Raspberry Pi I2C (Python)

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Introduction: Raspberry Pi I2C (Python)

In this instructable, I will explain how to use I2C on the Pi, with the examples of the CMPS03 compass module and SRF08 Ultrasonic range, using python. I will explain right through installing the OS, to ensure that the dependencies and everything is installed.

I2C is a communication bus designed by Philips, for chips to communicate with each other on a PCB. It is commonly used, however, for connecting sensors, such as the two examples later in this instructable and port expanders, because you can have multiple devices on the same two pins. 

Step 1: Install R-Pi Image

Go to the Raspberry Pi website, and download the latest Raspbian image and follow the instructions burn it to the SD card.

http://www.raspberrypi.org/downloads

There is an easy setup guide on the wiki, just follow it through.

When you have got it installed, run the config tool, and get everything going nicely. In my case, I am running it headless via SSH, which is enabled as default, at pi@192.168.0.X (check on your router to find the IP).

Step 2: Enable I2C

On the Pi, I2C is disabled by default. The first thing to do, is run the command sudo nano /etc/modprobe.d/raspi-blacklist.conf . In this file, there is a comment, and two lines. Add a hash before the I2C line, to comment it out.

Original:

# blacklist spi and i2c by default (many users don't need them)

blacklist spi-bcm2708
blacklist i2c-bcm2708


Convert to this:

# blacklist spi and i2c by default (many users don't need them)

blacklist spi-bcm2708
#blacklist i2c-bcm2708

Step 3: Enable Kernel I2C Module

The next thing to do is add the I2C module to the kernel. Run the command sudo nano /etc/modules .You should see the following file:

# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.

snd-bcm2835


This should have the line i2c-devadded to the end.

Final file:

# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.

snd-bcm2835
i2c-dev

Step 4: Install Necessary Packages

There are a few packages that will need installing to use I2C. The first command to run is sudo apt-get install i2c-tools. If this fails, try running sudo apt-get update and try again, else run crying to your nearest nerd. The other package needed can be installed by running sudo apt-get install python-smbus.

To configure the software, we will add the Pi user to the I2C access group, by running the command sudo adduser pi i2c.
Now run sudo reboot to reboot, and test the new software.

To test the software, run the command i2cdetect -y 0 to see if there is anything connected. On my setup, it returned this output, because there was nothing connected:

0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Step 5: Example 1: CMPS03 Compass Module

We now have everything ready to start using I2C! 

To use the CMPS03 compass module, connect the power to V+ and 0V, from the Pi. I used the 5V line, which they recommend not doing because it might damage your pi, It worked for me, and has caused now damage, but I am not responsible if your's fries. 

Then, connect the SDA and SCL lines to the Pi SDA and SCL, and you are ready to roll. The wiring diagram is shown at http://www.robot-electronics.co.uk/htm/cmps3tech.htm. 

When you have connected it, run the command "i2cdetect -y 0". In my case, this returned: 

       0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: 60 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --


This shows that the module is on address 0x60.  You then need the following python file:

import smbus
import time
bus = smbus.SMBus(0)
address = 0x60

def bearing255():
        bear = bus.read_byte_data(address, 1)
        return bear

def bearing3599():
        bear1 = bus.read_byte_data(address, 2)
        bear2 = bus.read_byte_data(address, 3)
        bear = (bear1 << 8) + bear2
        bear = bear/10.0
        return bear

while True:
        bearing = bearing3599()     #this returns the value to 1 decimal place in degrees. 
        bear255 = bearing255()      #this returns the value as a byte between 0 and 255. 
        print bearing
        print bear255

        time.sleep(1)

This program should be saved as anything, but add ".py" on the end. Then, run the command with sudo python whateveryoucalledit.p and you should get values written to your screen in a long list. 

Step 6: SRF08 Range Sensor

The second example is the SRF08 range sensor, with built in light sensor. 

