Microchip’s MCP23017 provides 16 additional GPIO pins that can be purchased for as little as $1.99. The chip communicates using the I2C bus. (The companion MCP23S17 is available for SPI bus.) The I2C bus allows the chip to be remote from the Raspberry Pi, requiring only a four-wire ribbon cable (power, ground, and a pair of I2C bus lines). This chapter explores the features and limits of this peripheral.