Real-Time Embedded System PDF

Real-Time Embedded System By: Desalegn A.(MSc, SW Eng’g) Oda Bultum University 2015 E.C 1  Introduction to Real Time & Embedded Systems An embedded system is an electronic system that are designed to perform a dedicated function which is emb...

Real-Time Embedded System By: Desalegn A.(MSc, SW Eng’g) Oda Bultum University 2015 E.C 1  Introduction to Real Time & Embedded Systems An embedded system is an electronic system that are designed to perform a dedicated function which is embedded with in large system. A real time system are those that can provide guaranteed worst-case response times to noncritical events. Embedded systems differ from general-purpose computing devices mainly in two aspects: First, an embedded system is designed simply for a specific function, whereas a general-purpose computing device, such as smartphone, laptop, or desktop computer, is not; they can be used as Web servers or data warehouses, or can be used for writing articles, reading news, playing games, or running scientific experiments, to mention only a few applications. Second, an embedded system is traditionally built together with the software intended to run on it. Such a parallel model of developing hardware and software together is known as hardware-software co-design.  Definition of Real Time Embedded Systems The embedded systems which respond to real time situation with the help of its embedded software and hardware, within the specified time constraints are called real time embedded systems. A systems that need to respond to a service request within a certain amount of time. Definition of Real Time Embedded Systems A real time system is a system that must satisfy explicit (bounded) response-time constraints or risk severe consequences, including failure. A real time system is one whose logical correctness is based on both the correctness of the outputs and their timeliness. A real time system is any information processing activity or system which has to respond to externally generated input stimuli within a finite and specified period.  Definition of Real Time Embedded Systems Related Notions reactive system: continuous interaction with the environment (as opposed to information processing) embedded system: computer system encapsulated in its environment (device it controls), combination of computer hardware and software, dedicated to specific purpose safety-critical system: a failure may cause injury, loss of lives, significant financial loss  Characteristics of a Real-Time Embedded System Real time embedded systems must have the following characteristics; 1. Constant Response: A real-time embedded system always responds in the same manner to a certain situation, it is not allowed to deviate from its normal designated output. An air- conditioner is not allowed to throw hot air in summers. 2. Deadline: A deadline is crucial to the working of an embedded system, a missed deadline can cost lives and finances.  Characteristics of a Real-Time Embedded System 3. Accuracy: In case of any malfunctioning, the system failure can cause havoc For example, what would happen if the pacemaker can't maintain the heartbeat, patient would eventually die! 4. Quick Response: Is the most important characteristic of all, the real-time embedded system must be swift enough to respond to the changing external environment with immediate effect. Examples of real-time and embedded systems Missile launcher Medical equipment Handheld devices Avionics Industrial control systems Transportation control Examples of real-time and embedded systems Central Health System GPS Systems Fitness trackers Medical devices Automotive systems Examples of real-time and embedded systems Transit and fare Collection(air port in Ethiopian context) ATMs Factory robots Model of Real Time System The basic model of a real-time system presents the overview of all the components involved in a real-time system. Real-time system includes various hardware and software embedded in a such a way that the specific tasks can be performed in the time constraints allowed. The accuracy and correctness involved in real-time system makes the model complex. There are various models of real-time system which are more complex and are hard to understand. Model of Real Time System Basic model of real-time system which has some commonly used terms for hardware. Model of Real Time System Sensor: Sensor is used for the conversion of some physical events or characteristics into the electrical signals. These are hardware devices that takes the input from environment and gives to the system by converting it. For example, a thermometer takes the temperature as physical characteristic and then converts it into electrical signals for the system. Model of Real Time System Actuator: Actuator is the reverse device of sensor. Where sensor converts the physical events into electrical signals, actuator does the reverse. It converts the electrical signals into the physical events or characteristics. It takes the input from the output interface of the system. The output from the actuator may be in any form of physical action. E.g Some of the commonly used actuator are motors and heaters. Model of Real Time System Signal Conditioning Unit: When the sensor converts the physical actions into electrical signals, then computer can’t used them directly. Hence, after the conversion of physical actions into electrical signals, there is need of conditioning. Similarly while giving the output when electrical signals are sent