The SMBus alert response provides a quick identification for which device issued the interrupt. Without this alert response capability, the processor does not know which device pulled the interrupt line when there are multiple target devices connected.

OPT4001 is designed to respond to the SMBus alert response address, when in the latched window-style comparison mode. The OPT4001 does not respond to the SMBus alert response when in transparent mode.

The response behavior of the device to the SMBus alert response is shown in Figure 8-9. When the interrupt line to the processor is pulled to active, the controller can broadcast the alert response target address. Following this alert response, any target devices that generated an alert identify themselves by acknowledging the alert response and sending respective I²C address on the bus. The alert response can activate several different target devices simultaneously. If more than one target attempts to respond, bus arbitration rules apply. The device with the lowest address wins the arbitration. If the OPT4001 loses the arbitration, the device does not acknowledge the I²C transaction and the INT pin remains in an active state, prompting the I²C controller processor to issue a subsequent SMBus alert response. When the OPT4001 wins the arbitration, the device acknowledges the transaction and sets the INT pin to inactive. The controller can issue that same command again, as many times as necessary to clear the INT pin. See Section 8.4.2 for additional details of how the flags and INT pin are controlled. The controller can obtain information about the source of the OPT4001 interrupt from the address broadcast in the above process. The FLAG_H value is sent as the final LSB of the address to provide the controller additional information about the cause of the OPT4001 interrupt. If the controller requires additional information, the result register or the configuration register can be queried. The FLAG_H and FLAG_L fields are not cleared upon an SMBus alert response.

A. FH is the FLAG_H register

B. A1 and A0 are determined by the ADDR pin (only on SOT-5X3 version)

Figure 8-9 Timing Diagram for SMBus Alert Response