Why Timing Matters: The Role of RTOS in Advanced Embedded Systems

Table of contents

What is RTOS, and why is it so useful?

Where can you use RTOS?

How to choose RTOS, bare metal or Linux RT?

How does RTOS work?

Let’s сompare: Zephyr RTOS vs. FreeRTOS

Case studies
 

An RTOS is a type of operating system designed to manage tasks that must be completed within a strict time frame. General-purpose operating systems (Windows or MacOS) prioritise multitasking and user interaction, whereas an RTOS focuses on completing tasks exactly when needed. The RTOS ensures this timely execution of operations, helping to maintain system efficiency. 

An RTOS was created to manage hardware resources, perform activities within specific time restrictions, and guarantee that key procedures are finished on time. Here are some of its main features:

• Deterministic behavior. RTOS provides predictable response times, meaning that tasks will consistently be completed within a specified timeframe.

• Priority-based scheduling. Tasks are assigned priority levels: high-priority tasks are executed before lower-priority ones.

• Multitasking. An RTOS can handle multiple tasks simultaneously and switch between them based on their priorities for optimal resource utilisation.

• Fast dispatch latency. Switching time from one task to another is minimised in embedded real-time systems, enabling quick responses to events.

• Memory management. RTOS includes efficient memory management techniques to allocate and deallocate memory dynamically, prevent memory leaks and ensure that tasks have access to the necessary resources.

• Interrupt handling. An RTOS can respond to external events and interrupt quickly.

• Small footprint. Compared to general-purpose operating systems, RTOS typically occupies less memory and consumes fewer resources.

• Task synchronisation mechanisms. RTOS provides semaphores, mutexes, and message queues to coordinate task execution and resource sharing among multiple processes without conflicts.

• Modular design. Many RTOSs support modular development approaches for creating independent modules that can be tested individually before integration into the larger system. 
   

RTOS are categorised based on timing constraints and the consequences of missing deadlines. Here are the three different types of RTOS and their typical applications: 
 

Hard RTOS 

These systems have strict timing requirements, where tasks must be completed within a defined time frame. Missing a deadline can lead to catastrophic failures. It is commonly used in critical applications such as medicine (pacemakers and infusion pumps), aerospace (flight control systems and avionics), and industrial automation (robotics and manufacturing systems that rely on precise timing for operations). 

Firm RTOS

This type also has deadlines that need to be met, but occasional missed deadlines do not result in severe consequences. However, missing deadlines can degrade system performance. You can find firm RTOS in applications where timely responses, such as multimedia and telecommunications, are important but not critical. 
 

Soft RTOS

These systems provide a more relaxed approach to timing constraints. Tasks can miss deadlines without a significant impact on system functionality. It is suitable for applications where timing is less critical, including consumer electronics (smart home devices and appliances) and general computing multimedia systems.  

We encounter devices using RTOS in our daily lives, such as smart thermostats, fitness trackers, and game consoles. However, the application of these systems extends far beyond these familiar examples, covering a wide range of industries and technologies. 

The system is essential for controlling avionics systems, satellite operations, drones and radar systems in the aerospace sector. Its deterministic behavior ensures critical safety and mission success tasks are executed within strict time constraints. 

In the automotive industry, RTOS supports ADAS (advanced driver assistance systems), including lane departure warnings and collision avoidance, where real-time data processing is used for passenger safety and vehicle performance. 

The healthcare and medical device industry relies on the system for devices like infusion pumps, respiratory systems, and patient monitors. Real-time processing ensures timely responses for patient safety and treatment accuracy. 

In industrial automation, RTOS is used in PLCs (programmable logic controllers) to provide precise control and synchronisation of processes. 

In telecommunications, RTOS manages network protocols and ensures real-time data transfers in video conferencing, online gaming, and streaming, where low latency is crucial for smooth communication. 

In finance and retail, particularly in high-frequency trading, RTOS helps systems react instantly to market changes, though some use non-RTOS solutions that still require real-time processing principles. 

