Designing Embedded Hardware, 2nd Edition

Chapter 8. Adding Peripherals Using I²C

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.

—Antoine de Saint-Exupéry

In the last chapter, we looked at the low-cost SPI interface used to connect peripheral chips to microcontrollers. In this chapter, we'll examine the alternate serial interface for connecting peripherals, I²C.

Overview of I²C

I ² C (Inter-Integrated Circuit) bus is a very cheap yet effective network used to interconnect peripheral devices within small-scale embedded systems. It is sometimes also known as IIC and has been in existence for more than 20 years. It is the equivalent of SPI, but its operation is somewhat different.

I²C uses two wires to connect multiple devices in a multi-drop bus. The bus is bidirectional, low-speed, and synchronous to a common clock. Devices may be attached or detached from the I²C bus without affecting other devices. Several manufacturers, such as Microchip, Philips, Intel, and others produce small microcontrollers with I²C built in. The data rate of I²C is somewhat slower than SPI, at 100 kbps in standard mode, and 400 kbps in fast mode.

The two wires used to interconnect with I²C are SDA (serial data) and SCL (serial clock). Both lines are open-drain.^[*] They are connected to a positive supply via a pull-up resistor and therefore remain high when not in use. A device using the I²C bus to communicate drives the lines low or leaves them pulled high as appropriate. Each device ...