to the actuator, then also conditioning is required. Therefore, Signal conditioning is of two types: Input Conditioning Unit: It is used for conditioning the electrical signals coming from sensor. Output Conditioning Unit: It is used for conditioning the electrical signals coming from the system. Model of Real Time System Interface Unit: Interface units are basically used for the conversion of digital to analog and vice-versa. Signals coming from the input conditioning unit are analog and the system does the operations on digital signals only, then the interface unit is used to change the analog signals to digital signals. Similarly, while transmitting the signals to output conditioning unit the interface of signals are changed i.e. from digital to analog. Interface unit is also of two types: Input Interface: It is used for conversion of analog signals to digital. Output Interface: It is used for conversion of digital signals to analog. Types of Real Time Embedded Systems A real time embedded system, being made up of real time operating system, embedded software and hardware. Real Time embedded systems have the following three types. Types of Real Time Embedded Systems 1. Soft Real Time Embedded Systems A soft real-time constraint is a constraint that a system should meet, but when the deadline is occasionally missed, it won’t cause any disastrous result, and the delivered service is still useful to a certain extent. Try to reach deadlines but do not fail if a deadline is missed. However, they may degrade their quality of service in such an event to improve responsiveness. Examples of soft real-time systems: audio and video delivery software for entertainment Types of Real Time Embedded Systems 1. Soft Real Time Embedded Systems Characteristics of soft real time embedded systems; Timeliness of a task poses a positive impact on the system, but it is not crucial for the performance of the system. Missing a deadline would not degrade the performance of the whole embedded system. Example of Soft Real Time Embedded Systems A data acquisition system can tolerate delays and hence its a soft real time embedded system. websites, computer games, cellular networks, online database and multimedia transmission and reception. Types of Real Time Embedded Systems 2. Hard Real Time Embedded Systems A hard real-time constraint is a constraint that a system must meet. Hard real-time software systems have a set of strict deadlines, and missing a deadline is considered a system failure. If the deadline is missed, it will either cause the system failure or result in a zero usefulness of the delivered service. Examples of hard real-time systems: airplane sensor and autopilot systems, & spacecrafts. Types of Real Time Embedded Systems 2. Hard Real Time Embedded Systems characteristics of hard real time embedded systems; Time is crucial. The output must be completely on time, the prescribed deadline can not be missed in any case. e.g You can't submit your exam paper after the time is over, or can you? In case a deadline is missed, it would be regarded as a system failure. Types of Real Time Embedded Systems 3. Firm real-time systems Firm real-time systems treat information delivered/ computations made after a deadline as invalid. Like soft real-time systems, they do not fail after a missed deadline, and they may degrade QoS if a deadline is missed. Examples of firm real-time systems: financial forecast systems, robotic assembly lines. Modeling timing constraints Timing constraints is a vital attribute in real-time systems. Timing constraints decides the total correctness of the result in real-time systems.  The correctness of results in real-time system does not depends only on logical correctness but also the result should be obtained within the time constraint. Several events happening in real time system and these events are scheduled by schedulers using timing constraints. Modeling timing constraints Timing constraints are broadly classified into two categories: 1. Performance Constraints  The constraints enforced on the response of the system is known as Performance Constraints.  It is how quickly and accurately the system is responding. 2. Behavioral Constraint  The constraints enforced on the stimuli generated by the environment is known as Behavioral Constraints. Modeling timing constraints  The both performance and behavioral constraints are classified into three categories I. Delay Constraint II. Deadline Constraint III. Duration Constraint D >= d Modeling timing constraints D >= d  Delay Constraint  It defines the minimum time interval between occurrence of two consecutive events in the real-time system  If an event occurs before the delay constraint, then it is called a delay violation.  The time interval between occurrence of two events should be greater than or equal to delay constraint.  If D is the actual time interval between occurrence of two events and d is the delay constraint, then D>=d Modeling timing constraints  Deadline Constraint  A deadline constraint describes the maximum time interval between occurrence of two consecutive events in the real-time system. i.e. the latest time by which two consecutive events in the real-time system.  If an event occurs after the deadline constraint, then the result of event is considered incorrect.  The time interval between occurrence of two events should be less than or equal to deadline constraint.  If D is the actual time interval between occurrence of two events and d is the deadline constraint, then D