It provides 16 configurable push-pull I/O pins in a compact 24-SOIC package, saving microcontroller GPIOs on dense boards. The part includes a power-on reset (POR) feature that initializes all I/O as inputs, and an interrupt output pin signals the host when an input changes state — useful for keeping the main processor in a low-power sleep mode until a button press or sensor edge occurs.
Supply range and I/O drive — sizing the rail and the load
The total package dissipation limits how many pins can run at full drive simultaneously; derate per the datasheet's thermal curves above 85°C ambient.
Interrupt output — why it matters for low-power designs
The interrupt output (INT) is a key feature for battery-operated or power-conscious systems. Instead of the host MCU polling the expander over I²C to check for input changes, the INT pin pulls low when any input configured as an interrupt source changes state. The host can stay in a deep-sleep mode and wake only on the interrupt edge, saving significant quiescent current. The interrupt is cleared by reading the input port register, so the I²C transaction is minimal.
The base product number PCA9555 covers multiple package variants (SOIC, TSSOP, QFN), so if the 24-SOIC footprint is a constraint, the PW (TSSOP) or RGY (QFN) siblings are pin-compatible and share the same I²C address map and register set.