Chapter 3: Operating in Real Time

This chapter addresses the need for embedded systems to generate real-time responses to inputs from sensors and other sources. The concepts of Real-Time Operating Systems (RTOSes) and their key features are introduced, as well as some challenges that commonly occur when implementing multitasking real-time applications. The chapter concludes with a discussion of the important characteristics of some popular open source and commercial RTOS implementations.

After completing this chapter, you will understand what it means for a system to operate in real time and will know the key attributes a real-time system must exhibit. You will understand the RTOS features that embedded systems rely upon, as well as some problems that frequently crop up in real-time embedded system designs. You will also have learned the key features of several popular RTOS implementations.

We will cover the following topics in this chapter:

• What does real-time mean?
...

Technical requirements

The files for this chapter are available at https://github.com/PacktPublishing/Architecting-High-Performance-Embedded-Systems.

What does real-time mean?

Real-time means computing with a deadline. In a real-time embedded system, the time it takes to respond to an input is a critical component of system performance. If the system produces a correct response, but the response is not generated within the required time limit, the effect may range from a mild nuisance to a catastrophic impact for a safety-related system.

The response of a real-time embedded system to inputs must be both correct and timely. Most standard software development approaches focus on the correctness of the response produced by a piece of code rather than being overly concerned with the timeliness of the response. Non-real-time software development approaches attempt to develop code that executes as quickly as possible, but usually do not provide a hard time limit specifying when the response must be provided. Real-time systems are considered to have failed if the timing constraints are not met, even during stressing and rare combinations...

Attributes of a real-time embedded system

The hardware and software of a real-time embedded system must exhibit some specific characteristics to ensure the system reliably meets its performance goals of producing reliably correct and timely outputs. Most real-time embedded systems that perform functions of moderate to high complexity must divide the processing work into multiple tasks that execute in an apparently (to the user) simultaneous manner, including managing the operation of hardware such as an automobile engine while regularly updating information displayed to the driver.

At the finest-grained level of processor operation, most embedded systems rely on the use of interrupts to notify the processor when an operation is required by an I/O device. In a real-time application, the handling of interrupts can become a critical factor in ensuring proper system operation. At the simplest level, any time an interrupt is being processed, the code algorithm that was paused to handle...

Understanding key RTOS features and challenges

Several standard capabilities are included in most of the RTOS implementations that are in wide use today. Some of these features enable efficient communication among tasks in a manner consistent with real-time operation. While common, not all of the following features are universally available in all RTOSes.

Mutexes

A mutex, which stands for mutual exclusion, is a mechanism for managing access to a shared resource among tasks. A mutex is conceptually identical to a global variable that can be read and written by all tasks. The variable has the value 1 when the shared resource is free, and 0 when it is in use by a task. When a task needs to gain access to the resource, it reads the variable and, if it is free, with the value 1, sets it to 0 to indicate the mutex is owned by a task. The task is then free to interact with the resource. When the interaction is complete, the task sets the mutex to 1, thereby releasing ownership.

If...

Popular real-time operating systems

When selecting an RTOS for a particular real-time embedded system architecture and application domain, it is important to consider a variety of technical and non-technical factors in the selection process. Almost all popular RTOSes support prioritized preemptive multitasking, mutexes, semaphores, queues, event flags, timers, and dynamic memory allocation. All of the RTOSes listed in this section include these features.

Some key technical attributes that differentiate among the various RTOSes are as follows:

• Feature richness: Some RTOSes are intended be as small as possible, consuming the absolute minimum quantity of ROM, RAM, and processor cycles in tiny microcontrollers. Other RTOSes are designed to support a large number of tasks and complex protocol stacks such as TCP/IP running on a 32-bit processor.
• Memory protection and virtual memory management: Simple microcontrollers and low-end microprocessors typically support only direct...

Summary

This chapter described the methods RTOSes use to ensure real-time responses to inputs. Key features available in common RTOSes were introduced, along with some challenges that commonly arise when implementing multitasking real-time applications. The chapter concluded with a listing of the key features of some popular open source and commercial RTOS implementations.

Having completed this chapter, you now understand what it means for a system to operate in real time and understand the key attributes a real-time embedded system must exhibit. You understand the RTOS features that embedded systems rely upon, as well as some challenges that frequently occur in real-time embedded system designs. You are also familiar with the key features of several popular RTOS implementations.

The next chapter introduces the concepts involved in the design of real-time embedded systems using FPGAs and works through a simple FPGA application example.