The ADXL345 is a 3-axis accelerometer which we use for our motion sensing tests. Our source code is from SparkFuns ADXL Quickstart Guide and we have modified to to include comments that tell us which pins are necessary to make it work on the Arduino UNO and The Arduino Mega 2560.  We also added in some debugging code that enables us to see something happening when the program is running if the ADXL345 is not initialized.

Our Initial tests were on the Mega 2560 and we had difficulty getting anything to read from the device. After some trial and error we grabbed an UNO and with no problem starting getting reads. We figured the problem was somewhere in reading from the device and so we checked the documentation for the SPI library and found a chart that informed us of the proper pins to use of the different Arduino boards.

We reconnected the ADXL345 to the Arduino Mega 2560 with the jumpers connected to the appropriate pins and were able to get readings off the device.  It does seem to take a bit of time for the ADXL345 to initialize, at least in our initial tests.

//Add the SPI library so we can communicate with the ADXL345 sensor
#include <SPI.h>

//Assign the Chip Select signal to pin 10.
int CS=10;

/* Please follow following Pinouts
 * ADXL345 | UNO | Mega 2560
 * SCL | 13 | 52
 * SDA | 11 | 51
 * SDO | 12 | 50
 * CS | 10 | 10
 * GND | GND | GND
 * VCC | 3.3V | 3.3v

//ADXL345 Register Addresses
#define DEVID 0x00 //Device ID Register
#define THRESH_TAP 0x1D //Tap Threshold
#define OFSX 0x1E //X-axis offset
#define OFSY 0x1F //Y-axis offset
#define OFSZ 0x20 //Z-axis offset
#define DURATION 0x21 //Tap Duration
#define LATENT 0x22 //Tap latency
#define WINDOW 0x23 //Tap window
#define THRESH_ACT 0x24 //Activity Threshold
#define THRESH_INACT 0x25 //Inactivity Threshold
#define TIME_INACT 0x26 //Inactivity Time
#define ACT_INACT_CTL 0x27 //Axis enable control for activity and inactivity detection
#define THRESH_FF 0x28 //free-fall threshold
#define TIME_FF 0x29 //Free-Fall Time
#define TAP_AXES 0x2A //Axis control for tap/double tap
#define ACT_TAP_STATUS 0x2B //Source of tap/double tap
#define BW_RATE 0x2C //Data rate and power mode control
#define POWER_CTL 0x2D //Power Control Register
#define INT_ENABLE 0x2E //Interrupt Enable Control
#define INT_MAP 0x2F //Interrupt Mapping Control
#define INT_SOURCE 0x30 //Source of interrupts
#define DATA_FORMAT 0x31 //Data format control
#define DATAX0 0x32 //X-Axis Data 0
#define DATAX1 0x33 //X-Axis Data 1
#define DATAY0 0x34 //Y-Axis Data 0
#define DATAY1 0x35 //Y-Axis Data 1
#define DATAZ0 0x36 //Z-Axis Data 0
#define DATAZ1 0x37 //Z-Axis Data 1
#define FIFO_CTL 0x38 //FIFO control
#define FIFO_STATUS 0x39 //FIFO status

//This buffer will hold values read from the ADXL345 registers.
char values[10];
char output[20];
//These variables will be used to hold the x,y and z axis accelerometer values.
int x,y,z;
double xg, yg, zg;
int tapType=0;

void setup(){ 
 //Initiate an SPI communication instance.
 //Configure the SPI connection for the ADXL345.
 //Create a serial connection to display the data on the terminal.
 Serial.print("Hello Word!");
 //Set up the Chip Select pin to be an output from the Arduino.
 pinMode(CS, OUTPUT);
 //Before communication starts, the Chip Select pin needs to be set high.
 digitalWrite(CS, HIGH);
 //Create an interrupt that will trigger when a tap is detected.
 attachInterrupt(0, tap, RISING);
 //Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register.
 writeRegister(DATA_FORMAT, 0x01);

 //Send the Tap and Double Tap Interrupts to INT1 pin
 writeRegister(INT_MAP, 0x9F);
 //Look for taps on the Z axis only.
 writeRegister(TAP_AXES, 0x01);
 //Set the Tap Threshold to 3g
 writeRegister(THRESH_TAP, 0x38);
 //Set the Tap Duration that must be reached
 writeRegister(DURATION, 0x10);
 //100ms Latency before the second tap can occur.
 writeRegister(LATENT, 0x50);
 writeRegister(WINDOW, 0xFF);
 //Enable the Single and Double Taps.
 writeRegister(INT_ENABLE, 0xE0); 
 //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register.
 writeRegister(POWER_CTL, 0x08); //Measurement mode
 readRegister(INT_SOURCE, 1, values); //Clear the interrupts from the INT_SOURCE register.

void loop(){
 //Reading 6 bytes of data starting at register DATAX0 will retrieve the x,y and z acceleration values from the ADXL345.
 //The results of the read operation will get stored to the values[] buffer.
 readRegister(DATAX0, 6, values);

 //The ADXL345 gives 10-bit acceleration values, but they are stored as bytes (8-bits). To get the full value, two bytes must be combined for each axis.
 //The X value is stored in values[0] and values[1].
 x = ((int)values[1]<<8)|(int)values[0];
 //The Y value is stored in values[2] and values[3].
 y = ((int)values[3]<<8)|(int)values[2];
 //The Z value is stored in values[4] and values[5].
 z = ((int)values[5]<<8)|(int)values[4]; //Convert the accelerometer value to G's. //With 10 bits measuring over a +/-4g range we can find how to convert by using the equation: // Gs = Measurement Value * (G-range/(2^10)) or Gs = Measurement Value * (8/1024) xg = x * 0.0078; yg = y * 0.0078; zg = z * 0.0078; Serial.println("Before if:"); Serial.println(tapType); if(tapType > 0)
 if(tapType == 1){
 Serial.println("After if:");
 attachInterrupt(0, tap, RISING);
 int Type=0; 

//This function will write a value to a register on the ADXL345.
// char registerAddress - The register to write a value to
// char value - The value to be written to the specified register.
void writeRegister(char registerAddress, char value){
 //Set Chip Select pin low to signal the beginning of an SPI packet.
 digitalWrite(CS, LOW);
 //Transfer the register address over SPI.
 //Transfer the desired register value over SPI.
 //Set the Chip Select pin high to signal the end of an SPI packet.
 digitalWrite(CS, HIGH);

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.
// char registerAddress - The register addresse to start the read sequence from.
// int numBytes - The number of registers that should be read.
// char * values - A pointer to a buffer where the results of the operation should be stored.
void readRegister(char registerAddress, int numBytes, char * values){
 //Since we're performing a read operation, the most significant bit of the register address should be set.
 char address = 0x80 | registerAddress;
 //If we're doing a multi-byte read, bit 6 needs to be set as well.
 if(numBytes > 1)address = address | 0x40;
 //Set the Chip select pin low to start an SPI packet.
 digitalWrite(CS, LOW);
 //Transfer the starting register address that needs to be read.
 //Continue to read registers until we've read the number specified, storing the results to the input buffer.
 for(int i=0; i<numBytes; i++){
 values[i] = SPI.transfer(0x00);
 //Set the Chips Select pin high to end the SPI packet.
 digitalWrite(CS, HIGH);

void tap(void){
 //Clear the interrupts on the ADXL345
 readRegister(INT_SOURCE, 1, values); 
 if(values[0] & (1<<5))tapType=2;
 else tapType=1;;

Here are some additional links to information on implementing the ADXL345 on the Arduino platform:

MEMS (Part 1) – Guide to using accelerometer ADXL345