Two ADXL345 accelerometers on Arduino UNO

Hi all,
I’m trying to connect two ADXL345 accelerometers to an Arduino UNO (via I2C), but as I am an absolute beginner at Arduino, I’m getting stuck.

My setup is as following:
Imgur

And this is the code I use (I tried to keep it as simple as possible):

#include <Wire.h>  // Get I2C library

//  ADXL345 1
#define ACC1 (0x53)     //define ADXL345 address
#define A1_TO_READ (6) //read bytes (2) every time

//  ADXL345 2
#define ACC2 (0x1D)     //define ADXL345 address
#define A2_TO_READ (6) //read bytes (2) every time 

// for the offset
int a1_offx = 0;
int a1_offy = 0;
int a1_offz = 0;

// for the offest
int a2_offx = 0;
int a2_offy = 0;
int a2_offz = 0;

char str[512];

void getAccelerometerData1(int * result) {
  int regAddress = 0x32;    //The setting of data of the first axis of ADXL345
  byte buff[A1_TO_READ];

  readFrom(ACC1, regAddress, A1_TO_READ, buff); //read the data from adxl345

  //the value of every axis has 10 resolution, which means 2 bytes
  //we have to convert 2 bytes into 1 int value.
  result[0] = (((int)buff[1]) << 8) | buff[0] + a1_offx;
  result[1] = (((int)buff[3]) << 8) | buff[2] + a1_offy;
  result[2] = (((int)buff[5]) << 8) | buff[4] + a1_offz;
}

void getAccelerometerData2(int * result) {
  int regAddress = 0x32;    //The setting of data ofthe first axis of ADXL345
  byte buff[A2_TO_READ];

  readFrom(ACC2, regAddress, A2_TO_READ, buff); //read the data from adxl345

  //the value of every axis has 10 resolution, which means 2 bytes
  //we have to convert 2 bytes into 1 int value.
  result[0] = (((int)buff[1]) << 8) | buff[0] + a2_offx;
  result[1] = (((int)buff[3]) << 8) | buff[2] + a2_offy;
  result[2] = (((int)buff[5]) << 8) | buff[4] + a2_offz;
}

void setup()
{
  Serial.begin(9600);
  Wire.begin();
  writeTo(ACC1, 0x2D, 24);
  writeTo(ACC2, 0x2D, 24);
}

// loop reading and printing data
void loop()
{
  int acc1[3];
  int acc2[3];
  getAccelerometerData1(acc1);
  getAccelerometerData2(acc1);

  sprintf(str, "%d,%d,%d,%d,%d,%d", acc1[0], acc1[1], acc1[2], acc2[0], acc2[1], acc2[2]);
  //  Serial.print(str);
  Serial.print(str);
  Serial.write(10);
  delay(10); // delay is needed in order not to clog the port
}

// functions
// write val into the address register of accelerometer
void writeTo(int DEVICE, byte address, byte val) {
  Wire.beginTransmission(DEVICE); //send to sensor
  Wire.write(address);        // send register address
  Wire.write(val);        // send the value which needed to write
  Wire.endTransmission(); //end transmission
}

// read data from the buffer array of address registers in the accelerometer sensor
void readFrom(int DEVICE, byte address, int num, byte buff[]) {
  Wire.beginTransmission(DEVICE); //start to send to accelerometer sensor
  Wire.write(address);            //send address which are read
  Wire.endTransmission();       //end transmission

  Wire.beginTransmission(DEVICE); //start to send to ACC
  Wire.requestFrom(DEVICE, num);    // require sending 6 bytes data from accelerometer sensor

  int i = 0;
  while (Wire.available())   //Error when the return value is smaller than required value
  {
    buff[i] = Wire.read(); // receive data
    i++;
  }
  Wire.endTransmission(); //end transmission
}

The problem is that my output is messed up: there is data printed into the serial monitor, but the same line is repeated over and over (19969,19713,19457,214,2,513) and it doesn’t change when I move one of the accelerometers.

What am I doing wrong?

Greetings!

You can't use the alternate I2C address without pulling the alt address pin LOW on one accel.

From the data sheet.

With the ALT ADDRESS pin high, the 7-bit I2C address for the device is 0x1D, followed by the R/W bit. This translates to 0x3A for a write and 0x3B for a read. An alternate I2C address of 0x53 (followed by the R/W bit) can be chosen by grounding the ALT ADDRESS pin (Pin 12). This translates to 0xA6 for a write and 0xA7 for a read.

Since the alt address pin is not broken out on the modules in the picture (in your link), you might need to use the SPI interface and choose which accel is active by the chip select (see the data sheet).