16 GPIOs over two wires — the I²C bus expander
The PCA9555DW: Clocked at 400 kHz, it suits designs where the MCU has run out of pins but needs to monitor switches, drive LEDs, or control configuration straps on a remote board.
Interrupt and POR — two features that save firmware cycles
An interrupt output (INT) signals the host when an input pin changes state, so the MCU does not have to poll the expander continuously — useful for button panels or limit-switch arrays where latency matters. The power-on reset (POR) feature ensures all I/Os default to inputs at power-up, preventing output glitches before the host configures the direction register.
Output drive capability
For higher-current loads, the push-pull outputs can drive the base of an external transistor or a relay coil via a driver IC.