The I²C bus employs two signals, SDA (data) and SCL (clock), to communicate between integrated circuits in a system using serial data transmission. The address and data 8-bit bytes are transferred MSB first. In addition, each byte transferred on the bus is acknowledged by the receiving device with an acknowledge bit. Each transfer operation begins with the master device driving a start condition on the bus and ends with the master device driving a stop condition on the bus. The bus uses transitions on the data pin (SDA) while the clock is at logic high to indicate start and stop conditions. A high-to-low transition on SDA indicates a start, and a low-to-high transition indicates a stop. Normal data-bit transitions must occur within the low time of the clock period.

The master device drives a start condition followed by the 7-bit slave address and the read/write (R/W) bit to open communication with another device and then waits for an acknowledgment condition. The slave device holds SDA low during the acknowledge clock period to indicate acknowledgment. When this occurs, the master device transmits the next byte of the sequence. Each slave device is addressed by a unique 7-bit slave address plus the R/W bit (1 byte). All compatible devices share the same signals via a bidirectional bus using a wired-AND connection.

There is no limit on the number of bytes that can be transmitted between start and stop conditions. When the last word transfers, the master device generates a stop condition to release the bus. Figure 8-66 shows a generic data transfer sequence.

Figure 8-66 Typical I²C Sequence

In the system, use external pullup resistors for the SDA and SCL signals to set the logic high level for the bus. The SDA and SCL voltages must not exceed the device supply voltage, IOVDD.