The devices on the I2C bus are either masters or slaves. The master is the device that is responsible for driving the SCL clock line, while the slaves are the devices that respond to the master. A slave cannot initiate a transfer over the I2C bus; this can only be done by master. Multiple masters and slaves can be used on the I2C bus; however there is normally only one master.
USING I2C AS MULTIPLE MASTERS/SLAVES BUS
This means that more than one device capable of controlling the bus can be connected to it. Microcontrollers are usually the master. The four operational modes are as follows: