Introducing Real-Time Systems
Real-time systems come in a wide variety of implementations and use cases. This book focuses on how to use a real-time OS (RTOS) to create real-time applications on a microcontroller unit (MCU).
In this chapter, we'll start with an overview of what an RTOS is and get an idea of the wide range of systems that can have real-time requirements. From there, we'll look at some of the different ways of achieving real-time performance, along with an overview of the types of systems (such as hardware, firmware, and software) that may be used. We'll wrap up by discussing when it is advisable to use an RTOS in an MCU application and when it might not be necessary at all.
In a nutshell, we will cover the following topics in this chapter:
- What is "real-time" anyway?
- Defining RTOS
- Deciding when to use an RTOS
There are no software or hardware requirements for this chapter.
What is real-time anyway?
Any system that has a deterministic response to a given event can be considered "real-time." If a system is considered to fail when it doesn't meet a timing requirement, it must be real-time. How failure is defined (and the consequences of a failed system) can vary widely. It is extremely important to realize that real-time requirements can vary widely, both in the speed of the timing requirement and also the severity of consequences if the required real-time deadlines are not met.
The ranges of timing requirements
To illustrate the range of timing requirements that can be encountered, let's consider a few different systems that acquire readings from analog-to-digital converters...
OSes (such as Windows, Linux, and macOS) were created as a way to provide a consistent programming environment that abstracted away the underlying hardware to make it easier to write and maintain computer programs. They provide the application programmer with many different primitives (such as threads and mutexes) that can be used to create more complex behavior. For example, it is possible to create a multi-threaded program that provides protected access to shared data:
The preceding application doesn't implement thread and mutex primitives, it only makes use of them. The actual implementations of threads and mutexes are handled by the OS. This has a few advantages:
- The application code is less complex.
- It is easier to understand—the same primitives are used regardless of the programmer, making it easier to understand code created by different people...
Deciding when to use an RTOS
Occasionally, when someone first learns of the term real-time OS, they mistakenly believe that an RTOS is the only way to achieve real-time behavior in an embedded system. While this is certainly understandable (especially given the name) it couldn't be further from the truth. Sometimes, it is best to think of an RTOS as a potential solution, rather than the solution to be used for everything. Generally speaking, for an MCU-based RTOS to be the ideal solution for a given problem, it needs to have a Goldilocks-level of complexity—not too simple, but not too complicated.
If there is an extremely simple problem, such as monitoring two states and triggering an alert when they are both present, the solution could be a straightforward hardware solution (such as an AND gate). In this case, there may be no reason to complicate things further, since...
In this chapter, we've covered how to identify real-time requirements, as well as the different platforms available for implementing real-time systems. At this point, you should have an appreciation for both the wide range of systems that can have real-time requirements, as well as the variety of ways there are to meet those real-time requirements.
In the next chapter, we'll start digging into MCU-based real-time firmware by taking a closer look at two different programming models—super loops and RTOS tasks.
As we conclude, here is a list of questions for you to test your knowledge with on this chapter's material. You will find the answers in the Assessments section of the appendix:
- Does a system with real-time requirements always need to be extremely fast?
- Is an RTOS always required for real-time systems?
- Is firmware the only way to satisfy real-time requirements?
- What is a real-time system?
- Name 3–4 types of real-time systems.
- When is it appropriate to use an RTOS to meet real-time requirements?