The MCP23X08 device provides 8-bit, general purpose, parallel I/O expansion for I2C bus or SPI applications. The two devices differ in the number of hardware address pins and the serial interface: MCP23008 – I2C interface, three address pinsMCP23S08 – SPI interface, two address pins
The MCP23X08 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. 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.

Additional Features:
8-bit port expander with interrupt output
3 address inputs for up to 8 devices on one bus
25mA sink/source capability per I/O
Supports 100kHz,400kHz and 1.7MHz I2C™Compatible modes
Internal power-on-reset (POR)