BeagleBone Cookbook

Chapter 8. Real-Time I/O

8.0 Introduction

Sometimes, when BeagleBone Black interacts with the physical world, it needs to respond in a timely manner. For example, your robot has just detected that one of the driving motors needs to turn a bit faster. Systems that can respond quickly to a real event are known as real-time systems. There are two broad categories of real-time systems: soft and hard.

In a soft real-time system, the real-time requirements should be met most of the time, where most depends on the system. A video playback system is a good example. The goal might be to display 60 frames per second, but it doesn’t matter much if you miss a frame now and then. In a 100 percent hard real-time system, you can never fail to respond in time. Think of an airbag deployment system on a car. You can’t even be 50 ms late.

Systems running Linux generally can’t do 100 percent hard real-time processing, because Linux gets in the way. However, the Bone has an ARM processor running Linux and two additional 32-bit programmable real-time units (PRUs [http://bit.ly/1EzTPZv]) available to do real-time processing. Although the PRUs can achieve 100 percent hard real-time, they take some effort to use.

This chapter shows several ways to do real-time input/output (I/O), starting with the effortless, yet slower BoneScript and moving up with increasing speed (and effort) to using the PRUs.

Note

In this chapter, as in the others, we assume that you are logged in as root (as indicated by ...