Hands-On RTOS with Microcontrollers

Building real-time embedded systems using FreeRTOS, STM32 MCUs, and SEGGER debug tools

Brian Amos

• Publisher: Packt Publishing
• Year: 2020
• ISBN:
  9781838829285
• Topic: Informatique
• Subtopic: Matériel

Build a strong foundation in designing and implementing real-time systems with the help of practical examples

Key Features

• Get up and running with the fundamentals of RTOS and apply them on STM32
• Enhance your programming skills to design and build real-world embedded systems
• Get to grips with advanced techniques for implementing embedded systems

Book Description

A real-time operating system (RTOS) is used to develop systems that respond to events within strict timelines. Real-time embedded systems have applications in various industries, from automotive and aerospace through to laboratory test equipment and consumer electronics. These systems provide consistent and reliable timing and are designed to run without intervention for years.This microcontrollers book starts by introducing you to the concept of RTOS and compares some other alternative methods for achieving real-time performance. Once you've understood the fundamentals, such as tasks, queues, mutexes, and semaphores, you'll learn what to look for when selecting a microcontroller and development environment. By working through examples that use an STM32F7 Nucleo board, the STM32CubeIDE, and SEGGER debug tools, including SEGGER J-Link, Ozone, and SystemView, you'll gain an understanding of preemptive scheduling policies and task communication. The book will then help you develop highly efficient low-level drivers and analyze their real-time performance and CPU utilization. Finally, you'll cover tips for troubleshooting and be able to take your new-found skills to the next level.By the end of this book, you'll have built on your embedded system skills and will be able to create real-time systems using microcontrollers and FreeRTOS.

What you will learn

• Understand when to use an RTOS for a project
• Explore RTOS concepts such as tasks, mutexes, semaphores, and queues
• Discover different microcontroller units (MCUs) and choose the best one for your project
• Evaluate and select the best IDE and middleware stack for your project
• Use professional-grade tools for analyzing and debugging your application
• Get FreeRTOS-based applications up and running on an STM32 board

Who this book is for

This book is for embedded engineers, students, or anyone interested in learning the complete RTOS feature set with embedded devices. A basic understanding of the C programming language and embedded systems or microcontrollers will be helpful.

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Section 1: Introduction and RTOS Concepts

What is a real-time system and what are the major components that make up a real-time operating system (RTOS)? These are the questions we'll be answering in the first section of this book. This prerequisite knowledge will serve as a foundation we'll build upon with working examples and hands-on exercises in later chapters. If you're already familiar with another RTOS, you can probably skim or skip this section.

This section comprises the following chapters:

• Chapter 1, Introducing Real-Time Systems
• Chapter 2, Understanding RTOS Tasks
• Chapter 3, Task Signaling and Communication Mechanisms

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

Technical requirements

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 (ADCs).

The first system we'll look at is a control system that is set up to control the temperature of a soldering iron (as seen in the following diagram). The parts of the system we're concerned with are the MCU, ADC, sensor, and heater.

The MCU is responsible for the following:

• Taking readings from a temperature sensor via the ADC
• Running a closed-loop control algorithm (to maintain a constant temperature at the soldering iron tip)
• Adjusting the output of the heater as needed

These can be seen in the following diagram:

Since the temperature of the tip doesn't change incredibly quickly, the MCU may only need to acquire 50 ADC samples per second (50 Hz). The control algorithm responsible for adjusting the heater (to maintain a constant temperature) runs at an even slower pace, 5 Hz:

The ADC will assert a hardware line, signaling a conversion has been completed and is ready for the MCU to transfer the reading to its internal memory. The MCU reading the ADC has up to 20 ms to transfer data from the ADC to internal memory before a new reading needs to be taken (as seen in the following diagram). The MCU also needs to be running the control algorithm to calculate the updated values for the heater output at 5 Hz (200 ms). Both of these cases (although not particularly fast) are examples of real-time requirements:

Now, on the other end of the ADC reading spectrum, we could have a high bandwidth network analyzer or oscilloscope that is going to be reading an ADC at a rate of tens of GHz! The raw ADC readings will likely be converted into the frequency domain and graphically displayed on a high-resolution front panel dozens of times a second. A system like this requires huge amounts of processing to be performed and must adhere to extremely tight timing requirements, if it is to function properly.

