// --------------------------------------------------------
// MMA7455_init
//
// Initialize the MMA7455.
// Set also the offset, assuming that the accelerometer is
// in flat horizontal position.
//
// Important notes about the offset:
// The sensor has internal registers to set an offset.
// But the offset could also be calculated by software.
// This function uses the internal offset registers
// of the sensor.
// That turned out to be bad idea, since setting the
// offset alters the actual offset of the sensor.
// A second offset calculation had to be implemented
// to fine tune the offset.
// Using software variables for the offset would be
// much better.
//
// The offset is influenced by the slightest vibration
// (like a computer on the table).
//
int MMA7455_init(uint8_t mode, unsigned char do_calibration)
{
int x, y, z, error;
xyz_union xyz;
uint8_t c1, c2;
// Initialize the sensor
//
// Sensitivity:
// 2g : GLVL0
// 4g : GLVL1
// 8g : GLVL1 | GLVL0
// Mode:
// Standby : 0
// Measurement : MODE0
// Level Detection : MODE1
// Pulse Detection : MODE1 | MODE0
// There was no need to add functions to write and read
// a single byte. So only the two functions to write
// and read multiple bytes are used.
c1 = mode;
error = MMA7455_write(MMA7455_MCTL, &c1, 1);
if (error != 0)
return (error);
// Read it back, to test the sensor and communication.
error = MMA7455_read(MMA7455_MCTL, &c2, 1);
if (error != 0)
return (error);
if (c1 != c2)
return (-99);
// Nothing to do here if calibration skipped
if (!do_calibration)
return (0);
// Clear the offset registers.
// If the Arduino was reset or with a warm-boot,
// there still could be offset written in the sensor.
// Only with power-up the offset values of the sensor
// are zero.
xyz.value.x = xyz.value.y = xyz.value.z = 0;
error = MMA7455_write(MMA7455_XOFFL, (uint8_t *) &xyz, 6);
if (error != 0)
return (error);
// The mode has just been set, and the sensor is activated.
// To get a valid reading, wait some time.
_delay_ms(100);
// Calcuate the offset.
//
// The values are 16-bits signed integers, but the sensor
// uses offsets of 11-bits signed integers.
// However that is not a problem,
// as long as the value is within the range.
// Assuming that the sensor is flat horizontal,
// the 'z'-axis should be 1 'g'. And 1 'g' is
// a value of 64 (if the 2g most sensitive setting
// is used).
// Note that the actual written value should be doubled
// for this sensor.
error = MMA7455_xyz (&x, &y, &z); // get the x,y,z values
if (error != 0)
return (error);
xyz.value.x = 2 * -x; // The sensor wants double values.
xyz.value.y = 2 * -y;
xyz.value.z = 2 * -(z-64); // 64 is for 1 'g' for z-axis.
error = MMA7455_write(MMA7455_XOFFL, (uint8_t *) &xyz, 6);
if (error != 0)
return (error);
// The offset has been set, and everything should be okay.
// But by setting the offset, the offset of the sensor
// changes.
// A second offset calculation has to be done after
// a short delay, to compensate for that.
_delay_ms(200);
error = MMA7455_xyz (&x, &y, &z); // get te x,y,z values again
if (error != 0)
return (error);
xyz.value.x += 2 * -x; // add to previous value
xyz.value.y += 2 * -y;
xyz.value.z += 2 * -(z-64); // 64 is for 1 'g' for z-axis.
// Write the offset for a second time.
// This time the offset is fine tuned.
error = MMA7455_write(MMA7455_XOFFL, (uint8_t *) &xyz, 6);
if (error != 0)
return (error);
return (0); // return : no error
}
int MMA7455::write(int start, const uint8_t *pData, int size)
{
return MMA7455_write(start, pData, size);
}