Dear Thomas, Lauszus,
Thanks for the time you took to reply.
I read the article and searched for a calibration programm related to the LSM303. I found this program:
#include <Wire.h>
#include <LSM303.h>
LSM303 compass;
LSM303::vector running_min = {2047, 2047, 2047}, running_max = {-2048, -2048, -2048};
// temp
float x_min; float x_max; float x_cur;
float y_min; float y_max; float y_cur;
float z_min; float z_max; float z_cur;
void setup() {
Serial.begin(9600);
Wire.begin();
compass.init();
compass.enableDefault();
}
void loop() {
compass.read();
running_min.x = min(running_min.x, compass.m.x);
running_min.y = min(running_min.y, compass.m.y);
running_min.z = min(running_min.z, compass.m.z);
running_max.x = max(running_max.x, compass.m.x);
running_max.y = max(running_max.y, compass.m.y);
running_max.z = max(running_max.z, compass.m.z);
Serial.print("M min ");
Serial.print("X: ");
Serial.print((int)running_min.x);
Serial.print(" Y: ");
Serial.print((int)running_min.y);
Serial.print(" Z: ");
Serial.print((int)running_min.z);
Serial.print(" M max ");
Serial.print("X: ");
Serial.print((int)running_max.x);
Serial.print(" Y: ");
Serial.print((int)running_max.y);
Serial.print(" Z: ");
Serial.println((int)running_max.z);
delay(100);
}
After running the programm and looking at the serial output, I started moving the board in all sorts of direction, and after some time I got a stable output:
/* Output of Calibrate.ino
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
*/
These values I entered in the programm "Heading.ino"
#include <Wire.h>
#include <LSM303.h>
LSM303 compass;
void setup() {
Serial.begin(9600);
Wire.begin();
compass.init();
compass.enableDefault();
// Calibration values. Use the Calibrate example program to get the values for
// your compass.
compass.m_min.x = -548; compass.m_min.y = -378; compass.m_min.z = -547;
compass.m_max.x = 248; compass.m_max.y = 413; compass.m_max.z = 314;
/* Output van Calibrate
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
M min X: -548 Y: -378 Z: -547 M max X: 248 Y: 413 Z: 314
*/
}
/* Oorspronkelijke code
void loop() {
compass.read();
int heading = compass.heading((LSM303::vector){0,-1,0});
Serial.println(heading);
delay(500);
}
*/
/*----------------------------------------------------------------------------
1 2 3 4 5 6
| | | | | |
$--HDG,x.x,x.x,a,x.x,a*hh<CR><LF>
------------------------------------------------------------------------------
Field Number:
1. Magnetic Sensor heading in degrees
2. Magnetic Deviation, degrees
3. Magnetic Deviation direction, E = Easterly, W = Westerly
4. Magnetic Variation degrees
5. Magnetic Variation direction, E = Easterly, W = Westerly
6. Checksum */
char nmeastr[17]; // HCHDG,000.00,,,,*
int checksum;
char hs[6]; // 000.00
void loop() {
strcpy(nmeastr,"HCHDG,");
//dtostrf(FLOAT,WIDTH,PRECSISION,BUFFER);
compass.read();
dtostrf(compass.heading((LSM303::vector){0,-1,0}), 5, 0, hs);
//Serial.print("hs ");
//Serial.print(hs);
//Serial.println();
strcat(nmeastr,hs);
strcat(nmeastr,",,,,");
//add a checksum
checksum=0;
for (int n=0; n < strlen(nmeastr); n++) {
checksum ^= nmeastr[n];
}
Serial.print("$");
Serial.print(nmeastr);
Serial.print("*");
Serial.print(checksum, HEX);
Serial.println();
/* 5hz == 200 */
delay(500);
}
While running this programm, serial output showed the heading as NMEA-string. Having it away from all sorts of electroncs and put a normal (old fashion 
) compass pointing in the same directoin, and laying flat on the floor,, the output showed:
$HCHDG, 0,,,,*5C
$HCHDG, 2,,,,*5E
$HCHDG, 5,,,,*59
$HCHDG, 1,,,,*5D
$HCHDG, 3,,,,*5F
$HCHDG, 1,,,,*5D
$HCHDG, 2,,,,*5E
$HCHDG, 4,,,,*58
$HCHDG, 1,,,,*5D
$HCHDG, 2,,,,*5E
$HCHDG, 2,,,,*5E
$HCHDG, 0,,,,*5C
$HCHDG, 3,,,,*5F
$HCHDG, 1,,,,*5D
$HCHDG, 2,,,,*5E
$HCHDG, 3,,,,*5F
$HCHDG, 3,,,,*5F
$HCHDG, 3,,,,*5F
$HCHDG, 3,,,,*5F
$HCHDG, 2,,,,*5E
$HCHDG, 4,,,,*58
$HCHDG, 4,,,,*58
$HCHDG, 5,,,,*59
Allthough not very accurated, it points north with some variance. Not too bad, but room for improvement, I think.
As soon as I start turning the lsm303 up and down , still pointing in the same direction, I would expect to see the same heading however the output shows:
$HCHDG, 14,,,,*49
$HCHDG, 14,,,,*49
$HCHDG, 16,,,,*4B
$HCHDG, 12,,,,*4F
$HCHDG, 10,,,,*4D
$HCHDG, 6,,,,*5A
$HCHDG, 5,,,,*59
$HCHDG, 6,,,,*5A
$HCHDG, 10,,,,*4D
$HCHDG, 7,,,,*5B
$HCHDG, 9,,,,*55
$HCHDG, 8,,,,*54
$HCHDG, 9,,,,*55
$HCHDG, 12,,,,*4F
$HCHDG, 13,,,,*4E
$HCHDG, 11,,,,*4C
$HCHDG, 10,,,,*4D
$HCHDG, 11,,,,*4C
IMHO, this heading info is too inaccurate to use as a compass. The autopilot will move the boat from left to right, like a drunken sailor
I'm not sure wether this is part of the lsm303 (in that case it is not a sufficiënt solution for a compass for an auto pilot) or filtering or something else is required to get a more stable output.
Many thanks, George