S S symmetry Article Timing Comparison of the Real-Time Operating Systems for Small Microcontrollers Ioan Ungurean 1,2 1 Faculty of Electrical Engineering and Computer Science; Stefan cel Mare University of Suceava, 720229 Suceava, Romania; [email protected] 2 MANSiD Integrated Center, Stefan cel Mare University, 720229 Suceava, Romania Received: 9 March 2020; Accepted: 1 April 2020; Published: 8 April 2020 Abstract: In automatic systems used in the control and monitoring of industrial processes, fieldbuses with specific real-time requirements are used. Often, the sensors are connected to these fieldbuses through embedded systems, which also have real-time features specific to the industrial environment in which it operates. The embedded operating systems are very important in the design and development of embedded systems. A distinct class of these operating systems is real-time operating systems (RTOSs) that can be used to develop embedded systems, which have hard and/or soft real-time requirements on small microcontrollers (MCUs). RTOSs offer the basic support for developing embedded systems with applicability in a wide range of fields such as data acquisition, internet of things, data compression, pattern recognition, diversity, similarity, symmetry, and so on. The RTOSs provide basic services for multitasking applications with deterministic behavior on MCUs. The services provided by the RTOSs are task management and inter-task synchronization and communication. The selection of the RTOS is very important in the development of the embedded system with real-time requirements and it must be based on the latency in the handling of the critical operations triggered by internal or external events, predictability/determinism in the execution of the RTOS primitives, license costs, and memory footprint. In this paper, we measured and compared the timing performance for synchronization throughout an event, semaphore, and mailbox for the following RTOSs: FreeRTOS 9.0.0, FreeRTOS 10.2.0, rt-thread, Keil RTX, uC/OS-II, and uC/OS-III. For the experimental tests, we developed test applications for two MCUs: ARM Cortex™-M4 and ARM Cortex™-M0+ based MCUs. Keywords: real time systems; real time operating systems; task synchronization; microcontrollers 1. Introduction In the automation systems designed and developed to monitor and control industrial processes, it is very important that they comply with the real-time requirements of the industrial installations. Typically, these systems contain industrial networks (fieldbuses) to which embedded devices are connected. These devices can acquire data from the sensors and receive/send data through the fieldbuses. The embedded systems used in the automation systems must have hard and/or soft real-time features and they must react in the imposed deadline to the data/events received throughout the fieldbuses or data acquired from the sensors connected to the embedded devices [1]. Usually, the design and development of the embedded systems, from the software point of view, is based on an embedded operating system. The real time operating systems (RTOSs) are a particular category of embedded operating systems that were designed to provide support in the design and develop of embedded systems with real-time capabilities. These operating systems have been developed especially for small microcontrollers (MCUs) on 8, 16, or 32 bits that are used to design and develop embedded systems [2]. Examples of RTOSs include FreeRTOS, RT-Thread, eCOS, Symmetry 2020, 12, 592; doi:10.3390/sym12040592 www.mdpi.com/journal/symmetry Symmetry 2020, 12, 592 2 of 19 LynxOS, QNX, VxWorks, OSEK (Open Systems and their Interfaces for the Electronics in Motor Vehicles), uC-OS/II, uC-OS/III, KEIL RTX, etc. [2]. These operating systems are designed to provide a deterministic and predictable time in the handling of the internal or external events [3,4]. In addition, RTOSs are widely used in the design and development of applications based on the Internet of Things (IoT) and Industrial Internet of Things (IIoT) concepts [5]. RTOSs are part of a category of operating systems designed for embedded systems, especially for systems with a small memory for code and data. Usually, these RTOSs are designed for MCUs that do not use virtual memory [6]. Linux and Windows that use have real time characteristics but there are variants for real time systems for these systems. For example, RTLinux is a patch for Linux with real time capabilities where real time tasks do not use virtual memory and have direct access to the hardware [7]. Windows Embedded Industry is the Windows based operating system for embedded systems with real time capabilities. We will consider small MCUs, the MCUs that do not have virtual memory and cannot use operating systems based on Linux, Windows, or Android (for example, these operating systems can be used on ARM Cortex Ax MCUs). With these MCUs, specialized applications with real-time capabilities can be developed. This paper aims to make a