/* 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

/* 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 e1;
   char filename[20] ;
   long funcs;
    
   int heading_byte, heading_word_h, heading_word_l;
   int bearing_long, bearing_degrees;
   
   sprintf(filename,"/dev/i2c-0");
   if ((file = open(filename,O_RDWR)) < 0) {
     e1 = errno;
     if (e1 != ENOENT) {
       fprintf(stderr,"Error: Could not open file '%s' : %sn",
               filename,strerror(e1));
       if(e1 == EACCES)
         fprintf(stderr,"Run as root?n");
     }
   } 
   
   if (ioctl(file,I2C_FUNCS,&funcs) < 0) {
     fprintf(stderr,
             "Error: Could not get the adapter functionality maxtrix: %sn",
             strerror(errno));
     exit(1);
   }
   if (! (funcs & I2C_FUNC_SMBUS_QUICK)) {
     fprintf(stderr,
             "Error: Can't use SMBus Quick Write command "
             "on this bus (ISA bus?)n");
     exit(1);
   }
   fprintf(stderr, "I2C functionality: %08Xn", funcs);
   
   if (ioctl(file,I2C_SLAVE,CMPS03_ADDR) < 0) {
     if (errno == EBUSY) {
       printf("device is busyn");
     }
     printf("Got error: %d\n", errno);
     exit(0);
   }
   
   /* Get software revision number */
   res = i2c_smbus_read_byte_data(file, CMPS03_SOFTWARE_REVISION);
   if (res < 0) {
     printf("Cannot read software revision leveln");
   } else {
     printf("Software revision level: %02xn", res);
   }
   
   /* Loop and read from the compass. */
   while (1) {
     /* The heading byte is 0-255 for the 360 degrees. */
     heading_byte = i2c_smbus_read_byte_data(file, CMPS03_BEARING_BYTE);
     if (heading_byte < 0) { printf("Error reading from compass."); 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 = i2c_smbus_read_byte_data(file, CMPS03_BEARING_WORD_HIGH);
     if (heading_word_h < 0) { printf("Error reading from compass."); exit(1);}
     heading_word_l = i2c_smbus_read_byte_data(file, CMPS03_BEARING_WORD_LOW);
     if (heading_word_l < 0) { printf("Error reading from compass."); 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);
   }
}