Embedded systems across various industries leverage RTOS for coordination of complex operations. In robotics, it allows mechanisms to respond dynamically to changing environments. RTOS in embedded systems also provides reliable performance for smart thermostats and automotive control systems. 

The choice between bare metal, RTOS and Linux RT depends on the specific requirements of your project, including real-time performance, hardware constraints and complexity. 

RTOS is ideal when real-time precision and deterministic responses are essential. It excels at prioritising high-priority tasks and enables real-time responses to sensor inputs, interrupts and data processing. For example, an RTOS would control flight operations such as balance, sensor input management and motor adjustments in a drone.  

Bare metal programming is best suited to applications that require direct access to hardware and minimal overhead. This approach is common for simple, low-level embedded systems with limited resources, such as small IoT devices or microcontroller-based applications. A temperature sensor in an IoT network that only needs to send temperature readings periodically is an ideal candidate for bare metal programming. 

Linux RT is better for those needing Linux's power and flexibility with real-time capabilities. Adding real-time patches to the standard Linux kernel supports tasks with real-time constraints while maintaining Linux's full functionality. It's perfect for high-performance, multitasking applications that require real-time responses, such as multimedia servers managing live video streams with smooth encoding, networking, and storage. 

The most important function of an RTOS is its scheduling mechanism. RTOSs use a priority-based scheduling algorithm to assign tasks of different priorities based on their importance. For example, in an airbag system of a car, if a sensor detects a collision, deploying the airbag must take priority over all other ongoing processes, which is exactly what the RTOS guarantees. 

RTOSs use two main types of scheduling: preemptive and cooperative. In preemptive scheduling, the system can interrupt a lower-priority task if a higher-priority task becomes ready to run. In contrast, cooperative scheduling requires tasks to voluntarily yield control back to the operating system, which is less common in real-time applications due to the possibility of delays. 

Also, RTOS could manage interrupts. When external events, such as sensor inputs, occur, the system can immediately pause the current task to handle the interrupt. This feature ensures the system can react instantly to critical events, such as stopping a machine when an emergency button is pressed. 

Memory management in an RTOS is also optimised for real-time tasks. This system typically uses static memory allocation to ensure no delays occur due to memory management during execution. It is essential for systems that cannot afford even microsecond delays. 

In addition to task scheduling, an RTOS often provides inter-task communication mechanisms, such as message queues, semaphores, and mutexes. These allow tasks and threads to communicate and synchronise efficiently without causing delays. 

Zephyr RTOS and FreeRTOS are two real-time operating systems commonly used in embedded systems developed by Promwad engineers. Both have certain advantages and are suitable for various applications, but they also have their specific characteristics. 
 

Zephyr RTOS

Zephyr RTOS, developed by the Linux Foundation, is a highly modular and scalable real-time operating system tailored for resource-constrained devices. Zephyr supports ARM, x86, RISC-V, ARC, and other architectures as an open-source solution. It is perfect for complex, multi-threaded applications that demand robust networking capabilities, strong security features, and modularity. 

One of Zephyr's key strengths is its configurability, thanks to the Kconfig system, which allows developers to customise this RTOS by selecting only the features they need. In addition, Zephyr offers secure boot and TrustZone, it is essential for secure IoT devices. The system also provides extensive networking support with TCP/IP, 6LoWPAN, and Bluetooth. 

Zephyr supports multi-core systems with native multi-processing capabilities and is suitable for complex applications. Moreover, its ecosystem is expanding rapidly, with backing from major industry players, leading to widespread adoption in IoT, industrial automation, and wearables. 

Major chip vendors, including Intel, NXP, Nordic Semiconductor, and STMicroelectronics, support Zephyr. Nordic Semiconductor uses Zephyr for its nRF52 and nRF91 series of SoCs, which are popular for Bluetooth and cellular IoT devices. Intel supports Zephyr for its Quark microcontrollers, which are used in low-power IoT solutions. 

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