LSM303D Raspberry Pi driver

Support Sensors
1

This is a driver I wrote for an LSM303D on the Raspberry Pi / Python.
The accelerometer data is in accx, accy, and accz.
The magnetometer data is in magx, magy, and magz.

Dave

#Driver for the LSM303D accelerometer and magnetometer/compass

#First follow the procedure to enable I2C on R-Pi.
#1. Add the lines "ic2-bcm2708" and "i2c-dev" to the file /etc/modules
#2. Comment out the line "blacklist ic2-bcm2708" (with a #) in the file /etc/modprobe.d/raspi-blacklist.conf
#3. Install I2C utility (including smbus) with the command "apt-get install python-smbus i2c-tools"
#4. Connect the I2C device and detect it using the command "i2cdetect -y 1".  It should show up as 1D or 1E (here the variable LSM is set to 1D).

#Driver by Fayetteville Free Library Robotics Group

from smbus import SMBus
busNum = 1
b = SMBus(busNum)

LSM = 0x1d

LSM_WHOAMI = 0b1001001 #Device self-id

def twos_comp_combine(msb, lsb):
    twos_comp = 256*msb + lsb
    if twos_comp >= 32768:
        return twos_comp - 65536
    else:
        return twos_comp

#Control register addresses -- from LSM303D datasheet

CTRL_0 = 0x1F #General settings
CTRL_1 = 0x20 #Turns on accelerometer and configures data rate
CTRL_2 = 0x21 #Self test accelerometer, anti-aliasing accel filter
CTRL_3 = 0x22 #Interrupts
CTRL_4 = 0x23 #Interrupts
CTRL_5 = 0x24 #Turns on temperature sensor
CTRL_6 = 0x25 #Magnetic resolution selection, data rate config
CTRL_7 = 0x26 #Turns on magnetometer and adjusts mode

#Registers holding twos-complemented MSB and LSB of magnetometer readings -- from LSM303D datasheet
MAG_X_LSB = 0x08 # x
MAG_X_MSB = 0x09
MAG_Y_LSB = 0x0A # y
MAG_Y_MSB = 0x0B
MAG_Z_LSB = 0x0C # z
MAG_Z_MSB = 0x0D

#Registers holding twos-complemented MSB and LSB of magnetometer readings -- from LSM303D datasheet
ACC_X_LSB = 0x28 # x
ACC_X_MSB = 0x29
ACC_Y_LSB = 0x2A # y
ACC_Y_MSB = 0x2B
ACC_Z_LSB = 0x2C # z
ACC_Z_MSB = 0x2D

#Registers holding 12-bit right justified, twos-complemented temperature data -- from LSM303D datasheet
TEMP_MSB = 0x05
TEMP_LSB = 0x06


if b.read_byte_data(LSM, 0x0f) == LSM_WHOAMI:
    print 'LSM303D detected successfully.'
else:
    print 'No LSM303D detected on bus '+str(busNum)+'.'

b.write_byte_data(LSM, CTRL_1, 0b1010111) # enable accelerometer, 50 hz sampling
b.write_byte_data(LSM, CTRL_2, 0x00) #set +/- 2g full scale
b.write_byte_data(LSM, CTRL_5, 0b01100100) #high resolution mode, thermometer off, 6.25hz ODR
b.write_byte_data(LSM, CTRL_6, 0b00100000) # set +/- 4 gauss full scale
b.write_byte_data(LSM, CTRL_7, 0x00) #get magnetometer out of low power mode


magx = twos_comp_combine(b.read_byte_data(LSM, MAG_X_MSB), b.read_byte_data(LSM, MAG_X_LSB))
magy = twos_comp_combine(b.read_byte_data(LSM, MAG_Y_MSB), b.read_byte_data(LSM, MAG_Y_LSB))
magz = twos_comp_combine(b.read_byte_data(LSM, MAG_Z_MSB), b.read_byte_data(LSM, MAG_Z_LSB))

print "Magnetic field (x, y, z):", magx, magy, magz

accx = twos_comp_combine(b.read_byte_data(LSM, ACC_X_MSB), b.read_byte_data(LSM, ACC_X_LSB))
accy = twos_comp_combine(b.read_byte_data(LSM, ACC_Y_MSB), b.read_byte_data(LSM, ACC_Y_LSB))
accz = twos_comp_combine(b.read_byte_data(LSM, ACC_Z_MSB), b.read_byte_data(LSM, ACC_Z_LSB))

print "Acceleration (x, y, z):", accx, accy, accz
2

Hello, Dave.

Cool; thank you for sharing!

-Jon

3

You’re welcome. Hopefully someone can use it :slight_smile:

4

We added a link to this thread as a Recommend link under the Resources tab of the relevant product pages.

-Jon

5

Hi Dave,
First, thanks for your code, it has been really helpful. We would like to use some parts of your driver in a larger project, and because we want to make things right we wanted to know under which licence this code is. (Just to let you know the project will use the MIT licence).
Thank you in advance for your answer.

6

Hello Nanoseb,

This code is free and open, and you can use it and modify it however you
like! I wrote the code myself based on the chip’s datasheet. Have fun and
good luck with your project!

Thanks!
Dave Kopp

7

This code was extremely helpful. The code got data from the sensor and printed them to the screen but my question is why do they give gigantic numbers for acceleration when it is stationary. The numbers will be like (x, y, z) : (4701, -2555, -14657). Im assuming that these are suppose to make 1 g but I’m not sure what manipulation you must do to get there? The temperature sensor also gives crazy numbers.

Thanks
Karl Lawson

8

really nice of you to share your code. but i had a question about the twos_comp_combine.

why do you use the if statement. i saw it another example but don’t really understand why you use it.

9

The data is held in “two’s complement” form. The if statement subtracts 65536 (which is 2^16) from the numbers which are supposed to be negative, and leaves the numbers which are supposed to be positive alone.

https://en.wikipedia.org/wiki/Two%27s_complement

Dave