What is an RTOS?
A Real Time Operating System, commonly known as an RTOS, is a software component that rapidly switches between tasks, giving the impression that multiple programs are being executed at the same time on a single processing core.
In actual fact the processing core can only execute one program at any one time, and what the RTOS is actually doing is rapidly switching between individual programming threads (or Tasks) to give the impression that multiple programs are executing simultaneously.
OS or RTOS?
The difference between an OS (Operating System) such as Windows or Unix and an RTOS (Real Time Operating System) found in embedded systems, is the response time to external events. OS’s typically provide a non-deterministic, soft real time response, where there are no guarantees as to when each task will complete, but they will try to stay responsive to the user. An RTOS differs in that it typically provides a hard real time response, providing a fast, highly deterministic reaction to external events. The difference between the two can be highlighted through examples – compare, for example, the editing of a document on a PC to the operation of a precision motor control.
When switching between Tasks the RTOS has to choose the most appropriate task to load next. There are several scheduling algorithms available, including Round Robin, Co-operative and Hybrid scheduling. However, to provide a responsive system most RTOS’s use a pre-emptive scheduling algorithm.
Tasks and Prioirities
In a pre-emptive system each Task is given an individual priority value. The faster the required response, the higher the priority level assigned. When working in pre-emptive mode, the task chosen to execute is the highest priority task that is able to execute. This results in a highly responsive system.
Why use an RTOS?
There are well-established techniques for writing good embedded software without the use of an RTOS. In some cases, these techniques may provide the most appropriate solution; however as the solution becomes more complex, the benefits of an RTOS become more apparent. These include:
Priority Based Scheduling: The ability to separate critical processing from non-critical is a powerful tool.
Abstracting Timing Information: The RTOS is responsible for timing and provides API functions. This allows for cleaner (and smaller) application code.
Maintainability/Extensibility: Abstracting timing dependencies and task based design results in fewer interdependencies between modules. This makes for easier maintenance.
Modularity: The task based API naturally encourages modular development as a task will typically have a clearly defined role.
Promotes Team Development: The task-based system allows separate designers/teams to work independently on their parts of the project.
Easier Testing: Modular task based development allows for modular task based testing.
Code Reuse: Another benefit of modularity is that similar applications on similar platforms will inevitably lead to the development of a library of standard tasks.
Improved Efficiency: An RTOS can be entirely event driven; no processing time is wasted polling for events that have not occurred.
Idle Processing: Background or idle processing is performed in the idle task. This ensures that things such as CPU load measurement, background CRC checking etc will not affect the main processing.
What should be considered when choosing an RTOS?
Responsiveness: The RTOS scheduling algorithm, interrupt latency and context switch times will significantly define the responsiveness and determinism of the system. The most important consideration is what type of response is desired - Is a hard real time response required? This means that there are precisely defined deadlines that, if not met, will cause the system to fail. Alternatively, would a non-deterministic, soft real time response be appropriate? In which case there are no guarantees as to when each task will complete.
Available system resources: Micro kernels use minimum system resources and provide limited but essential task scheduling functionality. Micro kernels generally deliver a hard real time response, and are used extensively with embedded microprocessors with limited RAM/ROM capacity, but can also be appropriate for larger embedded processor systems.
Alternatively, a full featured OS like Linux or WinCE could be used. These provide a feature rich operating system environment, normally supplied with drivers, GUI’s and middleware components. Full featured OS’s are generally less responsive, require more memory and more processing power than micro kernels, and are mainly used on powerful embedded processors where system resources are plentiful.
Open source or professionally licensed: There are widely used, free open source RTOS’s available, distributed under GPL or modified GPL licenses. However, these licenses may contain copy left restrictions and offer little protection. Professionally licensed RTOS products remove the copy left restrictions, offer full IP infringement indemnification and warranties. In addition, you have a single company providing support and taking responsibility for the quality of your product.
Quality: What emphasis does the RTOS supplier place on quality within their organisation? Quality is more than just a coding standard. Are the correct procedures in place to guarantee the quality of future products and support? Well-managed companies that take quality seriously tend to be ISO 9001 certified.
Safety Certification: Pre-certified and certifiable RTOS’s are available for applications that require certification to international design standards such as DO-178C and IEC 61508. These RTOS’s provide key safety features, and the design evidence required by certification bodies to confirm that the process used to develop the RTOS meets the relevant design standard.
Licensing: It’s not only the RTOS functionality and features that you’ll need to consider, but the licensing model that will work best for your project budget and the company’s “return on investment”.
RTOS Vendor: The company behind the RTOS is just as important as selecting the correct RTOS itself. Ideally you want to build a relationship with a supplier that can support not only your current product, but also your products of the future. To do this you need to select a proactive supplier with a good reputation, working with leading silicon manufacturers to ensure they can support the newest processors and tools.
Trust, quality of product, and quality of support is everything.
How does the RTOS affect the development of the design?
The choice of RTOS can greatly affect the development of the design. By selecting an appropriate RTOS the developer gains:
- A Task based design that enhances modularity, simplifies testing and encourages code reuse;
- An environment that makes it easier for engineering teams to develop together;
- Abstraction of timing behaviour from functional behaviour, which should result in smaller code size and more efficient use of available resources.
Peripheral support, memory usage and real-time capability are key features that govern the suitability of the RTOS. Using the wrong RTOS, particularly one that does not provide sufficient real time capability, will severely compromise the design and viability of the final product.
The RTOS needs to be of high quality and easy to use. Developing embedded projects is difficult and time consuming - the developer does not want to be struggling with RTOS related problems as well. The RTOS must be a trusted component that the developer can rely on, supported by in-depth training and good, responsive support.