During a recent project, I needed several switches for setting control values. The AVRs did not have enough I/O pins, so I had to think of something. I could have tried a complex input system with a keyboard and display, but the ATtiny2313 would have run out of resources. Fortunately, Atmel has provided a way around this problem by including an interface that can link to additional chips (such as memory or I/O ports) with a simple two wire interface. That's right, by using just two I/O pins on an AVR we can access many additional I/O pins, and other resources as well.
This two wire interface is formally known as the Inter-Integrated Circuit bus, or just the I2C bus and was invented by NXP when it was still Philips Semiconductors. If you're reading this Instructable then you've probably heard of the I2C bus and may even have used it on a PIC or other microcontroller. While conceptually very simple, and supported by hardware resources on the AVRs, software drivers are still necessary to use the I2C bus. Atmel provides Application Notes (see the Resources later in this Instructable), but these are incomplete and don't show any examples beyond communicating with another AVR device.
It is not the purpose of this Instructable to teach anyone how to create I2C drivers for the AVRs. Rather, I'll provide expanded versions of the Atmel drivers for ATtiny2313 and ATmega168 devices, I'll explain the requirements and restrictions that apply when using these, and I'll show you working examples of I2C devices. After you work through this Instructable you'll be able to use the I2C bus successfully in your AVR projects. Obviously, you can ignore the drivers for either tiny or MEGA if you're only interested in one of them. For those interested in learning more about the I2C bus, I'll provide links to appropriate material.