The MCP23X17 consists of multiple 8-bit configuration registers for input, output and polarity selection. The system master can enable the I/Os as either inputs or outputs by writing the I/O configuration bits (IODIRA/B). 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 16-bit I/O port functionally consists of two 8-bit ports (PORTA and PORTB).

16-bit input/output port expander with interrupt output
Cascadable for up to 8 devices on one bus
25mA sink/source capability per I/O
Supports 100kHz, 400kHz and 1.7MHz I2C™Compatible compatible modes