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This 8-bit I/O expander for the two-line bidirectional bus (I²C) is designed for 2.3-V to 5.5-V V_CC operation. The device provides general-purpose remote I/O expansion for most microcontroller families via the I²C interface [serial clock (SCL) and serial data (SDA)].

The PCA9557 consists of one 8-bit configuration (input or output selection), input port, output port, and polarity inversion (active-high) registers. At power on, the I/Os are configured as inputs. However, the system master can enable the I/Os as either inputs or outputs by writing to the I/O configuration bits. The data for each input or output is kept in the corresponding input or output register. The polarity of the input port register can be inverted with the polarity inversion register. All registers can be read by the system master.

The device outputs (latched) have high-current drive capability for directly driving LEDs. The device has low current consumption.

The system master can reset the PCA9557 in the event of a timeout or other improper operation by asserting a low in the active-low reset (RESET) input. The power-on reset puts the registers in their default state and initializes the I²C/SMBus state machine. Asserting RESET causes the same reset/initialization to occur without depowering the part.

Three hardware pins (A0, A1, and A2) are used to program and vary the fixed I²C address, allowing up to eight devices to share the same I²C bus or SMBus.