ADXL345 accelerometer and LPC1768 example

The ADXL345 is a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement at up to ±16g. Digital output data is formatted as 16-bit twos complement and is accessible through either a SPI (3- or 4-wire) or I2C digital interface.

The ADXL345 is well suited for mobile device applications. It measures the static acceleration of gravity in tilt-sensing applications, as well as dynamic acceleration resulting from motion or shock. Its high resolution (4 mg/LSB) enables measurement of inclination changes less than 1.0°.

Several special sensing functions are provided. Activity and inactivity sensing detect the presence or lack of motion and if the acceleration on any axis exceeds a user-set level. Tap sensing detects single and double taps. Free-fall sensing detects if the device is falling. These functions can be mapped to one of two interrupt output pins. An integrated, patent pending 32-level first in, first out (FIFO) buffer can be used to store data to minimize host processor intervention.

Features

  • Ultralow power: as low as 23 μA in measurement mode and 0.1 μA in standby mode at VS = 2.5 V (typical)
  • Power consumption scales automatically with bandwidth
  • User-selectable resolution
    • Fixed 10-bit resolution
    • Full resolution, where resolution increases with g range, up to 13-bit resolution at ±16 g (maintaining 4 mg/LSB scale factor in all g ranges)

Parts List

Label Part Type
MBED1 mbed LPC 1768
Part1 Adafruit ADXL345

Layout

 

mbed and ADXL345
mbed and ADXL345

Code

You will need to import the library from https://os.mbed.com/components/ADXL345-Accelerometer/ and I used the I2C example as well, which you can see below

#include "ADXL345_I2C.h"
 
 ADXL345_I2C accelerometer(p28, p27);
 Serial pc(USBTX, USBRX);
 
 int main() 
 {
       pc.baud(115200);
     int readings[3] = {0, 0, 0};
 
     pc.printf("Starting ADXL345 test...\n");
     wait(.001);
     pc.printf("Device ID is: 0x%02x\n", accelerometer.getDeviceID());
     wait(.001);
 
     // These are here to test whether any of the initialization fails. It will print the failure
    if (accelerometer.setPowerControl(0x00))
    {
         pc.printf("didn't intitialize power control\n"); 
         return 0;  
    }
     //Full resolution, +/-16g, 4mg/LSB.
     wait(.001);
 
     if(accelerometer.setDataFormatControl(0x0B))
     {
        pc.printf("didn't set data format\n");
        return 0;  
     }
     wait(.001);
 
     //3.2kHz data rate.
     if(accelerometer.setDataRate(ADXL345_3200HZ))
     {
        pc.printf("didn't set data rate\n");
        return 0;    
     }
     wait(.001);
 
     //Measurement mode.
 
     if(accelerometer.setPowerControl(MeasurementMode)) 
     {
        pc.printf("didn't set the power control to measurement\n"); 
        return 0;   
     } 
 
     while (1) 
     {
        wait(0.5);
        accelerometer.getOutput(readings);
        pc.printf("%i, %i, %i\r\n", (int16_t)readings[0], (int16_t)readings[1], (int16_t)readings[2]);
     }
 
 }

Testing

Using a terminal program like Teraterm you should see something like this

-320, 226, 164
-254, -8, 94
-12, -168, -70
-172, -210, 60
-76, -38, -162
-198, 322, 130
-42, -254, -8
-126, 228, 0
-140, 70, -92
-342, 66, 46
-122, 156, 158
-78, 148, 204

Links

GY-291 ADXL345 3-Axis Digital Gravity Sensor Acceleration Module IIC/SPI transmission