Somewhere in the middle of the spectrum, you'll find systems such as closed-loop motion controllers, which will typically need to execute their PID control loops between hundreds of Hz to tens of kHz in order to provide stability in a fast-moving system. So, how fast is real-time? Well, as you can see from the ADC examples alone, it depends.

In some of the previous cases, such as the oscilloscope or soldering iron, failure to meet a timing requirement results in poor performance or incorrect data being reported. In the case of the soldering iron, this might be poor temperature control (which could cause damage to components). For the test equipment, missing deadlines could cause erroneous readings, which is a failure. This may not seem like a big deal to some people, but for the users of that equipment, who are relying on the accuracy of the data being reported, it is likely to matter a great deal. Some laboratory equipment that is used in standard verification provides checks for product conformance. If there is an undetected malfunction in the equipment that results in an inaccurate measurement, an incorrect value could be reported. It may be possible for a suspect test to be rerun. Eventually, however, if retesting is required too often and reliable readings can't be counted on, then the test equipment will start to become suspect and viewed as unreliable and sales will decline—all because a real-time requirement wasn't being consistently met.

In other systems, such as the flight control of a UAV or motion control in industrial process control, failing to run the control algorithm in a timely manner could result in something more physically catastrophic, such as a crash. In this case, the consequences are potentially life-threatening.

Thankfully, there are steps that can be taken to avoid all of these failure scenarios.

The ways of guaranteeing real-time behavior

One of the easiest ways to ensure a system does what it is meant to do is to make sure it is as simple as possible while still meeting the requirements. This means resisting the urge to over-complicate a simple task. If a toaster is meant to toast a slice of bread, don't put a display on it and make it tell you the weather too; just have it turn on a heating element for the right amount of time. This simple task has been accomplished for years without requiring any code or programmable devices whatsoever.

As programmers, if we come across a problem, we have a tendency to immediately reach for the nearest MCU and start coding. However, some functions of a product (especially true if a product has electro-mechanical components) are best handled without code at all. A car window doesn't really need an MCU with a polling loop to run, turning on motors through drivers and watching sensors for feedback to shut them off. This task can actually be handled by a few mechanical switches and diodes. If a feedback-reporting mechanism is required for a given system—such as an error that needs to be asserted in the case of a stuck window—then there may be no choice but to use a more complex solution. However, our goal as engineers should always be the same—solve the problem as simply as possible, without adding additional complexity.

If a problem can be solved by hardware alone, then explore that possibility with the team first, before breaking out the MCU. If a problem can be handled by using a simple while loop to perform some polling of the sensor status, then simply poll the sensor for the status; there may be no need to start coding interrupt service routines (ISRs). If the functionality of the device is single-purposed, there are many cases where a full-blown RTOS can simply get in the way—so don't use one!

Types of real-time systems

There are many different ways of achieving real-time behavior. The following section is a discussion on the various types of real-time systems you might encounter. Also note that it is possible to have combinations of the following systems working together as subsystems. These different subsystems can occur at a product, board, or even chip level (this approach is discussed in Chapter 16, Multi-Processor and Multi-Core Systems).

Hardware

The original real-time system, hardware, is still the go-to for extremely tight tolerance and/or fast timing requirements. It can be implemented with discrete digital logic, analog components, programmable logic, or an application-specific integrated component (ASIC). Programmable logic devices (PLDs), complex programmable logic devices (CPLDs), and field-programmable gate arrays (FPGAs) are the various members of the programmable logic device portion of this solution. Hardware-based real-time systems can cover anything from analog filters, closed loop control, and simple state machines to complex video codecs. When implemented with power saving in mind, ASICs can be made to consume less power than an MCU-based solution. In general, hardware has the advantage of performing operations in parallel and instantly (this is, of course, an over-simplification), as opposed to a single-core MCU, which only gives the illusion of parallel processing.

The downsides for real-time hardware development generally include the following:

• The inflexibility of non-programmable devices.
• The expertise required is generally less commonly available than software/firmware developers.
• The cost of full-featured programmable devices (for example, large FPGAs).
• The high cost of developing a custom ASIC.

Bare-metal firmware

Bare-metal firmware is considered (for our purposes) to be any firmware that isn't built on top of a preexisting kernel/scheduler of some type. Some engin...