reading from MMA8452Q

John Nivard

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Apr 19, 2013, 11:42:11 AM4/19/13

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Hai

I am starting to get the MMA8452Q to work. It is connected to the i2c connector of the interface board and I have now 3 i2cdevices showing up. The MMA8452Q came mouted on a small board with only the interup lines and the data and clocl lines comming out. THe MMA is a xyz accelerometer.

#!/usr/bin/python3

#By John Nivard april 2013

import sys

#sys.path.append

from time import sleep

from quick2wire.i2c import I2CMaster, writing_bytes, reading

#

# reading OUT_X_MSB,OUT_X_LSB most significant bits MSB,

# OUT_X(Y,Z)_MSB and OUT_X(Y,Z)_LSB

# F_READ bit in CTRL_REG1 must be set for a fast read. In fast read mode

# the LSB registers are ignored

# Interrupt register configuration

#

#register address of relevant registers in the MMA8452Q

#

address = 0x1D # port number of the MMA8452Q

STATUS = 0x00 # Status register

OUT_X_MSB= 0x01 # [7:0] are 8 MSB of 12 bit sample

OUT_Y_MSB= 0x01 # [7:0] are 8 MSB of 12 bit sample

OUT_Z_MSB= 0x01 # [7:0] are 8 MSB of 12 bit sample

CTRL_REG1= 0x2A # []

BUF = 0

with I2CMaster () as i2c :

#set fast read active bit 1 and set active bit 0 page 38 of MMA8452Q

i2c.transaction(writing_bytes(address, CTRL_REG1, 0b00000011))

BUF = i2c.transaction(reading (address, 1))

print ("CTRL_REG1=" , BUF)

while True:

#write address to read OUT_X_MSB

i2c.transaction(writing_bytes(address, OUT_X_MSB))

BUF = i2c.transaction(reading (address, 1))

#

print (BUF)

sleep (0.5)

The output is as follows

[b' \xff']

It is not changing when I move the meter

I think my set up is ok but I am not 100% shure of it

John

Andrew Scheller

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Apr 19, 2013, 11:58:34 AM4/19/13

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> I am starting to get the MMA8452Q to work. It is connected to the i2c
> connector of the interface board and I have now 3 i2cdevices showing up. The

[snip]

> The output is as follows
> [b' \xff']
> [b' \xff']
> [b' \xff']
>
> It is not changing when I move the meter
> I think my set up is ok but I am not 100% shure of it

I thought you'd already determined that this chip wouldn't work with
the RaspPi because of the I2C repeated-start issue??

Andrew

John Nivard

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Apr 19, 2013, 12:27:44 PM4/19/13

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Hai Andrew,

Romilly did think there was nothing to prefent the MMA to communicate with the PI.

So I am starting to make a try. I am not shure if aI do the write and read in the good way,

The first thing I do is reading the WHO_IAM register but this give thge same result [b' \xff']

#!/usr/bin/python3

#By John Nivard april 2013

import sys

#sys.path.append

from time import sleep

from quick2wire.i2c import I2CMaster, writing_bytes, reading

#

# reading OUT_X_MSB,OUT_X_LSB most significant bits MSB,

# OUT_X(Y,Z)_MSB and OUT_X(Y,Z)_LSB

# F_READ bit in CTRL_REG1 must be set for a fast read. In fast read mode

# the LSB registers are ignored

# Interrupt register configuration

#

#register address of relevant registers in the MMA8452Q

#

address = 0x1D # port number of the MMA8452Q

STATUS = 0x00 # Status register

OUT_X_MSB= 0x01 # [7:0] are 8 MSB of 12 bit sample

OUT_Y_MSB= 0x03 # [7:0] are 8 MSB of 12 bit sample

OUT_Z_MSB= 0x05 # [7:0] are 8 MSB of 12 bit sample

CTRL_REG1= 0x2A # []

SYSMODE = 0x0B # current system mode

WHO_AM_I = 0x0D #

BUF = 0

with I2CMaster () as i2c :

#WHO_AM_I

i2c.transaction(writing_bytes(address, WHO_AM_I))

BUF = i2c.transaction(reading (address, 1))

print ("WHO_AM_I=" , BUF)

#read current system mode

i2c.transaction(writing_bytes(address, SYSMODE))

BUF = i2c.transaction(reading (address, 1))

print ("SYSMODE=" , BUF)

#set fast read active bit 1 and set active bit 0 page 38 of MMA8452Q

#i2c.transaction(writing_bytes(address, CTRL_REG1, 0b00000011))

#BUF = i2c.transaction(reading (address, 1))

#print ("CTRL_REG1=" , BUF)

while True:

#write address to read OUT_X_MSB

i2c.transaction(writing_bytes(address, OUT_X_MSB))

BUF = i2c.transaction(reading (address, 1))

#

print (BUF)

sleep (0.5)

Thomas Preston

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Jan 17, 2014, 9:30:54 AM1/17/14

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Did anyone get anywhere with this? I'm having the same issue. I can read from the chip using SMBus (python-smbus) but this only works in Python 2. I need Python 3 support.

DEFAULT_I2C_ADDRESS = 0x1d
DEFAULT_I2C_BUS = 1

# register addresses
SYSMOD = 0x0B
XYZ_DATA_CFG = 0x0E
CTRL_REG1 = 0x2A


class MMA8452Q(object):

"""MMA8452Q Accelerometer."""

def __init__(self, address=DEFAULT_I2C_ADDRESS, bus_num=DEFAULT_I2C_BUS):

self.i2c_address = address

self.i2c_bus = smbus.SMBus(bus_num)

self.g_mul = 0 # G-force multiplier (to get G-force from interval)

def init(self):

"""Initalises the accelerometer."""

# Sleep rate 50 Hz, Data rate 800 Hz,
# No reduced noise mode, Normal read mode, Active

config = 0x3

self.i2c_bus.write_byte_data(self.i2c_address, CTRL_REG1, config)

# Setup range

# register = 0x0E # XYZ_DATA_CFG

# data = 0x00 # Range 2G, no high pass filtering

xyz_config = 0x00

self.i2c_bus.write_byte_data(self.i2c_address,

XYZ_DATA_CFG,

xyz_config)

self.g_mul = 2.0 / 128 # 2G over 128 counts

# System state

# register = 0x0B # SYSMOD

sysmod_config = 0x01 # Awake mode

self.i2c_bus.write_byte_data(self.i2c_address, SYSMOD, sysmod_config)

# Reset ready for reading

self.i2c_bus.write_byte(self.i2c_address, 0x00)

def get_xyz(self):

status, x, y, z = self.i2c_bus.read_i2c_block_data(self.i2c_address,

0,

4)

# convert from unsigned to signed values

bytes = struct.pack('BBB', x, y, z)

x, y, z = struct.unpack('bbb', bytes)

# conver to g-force

return x * self.g_mul, y * self.g_mul, z * self.g_mul

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