comparison of RTOSs that are used on small MCUs, such as those based on ARM Cortex Mx architectures. These MCUs use RTOSs with a small memory footprint and without a memory management system such as virtual memory. Applications based RTOS for small MCUs are used in domains such as automotive [8], industrial automation [9], telecommunications [10], avionics [11], military systems [12], Internet of things, Industrial Internet of things [13], and so on. An area of applicability is that of a symmetry concept by developing embedded systems for data acquisition, data compression, pattern recognition, diversity, and sustainability. Usually, the software applications for these domains are a combination of soft and hard real-time tasks [6]. Several features, services, and capabilities are used to compare RTOSs. The most important ones are the maximum time for deactivation of the interrupts and the worst-case execution time (WCET) for tasks and RTOS services such as the routines of service of the interruptions, the time of executing the receipts, etc. [14,15]. Other features used in the comparison are modularity, scalability, memory footprint, latency, response time, and jitter [6,14]. An RTOS provides the basic services for developing multitasking software applications. The fundamental service provided by an RTOS is task management. Each task has associated a priority and the task with the highest priority from the ready state will enter in running state. There are two types of scheduling: preemptive and non-preemptive. At the pre-emptive scheduling, a task from the running state can be preempted by a higher priority task that enters in the ready state, while at the non-preemptive scheduling, the task from the running state must voluntarily release the processor. Most RTOSs use pre-emptive scheduling because it allows for achieving a shorter response time for critical operations. In addition, most RTOSs include synchronization and inter-task communication services such as semaphores, events, mailboxes, and message queues [6]. RTOSs can be compared from several perspectives, some of them are: the time for synchronization and inter-task communication or the response time of the task with the highest priority after the expected event occurs. In [14], we presented a first comparative test for the time of task context switching where task switching is triggered by an event, semaphore, or mailbox. In this paper, we measured, compared, and analyzed the timing performances for the following RTOSs: FreeRTOS 9.0.0, FreeRTOS 10.2.0, rt-thread, Keil RTX, uC/OS-II, and uC/OS-III. All of these RTOSs support preemptive scheduling. For testing, we used two MCUs: ARM Cortex™-M4 and ARM Cortex™-M0+ based MCUs. The paper extends the experimental tests presented in [14]. In [14], there are measured and compared timing performances for task context switching from a lower priority task to a higher priority task triggered by an event, semaphore, and mailbox. In this paper, the tests are completed by measuring the timing performances for the task context switching from a higher priority task to a lower priority task Symmetry 2020, 12, 592 3 of 19 and the timing performance for the primitives (non-blocking) used to wait and send/signal an event, semaphore, and mailbox. Furthermore, we use the newest compiler provided by the KEIL MDK-ARM. The main contribution of this paper is that the experimental tests are performed in actual MCUs, and not just simulations using software tools as found in related works. A test pin (an output GPIO (general-purpose input/output) selected according to the MCU and board used) is used to signal the beginning and the end of the operation for which we want to measure the time and the time is measured with an oscilloscope on this test pin. This paper aims to compare the timing for synchronization of the tasks for the most used RTOSs on small microcontrollers. To achieve this goal, RTOSs are executed on the same hardware configuration. This paper is structured as follows: Section2 presents some performances tests presented in the specialized literature and the motivation for the target RTOS’s selection, Section3 describes the test setup. Section4 contains discussions related to the experimental results. The conclusions are drawn in Section5. 2. Related Works Interesting studies related to the market for embedded systems are published by EETimes.com and Embedded.com every year. Unfortunately, the last study was published in 2017 and it has not updated [16]. In the last market study, they conclude that 68% of the ongoing embedded projects use and embedded operating system, and of these, 41% use an open-source embedded operating system. If we exclude the embedded system based on Linux or Windows, the most used RTOS are FreeRTOS (20% of the ongoing embedded projects that use an operating systems), in house solution (19%), Texas Instruments RTOS (5%), Texas Instruments DSP/BIOS (5%), Micrium uC-OS/III (5%), Keil RTX (4%), Micrium uC-OS/II (4%), and Wind River VxWorks (4%).