I recently had to read the Analog Devices ADXL345 accelerometer, using SPI. So I used the Raspberry Pi. I only needed to verify the functionality in Python, so the code do have some rough edges, when it comes to the calculation of the gravity, but the code that reads the device is correct.

Note: You need to have the python-dev package installed, along with the SpiDev. You can get the former, using apt-get, but you have to install SpiDev manually.

Another note, and this is important; You shouldn’t set the spi.mode = 3 field before calling the open() function. I have no idea what so ever why, but the SPI on the Raspberry Pi, will absolutely not work for me, if I do that.

The code is provided, here below:

#!/usr/bin/python
# -*- coding: utf-8 -*-
# Example on how to read the ADXL345 accelerometer.
# Kim H. Rasmussen, 2014
import time
import spidev

# Setup SPI
spi = spidev.SpiDev()
#spi.mode = 3    <-- Important: Do not do this! Or SPI won't work as intended, or even at all.
spi.open(0,0)
spi.mode = 3

# Read the Device ID (should be xE5)
id = spi.xfer2([128,0])
print 'Device ID (Should be 0xE5):\n'+str(hex(id[1])) + '\n'

# Read the offsets
xoffset = spi.xfer2([30 | 128,0])
yoffset = spi.xfer2([31 | 128,0])
zoffset = spi.xfer2([32 | 128,0])
print 'Offsets: '
print xoffset[1]
print yoffset[1]
print str(zoffset[1]) + "\n\nRead the ADXL345 every half second:"

# Initialize the ADXL345
def initadxl345():
    # Enter power saving state
    spi.xfer2([45, 0])

    # Set data rate to 100 Hz
    spi.xfer2([44, 10])

    # Enable full range (10 bits resolution) and +/- 16g 4 LSB
    spi.xfer2([49, 16])

    # Enable measurement
    spi.xfer2([45, 8])

# Read the ADXL x-y-z axia
def readadxl345():
    rx = spi.xfer2([242,0,0,0,0,0,0])

    # 
    out = [rx[1] | (rx[2] << 8),rx[3] | (rx[4] << 8),rx[5] | (rx[6] << 8)]
    # Format x-axis
    if (out[0] & (1<<16 - 1 )):
        out[0] = out[0] - (1<<16)
    out[0] = out[0] * 0.004 * 9.82
    # Format y-axis
    if (out[1] & (1<<16 - 1 )):
        out[1] = out[1] - (1<<16)
    out[1] = out[1] * 0.004 * 9.82
    # Format z-axis
    if (out[2] & (1<<16 - 1 )):
        out[2] = out[2] - (1<<16)
    out[2] = out[2] * 0.004 * 9.82

    return out

# Initialize the ADXL345 accelerometer
initadxl345()

# Read the ADXL345 every half second
timeout = 0.5
while(1):
    axia = readadxl345()
    # Print the reading
    print axia[0]
    print axia[1]
    print str(axia[2]) + '\n'

    elapsed = time.clock()
    current = 0
    while(current < timeout):
        current = time.clock() - elapsed