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 pins
MCP23S08 – SPI interface, two address pinsThe 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.

8-bit port expander with interrupt output
2 address inputs for up to 4 devices on one bus
25mA sink/source capability per I/O
Supports up to 10MHz SPI™ clock speeds
Internal power-on-reset (POR)