Used in the website article by Tom Kibalo, "Adding More Digital I/O to your 16-Bit Experimenter Kit - Part 1 The Hardware."
The MCP23S08 is an 18 pin DIP (perfect for solderless breadboard environments) and works with either 3.3V or 5V supplies. We’ll focus here on using this part with the Experimenter, but it can also be used with any microcontroller that has a Synchronous Peripheral Interface (SPI).
Its also a general purpose, parallel I/O expansion device that uses the SPI for 8 bit port read, 8 bit port write and port initialization. Its SPI can work at any clock rate up to 10 MHZ. The MCP23S08 SPI is a slave device that needs to get its synchronous clock from the microcontroller.