This guide assumes that release 2015-01-31 or later of the Raspbian Operating System is being used.
An I2C bus is broken out to pins 3 (SDA) and 5 (SCL) on the P1 header. The
number of steps that need to be performed to configure this I2C bus for usage
by user pi on Raspbian without root privileges is highly dependent in the
version of Raspbian being used.
With Raspbian Jessie 2015-11-21 or later the complete configuration can be
performed with the raspi-config software configuration tool which can be run
from a terminal window as follows:
sudo raspi-config
In the raspi-config user interface navigate to Interfacing Options >> I2C
and answer the question "Would you like the ARM I2C interface to be enabled?"
with <Yes>. After the next reboot user pi will be able to use the I2C bus
without root privileges.
On older versions of Raspbian (prior to Raspbian Jessie 2015-11-21) the
raspi-config tool can still be used to configure the I2C bus, but additional
steps typically need to be performed.
To enable I2C ensure that /boot/config.txt contains the following line:
dtparam=i2c_arm=on
To enable userspace access to I2C ensure that /etc/modules contains the
following line:
i2c-dev
The default I2C baudrate is 100000. If required, this can be changed with the
i2c_arm_baudrate parameter. For example, to set the baudrate to 400000, add
the following line to /boot/config.txt:
dtparam=i2c_arm_baudrate=400000
If release 2015-05-05 or later of the Raspbian Operating System is being used,
this step can be skipped as user pi can access the I2C bus without root
privileges.
If an earlier release of the Raspbian Operating System is being used, create a
file called 99-i2c.rules in directory /etc/udev/rules.d with the following
content:
SUBSYSTEM=="i2c-dev", MODE="0666"
This will give all users access to I2C and sudo need not be specified when executing programs using i2c-bus. A more selective rule should be used if required.
After performing the above steps, reboot the Raspberry Pi.