The OPT3004 offers compatibility with both I²C and SMBus interfaces. The I²C and SMBus protocols are essentially compatible with one another. The I²C interface is used throughout this document as the primary example with the SMBus protocol specified only when a difference between the two protocols is discussed.

The OPT3004 is connected to the bus with two pins: an SCL clock input pin and an SDA open-drain bidirectional data pin. The bus must be controlled by a controller device that generates the serial clock (SCL), controls the bus access, and generates start and stop conditions. To address a specific device, the controller initiates a start condition by pulling the data signal line (SDA) from a high logic level to a low logic level while SCL is high. All targets on the bus shift in the target address byte on the SCL rising edge, with the last bit indicating whether a read or write operation is intended. During the ninth clock pulse, the target being addressed responds to the controller by generating an acknowledge bit by pulling SDA low.

Data transfer is then initiated and eight bits of data are sent, followed by an acknowledge bit. During data transfer, SDA must remain stable while SCL is high. Any change in SDA while SCL is high is interpreted as a start or stop condition. When all data are transferred, the controller generates a stop condition, indicated by pulling SDA from low to high while SCL is high. The OPT3004 includes a 28-ms timeout on the I²C interface to prevent locking up the bus. If the SCL line is held low for this duration of time, the bus state machine is reset.