The RESET input can be asserted to initialize the system while keeping the V_CC supply at its operating level. A reset can be accomplished by holding the RESET pin low for a minimum of t_W. The TCAL9539 registers and I²C/SMBus state machine are changed to their default state once RESET is low (0). When RESET is high (1), the I/O levels at the P port can be changed externally or through the controller. This input requires a pull-up resistor to V_CC, if no active connection is used. When RESET is toggled the input port register is updated to reflect the state of the GPIO pins.