Beagle Board - beagleboard.org
Zoran Roncevic
Published © GPL3+

BeagleBone with Accelerometer MM8452

Simple article for BeagleBone and Sparkfun MM8452 accelerometer. Python code is used.

BeginnerProtip30 minutes1,652
BeagleBone with Accelerometer MM8452

Things used in this project

Hardware components

BeagleBone Black
BeagleBoard.org BeagleBone Black
×1
MikroElektronika MicroBUS Cape
×1
SparkFun Triple Axis Accelerometer MMA8452Q
×1

Story

Read more

Code

mm8452.py

Python
Python library for MMA8452 breakout board
from Adafruit_I2C import Adafruit_I2C 

def twos_comp(val, bits):
    """compute the 2's compliment of int value val"""
    if (val & (1 << (bits - 1))) != 0: # if sign bit is set e.g., 8bit: 128-255
        val = val - (1 << bits)        # compute negative value
    return val
    
class MM8452_ACCEL :
    address = None
    i2C = None
    scale = None
    SCALE_2G = 2 
    SCALE_4G = 4
    SCALE_8G = 8
    ODR_800 = 0 # output data rate
    ODR_400 = 1
    ODR_200 = 2 
    ODR_100 = 3 
    ODR_50 = 4
    ODR_12 = 5
    ODR_6 = 6
    ODR_1 = 7 
    WHO_AM_I = 0x0D  
    STATUS = 0x00
    OUT_X_MSB = 0x01
    OUT_X_LSB = 0x02
    OUT_Y_MSB = 0x03
    OUT_Y_LSB = 0x04
    OUT_Z_MSB = 0x05
    OUT_Z_LSB = 0x06    
    CTRL_REG1 = 0x2A
    XYZ_DATA_CFG = 0x0E
	
    def __init__(self, address = 0x1D):
        self.address = address
        self.i2C = Adafruit_I2C(self.address)
    
    def whoAmI(self):
        reg_val = self.i2C.readU8(self.WHO_AM_I)
        return reg_val    
        
    def standby(self):
        reg_val = self.i2C.readU8(self.CTRL_REG1)
        reg_val = reg_val & 0xFE # clear the active bit
        self.i2C.write8(self.CTRL_REG1,reg_val)
        
    def active(self):
        reg_val = self.i2C.readU8(self.CTRL_REG1)
        reg_val = reg_val | 0x01 # set the active bit
        self.i2C.write8(self.CTRL_REG1,reg_val)
        
    def config(self, fsr = 2, odr = 0):
        self.scale = fsr
        self.standby()
        self.setScale(self.scale)
        self.setODR(odr)
        self.active()
        
    def setScale(self, scale):
        # must be in standby mode!
        reg_val = self.i2C.readU8(self.XYZ_DATA_CFG)
        reg_val = reg_val & 0xFC
        scale = scale >> 2
        reg_val = reg_val | scale
        self.i2C.write8(self.XYZ_DATA_CFG,reg_val)
        
    def setODR(self,odr):
        reg_val = self.i2C.readU8(self.CTRL_REG1)
        reg_val = reg_val & 0xCF # clear the active bit
        odr = odr << 3
        reg_val = reg_val | odr
        self.i2C.write8(self.CTRL_REG1,reg_val)       
        
    def available(self):
        reg_val = self.i2C.readU8(self.STATUS)
        reg_val = reg_val & 0x08
        reg_val = reg_val >> 3
        return reg_val
        
    def readX(self):
        reg_XH = self.i2C.readU8(self.OUT_X_MSB)
        reg_XL = self.i2C.readU8(self.OUT_X_LSB)
        reg_x = (reg_XH << 8) | (reg_XL)
        res = reg_x >> 4
        res = twos_comp(res,12)
        return res
        
    def readY(self):
        reg_YH = self.i2C.readU8(self.OUT_Y_MSB)
        reg_YL = self.i2C.readU8(self.OUT_Y_LSB)
        reg_x = (reg_YH << 8) | (reg_YL)
        res = reg_x >> 4
        res = twos_comp(res,12)
        return res
        
    def readZ(self):
        reg_ZH = self.i2C.readU8(self.OUT_Z_MSB)
        reg_ZL = self.i2C.readU8(self.OUT_Z_LSB)
        reg_x = (reg_ZH << 8) | (reg_ZL)
        res = reg_x >> 4
        res = twos_comp(res,12)
        return res
        

accel.py

Python
Example code for testing
from Adafruit_I2C import Adafruit_I2C 
from mm8452 import MM8452_ACCEL
import time



acc = MM8452_ACCEL()

print format(acc.whoAmI(),'02x')

acc.config(acc.SCALE_8G, acc.ODR_100)

time.sleep(0.1)

while 1:
    if acc.available():
        print acc.readX(), acc.readY() , acc.readZ()
        time.sleep(0.3)

Credits

Zoran Roncevic

Zoran Roncevic

3 projects • 119 followers
Hackster Live ambassador in Serbia. Organizer of Maker NS community.

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