Converting G values into floats from accelerometer

i having issue trying change g values accelerator int float. when change variable type int float, it's serial output looks while sitting still:

code: [select]

-26844 -17153 -26844
mph/s: -0.17
-20133 -17217 -26844
mph/s: -0.17
-26844 -17153 -26844
mph/s: -0.17
-20133 -17217 -26844
mph/s: -0.17
-20133 -17217 -26844
mph/s: -0.17
-20133 -17217 -26844
mph/s: -0.17
-20133 -17217 -26844
mph/s: -0.17

while when it's plain int variable type, looks normal:

code: [select]

0 0 1
mph/s: 0
0 0 1
mph/s: 0
0 0 1
mph/s: 0
0 0 1
mph/s: 0
0 0 1
mph/s: 0
0 0 1
mph/s: 0

however, require float version of g values can accurate calculation of mph/s.

here full arduino code using adxl345 accelerometer:

code: [select]

#include <wire.h>

#define device (0x1d)    //adxl345 device address
#define to_read (6)        //num of bytes going read each time (two bytes each axis)

byte buff[to_read] ;    //6 bytes buffer saving data read device
char str[512];                      //string buffer transform data before sending serial port

void setup()
{
  wire.begin();        // join i2c bus (address optional master)
  serial.begin(9600);  // start serial output

  //turning on adxl345
  writeto(device, 0x2d, 0);     
  writeto(device, 0x2d, 16);
  writeto(device, 0x2d, 8);
}

void loop()
{
  int regaddress = 0x32;    //first axis-acceleration-data register on adxl345
  int x, y, z;
  float xg, yg, zg;
  float mphs;

  readfrom(device, regaddress, to_read, buff); //read acceleration data adxl345

   //each axis reading comes in 10 bit resolution, ie 2 bytes.  least significat byte first!
   //thus, converting both bytes in 1 int.
  x =   ((((int)buff[1]) << 8) | buff[0]);
  y =   ((((int)buff[3]) << 8) | buff[2]);
  z =   ((((int)buff[5]) << 8) | buff[4]);

  // convert raw xyz values g values.
  xg = x * 0.0078;
  yg = y * 0.0078;
  zg = z * 0.0078;

  // convert forward axis (y) g value mph/s (miles per hour per second).
  mphs = yg * 21.93685129;

  //we send x y z values string serial port
  sprintf(str, "%d %d %d", xg, yg, zg);
  serial.print(str);
  serial.write(10);

  serial.print("mph/s: ");
  serial.println(mphs);

  //it appears delay needed in order not clog port
  delay(15);
}

void writeto(int device, byte address, byte val) {
   wire.begintransmission(device); //start transmission device
   wire.write(address);        // send register address
   wire.write(val);        // send value write
   wire.endtransmission(); //end transmission
}

void readfrom(int device, byte address, int num, byte buff[]) {
  wire.begintransmission(device); //start transmission device
  wire.write(address);        //sends address read from
  wire.endtransmission(); //end transmission

  wire.begintransmission(device); //start transmission device (initiate again)
  wire.requestfrom(device, num);    // request 6 bytes device

  int = 0;
  while(wire.available())    //device may send less requested (abnormal)
  {
    buff[i] = wire.read(); // receive byte
    i++;
  }
  wire.endtransmission(); //end transmission
}

code: [select]

  sprintf(str, "%d %d %d", xg, yg, zg);
xg, yg, , zg not ints. using integer format specifier floats not work. nor using %f specifier, since not supported on arduino.

on other hand, whole function call unnecessary. print variables 1 @ time.

the mphs value looks ok. float value truncated int matches. value not expect says more expectations.

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