When power (from 0 V) is applied to V_CC, an internal power-on reset holds the TCA9539-Q1 in a reset condition until V_CC has reached V_PORR. At that point, the reset condition is released and the TCA9539-Q1 registers and I²C-SMBus state machine initialize to their default states. After that, V_CC must be lowered to V_PORF and then back up to the operating voltage for a power-reset cycle. See Figure 8-3.