ADXL345:I can't measure pitch angle range in 0-180 degress.

Using Arduino Sensors
1

Hello,

I'm trying to measure pitch angle between 0-180 degress. I'm using Arduino Uno R3, my Arduino Compiler version is 1.6.8 and I'm using ADXL345 for the measurement.

When I put the sensor horizontal to table I'm measuring pitch angle about -12.68 degrees and when it becomes orthogonal with the table I'm measuring pitch angle about 112 degrees. Theese values have to be 0-90 degrees and this is the problem. My measurement is wrong and I don't know why.

I've used I2C communication between the devices. Here is my pin configuration:

ADXL345 SCL --> Arduino SCL
ADXL345 SDA --> Arduino SDA
Vss --> 3.3 V
Cs --> 3.3 V
GND --> GND

ADXL345 datasheet link: http://www.analog.com/media/en/technical-documentation/data-sheets/ADXL345.pdf

Here is my code:

#define ADXL345 0x53 // device adress
#define POWER_CTL 0x2D
#define DATA_FORMAT 0x31
#define DATAX0 0x32

#include <Wire.h>
byte buff[6];
int rawX, rawY, rawZ;
double x,y,z,pitch;

void setup()
{
  Serial.begin(9600);
  Wire.begin();

  //measurement mode
  Wire.beginTransmission(ADXL345);
  Wire.write(POWER_CTL);
  Wire.write(0x08);
  Wire.endTransmission();

  // DATA FORMAT = +-16g, Full Resolution (4 mg/LSB) 13 bits. (see table 1 and table 21)
  Wire.beginTransmission(ADXL345);
  Wire.write(DATA_FORMAT);
  Wire.write(0x0B);
  Wire.endTransmission();  
  
}

void loop()
{
  int i=0;
  Wire.beginTransmission(ADXL345);
  Wire.write(DATAX0);
  Wire.requestFrom(ADXL345, 6);
  while(Wire.available())
  {
    buff[i] = Wire.read();
    ++i;
  }
  Wire.endTransmission();

  rawX = ((int)buff[1] << 8) | buff[0];
  rawY = ((int)buff[3] << 8) | buff[2];
  rawZ = ((int)buff[5] << 8) | buff[4];

  //Gs = Measurement Value * (G range/2^n), n = bits
  //Gs = Measurement Value *(32/2^13) = 0.00390625
  
  x = rawX * 0.00390625;
  y = rawY * 0.00390625;
  z = rawZ * 0.00390625;
  
  pitch = atan2(x,sqrt(y*y+z*z));
  pitch *= 57.2957795+180; // radian to degree
  
  Serial.print("rawX = ");
  Serial.print(rawX);
  Serial.print("\trawY = ");
  Serial.print(rawY);
  Serial.print("\trawZ = ");
  Serial.print(rawZ);

  Serial.print("\tx = ");
  Serial.print(x);
  Serial.print("\ty = ");
  Serial.print(y);
  Serial.print("\tz = ");
  Serial.print(z);
  
  Serial.print("\tpitch=");
  Serial.println(pitch);
  delay(100);

}

Here some pictures about what am I doing:
When sensor is horizontally to the table:

I'm getting -12 degrees (which has supposed to be 0 degree)

When sensor is orthogonal with table:

I'm getting 112 degrees (which has supposed to be 90 degree)

Should I add offset values for each of axes raw values ?

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