As we have seen, making use of the higher-level bus protocols allows us to connect to more complex hardware quickly and easily. The I2C can be put to great use by using it to expand the available I/O on the Raspberry Pi, as well as providing additional circuit protection (and, in some cases, additional power to drive more hardware).
There are lots of devices available that provide I/O expansion over the I2C bus (and also SPI), but the most commonly used is a 28-pin device, MCP23017, which provides 16 additional digital input/output pins. Being an I2C device, it only requires the two signals (SCL and SDA connections, plus ground, and power) and will happily function with other I2C devices ...
