Beaglebone and ADXL345 example in Python

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beaglebone and adxl345 output
beaglebone and adxl345 output
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

PartLink
BeagleboneBeagleBone Black TI AM335x Cortex-A8 development BB-Black Rev.C
ADXL345GY-291 ADXL345 digital three-axis acceleration of gravity tilt module
Connecting cablesFree shipping Dupont line 120pcs 20cm male to male + male to female and female to female jumper wire

 

Connection

I used the following connection from the module above to my Beaglebone

PI ConnectionModule Connection
3v3P9.3
GndP9.1
SDAP9.20
SCLP9.19

 

This is a layout showing the connection

beaglebone and adxl345 output
beaglebone and adxl345 output

 

Code

You need to save this as adxl345.py, I used the cloud9 IDE

import smbus
from time import sleep
 
bus = smbus.SMBus(2)
 
# ADXL345 constants
EARTH_GRAVITY_MS2   = 9.80665
SCALE_MULTIPLIER    = 0.004
 
DATA_FORMAT         = 0x31
BW_RATE             = 0x2C
POWER_CTL           = 0x2D
 
BW_RATE_1600HZ      = 0x0F
BW_RATE_800HZ       = 0x0E
BW_RATE_400HZ       = 0x0D
BW_RATE_200HZ       = 0x0C
BW_RATE_100HZ       = 0x0B
BW_RATE_50HZ        = 0x0A
BW_RATE_25HZ        = 0x09
 
RANGE_2G            = 0x00
RANGE_4G            = 0x01
RANGE_8G            = 0x02
RANGE_16G           = 0x03
 
MEASURE             = 0x08
AXES_DATA           = 0x32
 
class ADXL345:
 
    address = None
 
    def __init__(self, address = 0x53):        
        self.address = address
        self.setBandwidthRate(BW_RATE_100HZ)
        self.setRange(RANGE_2G)
        self.enableMeasurement()
 
    def enableMeasurement(self):
        bus.write_byte_data(self.address, POWER_CTL, MEASURE)
 
    def setBandwidthRate(self, rate_flag):
        bus.write_byte_data(self.address, BW_RATE, rate_flag)
 
    # set the measurement range for 10-bit readings
    def setRange(self, range_flag):
        value = bus.read_byte_data(self.address, DATA_FORMAT)
 
        value &= ~0x0F;
        value |= range_flag;  
        value |= 0x08;
 
        bus.write_byte_data(self.address, DATA_FORMAT, value)
 
    # returns the current reading from the sensor for each axis
    #
    # parameter gforce:
    #    False (default): result is returned in m/s^2
    #    True           : result is returned in gs
    def getAxes(self, gforce = False):
        bytes = bus.read_i2c_block_data(self.address, AXES_DATA, 6)
 
        x = bytes[0] | (bytes[1] << 8)
        if(x & (1 << 16 - 1)):
            x = x - (1<<16)
 
        y = bytes[2] | (bytes[3] << 8)
        if(y & (1 << 16 - 1)):
            y = y - (1<<16)
 
        z = bytes[4] | (bytes[5] << 8)
        if(z & (1 << 16 - 1)):
            z = z - (1<<16)
 
        x = x * SCALE_MULTIPLIER 
        y = y * SCALE_MULTIPLIER
        z = z * SCALE_MULTIPLIER
 
        if gforce == False:
            x = x * EARTH_GRAVITY_MS2
            y = y * EARTH_GRAVITY_MS2
            z = z * EARTH_GRAVITY_MS2
 
        x = round(x, 4)
        y = round(y, 4)
        z = round(z, 4)
 
        return {"x": x, "y": y, "z": z}
 
if __name__ == "__main__":
    # if run directly we'll just create an instance of the class and output 
    # the current readings
    adxl345 = ADXL345()
 
    axes = adxl345.getAxes(True)
    print "ADXL345 on address 0x%x:" % (adxl345.address)
    print "   x = %.3fG" % ( axes['x'] )
    print "   y = %.3fG" % ( axes['y'] )
    print "   z = %.3fG" % ( axes['z'] )

Now create another file called test_adxl345.py and entering the following

from adxl345 import ADXL345
 
adxl345 = ADXL345()
 
axes = adxl345.getAxes(True)
print "ADXL345 on address 0x%x:" % (adxl345.address)
print "   x = %.3fG" % ( axes['x'] )
print "   y = %.3fG" % ( axes['y'] )
print "   z = %.3fG" % ( axes['z'] )

Output

Open a terminal in the Cloud9 IDE and enter python test_adxl345.py

debian@beaglebone:/var/lib/cloud9/iain$ python test_adxl345.py


ADXL345 on address 0x53:
x = -0.216G
y = -0.388G
z = 0.300G

 

 

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