Hi,
The Master requests data from the slave.
Understanding the I2C Bus (125.3 KB)
2.1 General I2C Operation
The I2C bus is a standard bidirectional interface that uses a controller, known as the master, to
communicate with slave devices. A slave may not transmit data unless it has been addressed by the master.
Each device on the I2C bus has a specific device address to differentiate between other devices that are on the same I2C bus. Many slave devices will require configuration upon startup to set the behavior of the device.
This is typically done when the master accesses the slave's internal register maps,
which have unique register addresses. A device can have one or multiple registers where data is stored, written, or read.
Tom... 
