The I²C bus is not enabled in all Raspberry Pi images; therefore, we need to enable the module and install some supporting tools. Newer versions of Raspbian use device trees to handle hardware peripherals and drivers.

In order to make use of the I²C bus, we need to enable the ARM I²C in the bootconfig.txt file.

You can do this automatically using the following command:

sudo raspi-config

Select Advanced Options from the menu and then select I²C, as shown in the following screenshot. When asked, select Yes to enable the interface and then click Yes to load the module by default:

The raspi-config menu

From the menu, select I2C and select Yes to enable the interface and to load the module by default.

The raspi-config program enables the I2C_ARM interface by altering /boot/config.txt to include dtparam=i2c_arm=on. The other bus (I2C_VC) is typically reserved for interfacing with Raspberry Pi HAT add-on boards (to read the configuration information from the on-board memory devices); however, you can enable this using dtparam=i2c_vc=on.

If you wish, you can also enable the SPI using the raspi-config list, which is another type of bus.

Next, we should include the I²C module to be loaded upon turning the Raspberry Pi on, as follows:

sudo nano /etc/modules

Add the following on separate lines and save (Ctrl + X, Y, Enter):

i2c-dev
i2c-bcm2708

Similarly, we can also enable the SPI module by adding spi-bcm2708.

Next, we will install some tools to use I²C devices directly from the command line, as follows:

sudo apt-get update
sudo apt-get install i2c-tools

Finally, shut down the Raspberry Pi before attaching the hardware in order to allow the changes to be applied, as follows:

sudo halt

You will need a PCF8591 module (retailers of these are listed in the Appendix, Hardware and Software List) or you can obtain the PCF8591 chip separately and build your own circuit (see the There's more... section for details on the circuit):

The PCF8591 ADC and sensor module from dx.com

Connect the GND, VCC, SDA, and SCL pins to the Raspberry Pi GPIO header as follows:

I2C connections on the Raspberry Pi GPIO header

You can use the same I²C tools/code with other I²C devices by studying the datasheet of the device to find out what messages to send/read and which registers are used to control your device.