When power (from 0 V) is applied to V_CC, an internal power-on reset holds the TCAL9539 in a reset condition until the supply has reached V_POR. At that time, the reset condition is released, and the TCAL9539 registers and I²C/SMBus state machine initializes to their default states. After that, V_CC must be lowered to below V_PORF and back up to the operating voltage for a power-reset cycle.