Thirty spokes meet at a nave;
Because of the hole we may use the wheel.
Clay is molded into a vessel;
Because of the hollow we may use the cup.
Walls are built around a hearth;
Because of the doors we may use the house.
Thus tools come from what exists,
But use from what does not.—Lao Tse, Tao Te Ching
In this chapter and the next, we’ll look at two low-cost interfaces used to connect peripheral chips to microcontrollers, within a single embedded system. These interfaces allow you to connect devices such as real-time clocks, nonvolatile memories for parameter storage, sensor interfaces, and much more. The interfaces are easy to use and cheap to implement, making them ideal for small, embedded applications. Some microcontrollers incorporate both types of interface, whereas others may only have one or the other. The one to use really depends on what your processor has to offer and the requirements of the particular peripheral you’re using.
The Serial Peripheral Interface (known as SPI) was developed by Motorola to provide a low-cost and simple interface between microcontrollers and peripheral chips. (SPI is sometimes also known as a four-wire interface.) It can be used to interface to memory (for data storage), analog-digital converters, digital-analog converters, real-time clock calendars, LCD drivers, sensors, audio chips, and even other processors. The range of components that support SPI is ...