adxl345 accelerometer z-axis is a constant -1

Hey,

I’m trying to get the basic acceleration values out of the adxl345 acclerometer (breakout board). I’m essentially using the code on the sparkfun forum on my arduino uno, but setting the range to +/-2g rather than +/-4g:
https://www.sparkfun.com/tutorials/240

The only change I’ve made is on the writeRegister Function I’m writing DATA_FORMAT to 0x00 rather than 0x01 (I realize this is the device default, but I’m writing it explicitly just to be sure).
When I put the accelerometer in the +/-4g range I get responsive values, but when I put it in the +/-2g range I get a constant -1 value for the z-axis. I’ve seen people having problems with the accelerometer being out of range (z-value=511), but I haven’t been able to find anyone else having this particular problem. I apologize if this is super simple, but I’m very new to programming, and any help would be greatly appreciated!

Here’s my code (you can ignore the temperature and humidity portion):

//Add the SPI library so we can communicate with the ADXL345 sensor
#include <SPI.h>
//Add the DHT11 Library to communicate with the sensor
#include <dht11.h>

dht11 DHT11;

#define DHT11PIN 3

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

//This is a list of some of the registers available on the ADXL345.
//To learn more about these and the rest of the registers on the ADXL345, read the datasheet!
char POWER_CTL = 0x2D;	//Power Control Register
char DATA_FORMAT = 0x31;
char DATAX0 = 0x32;	//X-Axis Data 0
char DATAX1 = 0x33;	//X-Axis Data 1
char DATAY0 = 0x34;	//Y-Axis Data 0
char DATAY1 = 0x35;	//Y-Axis Data 1
char DATAZ0 = 0x36;	//Z-Axis Data 0
char DATAZ1 = 0x37;	//Z-Axis Data 1

//This buffer will hold values read from the ADXL345 registers.
char values[10];
//These variables will be used to hold the x,y and z axis accelerometer values.
int x1,y1,z1;
//These Variables will be used to hold the temp and humidity values.
float temp,RH;

void setup(){ 
  //Initiate an SPI communication instance.
  SPI.begin();
  //Configure the SPI connection for the ADXL345.
  SPI.setDataMode(SPI_MODE3);
  //Create a serial connection to display the data on the terminal.
  Serial.begin(57600);
  
  //Set up the Chip Select pin to be an output from the Arduino.
  pinMode(CS, OUTPUT);
  //pinMode(CS2, OUTPUT);
  //Before communication starts, the Chip Select pin needs to be set high.
  digitalWrite(CS, HIGH);
  //digitalWrite(CS2, HIGH);
  
  //Put the ADXL345 into +/- 2G range by writing the value 0x00 to the DATA_FORMAT register.
  writeRegister(CS, DATA_FORMAT, 0x00);
  //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register.
  writeRegister(CS, POWER_CTL, 0x08);  //Measurement mode
}

void loop(){
  
  //-------------------------------Accel Sensor 1----------------------------------------//
  //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(CS, 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].
  x1 = ((int)values[1]<<8)|(int)values[0];
  //The Y value is stored in values[2] and values[3].
  y1 = ((int)values[3]<<8)|(int)values[2];
  //The Z value is stored in values[4] and values[5].
  z1 = ((int)values[5]<<8)|(int)values[4];
  
  //-------------------------------Temp/RH Sensor----------------------------------------//
  int chk = DHT11.read(DHT11PIN);
  temp = (float)DHT11.temperature;
  RH = (float)DHT11.humidity;
  
  //Print the results to the terminal.
  Serial.print(x1, DEC);
  Serial.print(',');
  Serial.print(y1, DEC);
  Serial.print(',');
  Serial.print(z1, DEC);
  Serial.print(',');
  Serial.print(temp);
  Serial.print(',');
  Serial.println(RH); 
  delay(10); 
}

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

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.
//Parameters:
//  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(int CS_Pin, 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_Pin, LOW);
  //Transfer the starting register address that needs to be read.
  SPI.transfer(address);
  //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_Pin, HIGH);
}

Example output:
x,y,z,temp,humid
-99,-147,-1,26.00,43.00
-105,-153,-1,26.00,43.00
-104,-155,-1,26.00,43.00
-104,-153,-1,26.00,33.00
-105,-152,-1,26.00,33.00
-83,-143,-1,26.00,33.00
-113,-129,-1,26.00,33.00
-80,-118,-1,26.00,33.00
-121,-131,-1,26.00,33.00
-106,-136,-1,26.00,33.00
-113,-141,-1,26.00,33.00
-87,-122,-1,26.00,33.00
-68,-145,-1,26.00,34.00
-83,-116,-1,26.00,34.00
-93,-143,-1,26.00,34.00
-92,-130,-1,26.00,33.00
-113,-135,-1,26.00,33.00

The likely reason for this, is that your I2C communication is not actually working, and the numbers you are getting are actually random noise.

A -1 signed int is FFFF hex, all the bits are 1. So you are somehow getting all 1 bits from the z reading.