90 degree offset with LSM303DLHC (using C)

Hello all,

This is my first post… usually I just search the forums, but I haven’t had any luck with searching for someone with this issue. In summary, I recently ported a project I’m working on from an Arduino with a LSM303DLM daughter board to a custom PCB running an ATMEGA644PA and an LSM303DLHC. The problem I am having is the final heading result is now off by 90 degrees (when the x-axis is pointing north, the heading is telling me I’m facing east).

Has anybody seen anything similar to this issue?

A couple of notes:

  1. I am aware of the new accelerometer address, and can successfully communicate with it via I2C.
  2. I have the high resolution bit enabled (but I have also tried it without this enabled).
  3. I downloaded the LSM303DLHC arduino library, and took note of the differences in the code vs. the LSM303DLM library. I believe I’ve updated my code accordingly.
  4. To the best of my knowledge, I’ve calibrated the compass correctly (the same as I did with the LSM303DLM, which worked fine).

Here are the relevant snippets of code:

[code]static void smrt_io_compass_init()
{
TWSR = 0; // clear bit-rate prescale bits
TWBR = 17; // produces an SCL frequency of 400 kHz with a 20 MHz CPU clock speed

//enable accelerometer
i2c_start();
i2c_write_byte(ACC_ADDRESS); // write acc
i2c_write_byte(0x20); // CTRL_REG1_A
i2c_write_byte(0x77); // normal power mode, 400 Hz data rate, all axes enabled

i2c_start();
i2c_write_byte(ACC_ADDRESS);
i2c_write_byte(0x23); // CTRL_REG4_A
i2c_write_byte(0x8);  // enable high resolution mode
i2c_stop();

//enable magnetometer
i2c_start();
i2c_write_byte(MAG_ADDRESS); // write mag
i2c_write_byte(0x02); // MR_REG_M
i2c_write_byte(0x00); // continuous conversion mode
i2c_stop();

}

void smrt_io_read_data(vector *a, vector *m)
{
smrt_io_read_data_raw(a, m);

// shift and scale
m->x = (m->x - m_min.x) / (m_max.x - m_min.x) * 2 - 1.0;
m->y = (m->y - m_min.y) / (m_max.y - m_min.y) * 2 - 1.0;
m->z = (m->z - m_min.z) / (m_max.z - m_min.z) * 2 - 1.0;

}

static void smrt_io_read_data_raw(vector *a, vector *m)
{
// read accelerometer values
i2c_start();
i2c_write_byte(ACC_ADDRESS); // write acc
i2c_write_byte(0xa8); // OUT_X_L_A, MSB set to enable auto-increment
i2c_start(); // repeated start
i2c_write_byte(ACC_ADDRESS+1); // read acc
unsigned char axl = i2c_read_byte();
unsigned char axh = i2c_read_byte();
unsigned char ayl = i2c_read_byte();
unsigned char ayh = i2c_read_byte();
unsigned char azl = i2c_read_byte();
unsigned char azh = i2c_read_last_byte();
i2c_stop();

// read magnetometer values
i2c_start();
i2c_write_byte(MAG_ADDRESS); // write mag
i2c_write_byte(0x03); // OUTXH_M
i2c_start();		  // repeated start
i2c_write_byte(MAG_ADDRESS + 1); // read mag
unsigned char mxh = i2c_read_byte();
unsigned char mxl = i2c_read_byte();
// for the DLM and DLHC, the register address for Z comes before Y
unsigned char mzh = i2c_read_byte();
unsigned char mzl = i2c_read_byte();
unsigned char myh = i2c_read_byte();
unsigned char myl = i2c_read_last_byte();
i2c_stop();

// combine high and low bytes, then shift right to discard lowest 4 bits (which are meaningless)
// GCC performs an arithmetic right shift for signed negative numbers, but this code will not work
// if you port it to a compiler that does a logical right shift instead.
a->x = ((int16_t)(axh << 8 | axl)) >> 4;
a->y = ((int16_t)(ayh << 8 | ayl)) >> 4;
a->z = ((int16_t)(azh << 8 | azl)) >> 4;

m->x = mxh << 8 | mxl;
m->y = myh << 8 | myl;
m->z = mzh << 8 | mzl;

}

// Returns a heading (in degrees) given an acceleration vector a due to gravity, a magnetic vector m, and a facing vector p.
int smrt_io_get_heading(const vector *a, const vector *m, const vector *p)
{
vector E;
vector N;

vector_normalize(a);

// cross magnetic vector (magnetic north + inclination) with "down" (acceleration vector) to produce "east"
vector_cross(m, a, &E);
vector_normalize(&E);

// cross "down" with "east" to produce "north" (parallel to the ground)
vector_cross(a, &E, &N);

// compute heading
int heading = round(atan2(vector_dot(&E, p), vector_dot(&N, p)) * 180 / M_PI);
if (heading < 0)
heading += 360;
return heading;

}[/code]

What procedure did you use to “calibrate the compass correctly”? Did the problem you report appear at that time? If not, when did the problem appear?

BTW I recommend the second procedure described in the following post(s) for calibrating the magnetometer: sailboatinstruments.blogspot.com … ation.html

Finally, whoever wrote the original code is clearly confused about the right-hand convention for the vector cross product. It is a shame that the comments in the code have been allowed to stand for so long (i.e. cross “north” with “down” to produce “east” – N X D => W).

Unfortunately, this problem appeared from the start (with the factory calibration). However, to calibrate, I am using a ported version of the Arduino calibration algorithm.

The tool in the link you provided looks useful, and not to painful to use. I think I will give that a try to be sure that my issue isn’t with the calibration.

Thanks.

Are you certain that you are orienting the PCB correctly? For some reason, north is defined in the code as the -Y direction. In any case, if everything else is working correctly, it is trivial to redefine north with respect to the PCB orientation.

Jim,

I think you are correct… when the PCB was laid out, it was placed where the heading calculation desired is the X direction. However, the vector ‘p’ initialization remained at {0,-1,0} (I don’t remember now, but my old Arduino setup must have had -Y as the heading reference). I believe that’s the issue. I figured it was down in that heading calculation area, but I never even thought to check the ‘p’ vector initialiazation.

Thank you very much for your help.

Hello, cazzy.

I’m glad you and Jim worked it out. The heading() function returns the heading of the -Y axis (the angular difference between the -Y axis and north in the horizontal plane) by default because that’s the axis that points away from you if you orient our carrier boards the way our pictures show:

If you want to use a different axis as a “pointer”, you can use the version of the heading() function that takes a user-defined vector. In your case, you can just use:

heading((vector){1,0,0});

Jim, you are right that I should have corrected the comments for the heading() function in the library long ago. The version on GitHub should be fixed now.

- Kevin

[quote=“kevin”]Hello, cazzy.

I’m glad you and Jim worked it out. The heading() function returns the heading of the -Y axis (the angular difference between the -Y axis and north in the horizontal plane) by default because that’s the axis that points away from you if you orient our carrier boards the way our pictures show:

If you want to use a different axis as a “pointer”, you can use the version of the heading() function that takes a user-defined vector. In your case, you can just use:

heading((vector){1,0,0});

Jim, you are right that I should have corrected the comments for the heading() function in the library long ago. The version on GitHub should be fixed now.

  • Kevin[/quote]

Thank you for confirming… this forum has been helpful (I’m sure I’ll be back!). Yep, the change to {1,0,0} has already been made. Thanks again.