Wire it in in exactly the same way as before, with power, SDA and SCL connected to the Pi. I found that this sensor would not work off 3.3V, but again, I bear no responsibility for you putting 5V through your Pi pins. You can even leave the compass module in as well, because I2C can handle multiple devices on one line. The wiring diagram can be seen here: http://www.robot-electronics.co.uk/htm/srf08tech.shtml . 

Run i2cdetect -y 0  

0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: 60 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: 70 -- -- -- -- -- -- --


Note that I have left the compass module connected. 

You will then need the following python file. It is more complex, becuase you have to write a command to the sensor to get it to begin reading. 

import smbus
import time
bus = smbus.SMBus(0)
address = 0x70

#SRF08 REQUIRES 5V

def write(value):
        bus.write_byte_data(address, 0, value)
        return -1

def lightlevel():
        light = bus.read_byte_data(address, 1)
        return light

def range():
        range1 = bus.read_byte_data(address, 2)
        range2 = bus.read_byte_data(address, 3)
        range3 = (range1 << 8) + range2

        return range3

while True:
        write(0x51)
        time.sleep(0.7)
        lightlvl = lightlevel()
        rng = range()
        print lightlvl
        print rng



This will print the light level on the built in light sensor and the current range, in cm. 

Step 7: Conclusion

I hope you have found this instructable useful, as it should provide you with the code you need to get I2C working nicely. I spent a long time trying to fathom the Adafruit I2C Library out, before realising that these simple commands are all that I need. The basic read and write commands are functions in my provided code, so that should see you through. 

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    47 Discussions

    Thank you! I've been trouble shooting for almost an hour and was about to give up when i saw your post. I thought I had killed my BMP180 while soldering it or that it was broke for some other reason. =)

    the i2cdetect -y 0 command didn't do it for me.
    I2c-detect -y 1 however did

    smbus is not available in Python 3. Does anybody know an alternative?

    2 replies

    Hi Lorenzo, this may be far too late, but it may still be helpful for persons having the same issue as you. I followed the instructions on this link and got smbus working on python 3. http://jtecheng.com/?p=959#comment-925

    Hi Tallis,

    I appreciate your help, but I am not working at the moment with RPi.

    Thank you

    Hii your post was very helpfull but i have a big question how can i do if i have a pic? im starting from 0 how to work with the raspberry and i want to conect with a pic 16f886 slave.. and the rp as a master but when i do the conection dosent read the pic :( please help!

    1 reply

    Do you have access to an oscilloscope? I've always had a lot more luck understanding what's going on when I can see what they're sending back and fourth. I have some experience with both PICs and the Raspberry Pi, but I've noticed that the PIC freaks out a bit more readily than a Raspberry Pi with i2c. Make sure that the speeds are set up accordingly (although this shouldn't be a problem with i2c). Another thing is, you could just transmit things over GPIO pins on your own. What are you trying to do? i2c may not be the only thing that can accomplish the goal you want.

    Hii your post was very helpfull but i have a big question how can i do if i have a pic? im starting from 0 how to work with the raspberry and i want to conect with a pic 16f886 slave.. and the rp as a master but when i do the conection dosent read the pic :( please help!

    Now here is a question from a newb who wants to get into I2C on the Pi.

    The provided page (http://www.robot-electronics.co.uk/htm/srf08tech.html) says that this comes with default address 0xE0 and it is modifiable to 0xFE with increments of 0x02. But the pi only supports up to 0x78. How does that work?

    1 reply
    0
    None
    M0J01

    2 years ago

    Oh man, Thank you so much for this instructable! Your tutorial was short and to the point, while remaining sufficient for my applications.

    Thank you so much!

    I am just learning i2c, thanks a lot for your info, but for the price of the i2c rangefinders I think I will stick with HC-SR04 Ultrasound Wave Detector Range Ultrasonic Sensor Distance Module for just over $1 each and use i2c for the i2c sensors.

    thanks, very usefull. just wonder if i'm able to communicate with a pcf8574 or mcp23017 i/o expander now.