP03 speech synthesiser 
 * By Bram Stolk, b.stolk at chello.nl
 *
 * Bram Stolk's notes:
 * In linux, you can do I2C stuff in user-space, if you enable
 * the device-interface to i2c. You can use plain read() and 
 * write() calls on your /dev/i2c-0 file.
 *
 * NOTE: Check if you have an i2c adapter active by doing:
 * cat /proc/bus/i2c
 * If this comes up empty, then you have no adapters. If the
 * file does not exist at all, you probably lack i2c kernel
 * support alltogether.
 */

#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <linux/i2c-dev.h>

/* Note that the documentation for the compass states its address as 0xC0.
 * However, this includes the low bit which specifies read or write.
 * Linux i2c does not include this bit in this address, so the actual
 * address is 0xC0 shifted down, 0x60.
 */
#define CMPS03_ADDR     0x60
#define I2C_SLAVE       0x0B    /* Change slave address 

/* The important registers on the compass. Internal/test registers omitted. */
#define CMPS03_SOFTWARE_REVISION 0x0
#define CMPS03_BEARING_BYTE 0x1
#define CMPS03_BEARING_WORD_HIGH 0x2
#define CMPS03_BEARING_WORD_LOW 0x3
#define CMPS03_CALIBRATE_CMD 0xF

int main(int argc, char *argv[]) {
   char *end;
   int res,file;
   int error;
   char filename[20] ;
   long funcs;
    
   int heading_byte, heading_word_h, heading_word_l;
   int bearing_long, bearing_degrees;
   
   sprintf(filename,"/dev/i2c-0");
   file = open(filename,O_RDWR);
   if (errno != 0)
   {
     switch (errno)
     {
     case EACCES:
         fprintf(stderr,"Run as root? \n");
         break;
     default:
         fprintf(stderr,"Error: Could not open file '%s' : %s \n", filename, strerror(errno));
         break;
     }
   } 
      
   if (ioctl(file,I2C_SLAVE,CMPS03_ADDR) < 0) {
     switch (errno) 
     {
     case EBUSY:  
         printf("device is busy \n");
         break;

     default:
         printf("Got error: %s \n", strerror(errno));
         exit(0);
      }
   }
   
   /* Get software revision number */
   res = read(file, CMPS03_SOFTWARE_REVISION,1);
   if (res < 0) {
     printf("Cannot read software revision level \n");
   } else {
     printf("Software revision level: %02x \n", res);
   }
   
   /* Loop and read from the compass. */
   while (1) {
     /* The heading byte is 0-255 for the 360 degrees. */
     heading_byte = read(file, CMPS03_BEARING_BYTE,1);
     if (heading_byte < 0) { printf("Error reading from compass. \n"); exit(1);}
     
     /* The high resolution heading is given in two registers, and is 10 * the 
      * heading in degrees, ie 359.9 degrees reads as 3599. */
     heading_word_h = read(file, CMPS03_BEARING_WORD_HIGH,1);
     if (heading_word_h < 0) { printf("Error reading from compass. \n"); exit(1);}
     heading_word_l = read(file, CMPS03_BEARING_WORD_LOW,1);
     if (heading_word_l < 0) { printf("Error reading from compass. \n"); exit(1);}
     
     /* Combine the two bytes, and get the heading in degrees. */
     bearing_long = heading_word_h * 256 + heading_word_l;
     bearing_degrees = bearing_long / 10;
     
     printf("Bearing: %d \n", bearing_degrees);
     
     /* Wait for a while. */
     usleep(200000);
   }
}