SBOS473I March 2009 – December 2018 TMP112
The device that initiates the transfer is called a master, and the devices controlled by the master are slaves. The bus must be controlled by a master device that generates the serial clock (SCL), controls the bus access, and generates the START and STOP conditions.
To address a specific device, a START condition is initiated, indicated by pulling the data-line (SDA) from a high- to low-logic level when the SCL pin is high. All slaves on the bus shift in the slave address byte on the rising edge of the clock, with the last bit indicating whether a read or write operation is intended. During the ninth clock pulse, the slave being addressed responds to the master by generating an acknowledge and pulling the SDA pin low.
A data transfer is then initiated and sent over eight clock pulses followed by an acknowledge bit. During the data transfer the SDA pin must remain stable when the SCL pin is high, because any change in the SDA pin when the SCL pin is high is interpreted as a START or STOP signal.
When all data have been transferred, the master generates a STOP condition indicated by pulling the SDA pin from low to high when the SCL pin is high.