1. An Introduction to Real time Systems by Dr. Amin Danial Asham

2. References  Real-time Systems Theory and Practice. By Rajib mall  Real Time Systems Design and Analysis. By Philip A. Laplante  Taking the Mystery out of the Infamous Formula, "SNR = 6.02N + 1.76dB," and Why You Should Care. By Walt Kester  Synchronous Programming of Reactive. By Nicolas Halbwachs

3. What is A System? A system is a mapping of a set of inputs into a set of outputs........... I1I1 I2I2 InIn O1O1 O2O2 OmOm.......... System

4. Digital Computing systems.......... Keyboard Camera Different types of Sensors Monitor Printers Control signals to Actuators.......... Computing System

5. Computing Systems A computing system is a combination of: Hardware: the touchable material components that comprise a computing system: microprocessor, ram, rom, display, graphic card, sound card, keyboard, mouse, printer…etc. Software: non-touchable instructions that guide the processor supported with other hardware components to carry out a certain process. Software expresses or embodies a certain idea or concept and hence drives the hardware to act accordingly.

6. Classes of Computing systems: Transformational System: receives inputs at the beginning of execution, process them, delivers the outputs, and then terminate executions. Example: like compiler, any problem solving program…etc. Interactive System: interacts continuously with the environment at its own speed. Example: web. Reactive System (Real Time): reacts to the environment continuously within a time frame constraint imposed by the environment. The real time system may fail in case it could not respond before the deadline. The response time is usually of the order of milliseconds or microseconds. Example: Auto Pilot, ABS….etc.

7. Real Time System Definition: A real-time system is one whose logical correctness is based on both the correctness of the outputs and their timeliness. A system is considered a real time system when the performance success is depending on the time in which it performs. In other words, this type of systems must meet the time constraints imposed by the environment by performing the required tasks before the determined deadlines. A real-time deadline must be met, regardless of system Load. In real time systems the delay of the output relative to the input is bounded over unlimited time period of operation whatever how long is the delay.

8. Deadlines Deadlines are instants in time. Based on the application and the physical properties of the system, deadlines are determined in microseconds, milliseconds, seconds…..etc.

9. Examples of Real Time systems In Digital Signal Processing (DSP) application, the system is real time if the input is collected, processed and the output is generated within the sampling period. That is the output delay relative to the input is not more that the period elapsed before the next input sample. This criterion is applied whether the samples are processed in groups or individually. If a DSP system takes 2.01 seconds to process 2 seconds of audio signal that means it is not a real time. It is considered a real time of it takes 1.99 seconds. From real life, if there is a queue of customers at the checkout desk in a store, and customers are processed such that the queue is growing that means they are not processed in real time. But, if the queue is bounded that means the customers are processed on average at the same rate as they join the queue which is a real time processing.

10. Classification of Real Time Systems Real-time systems, as well as their deadlines, are classified by the consequence of missing a deadline. 1.Hard: Missing a deadline is a total system failure. Ex. Avionics weapons delivery system in which pressing a button launches an air-to-air missile. Any delay beyond the deadline after pressing the button will cause the target to be missed, which is catastrophic situation.

11. Hard real-time task doesn’t need to be completed within the shortest possible time. The point here is that the hard real-time task must complete within the specified time bound. Classification of Real Time Systems (cont.)

12. 2.Firm: Infrequent deadline misses are tolerable, but may degrade the system's quality of service. The usefulness (Utility) of a result is zero after its deadline. A firm real-time system is one in which a few missed deadlines will not lead to total failure, but missing more than a few may lead to complete and catastrophic system failure.

13.  Examples of Firm real-time tasks Ex. Navigation controller for autonomous robot weed killer. Missing deadlines my cause the robot to veer out of control damaging the crops Video Conference: video and audio data are sent in data packets which travel through different routes in the network. Different packets experience different delays based on the different routes. For this some frames may arrive at the receiver before the preceding delayed frames, which become useless. For this the frames that are delayed by certain amount of time are discarded without processing since they are useless. Classification of Real Time Systems (cont.)

14. 3.Soft: The usefulness (utility) of a result degrades after its deadline, thereby degrading the system's quality of service. The timing constraints on soft real-time tasks are not expressed as absolute values. Instead the constraints are expressed either in terms of the average response time required.

15.  Examples of Soft real-time tasks: Seat reservation in a railway reservation application: the average response time for a reservation request is within 20 sec. the response may be either a printed ticket or an apology message. The constraint in this case is that at least of 95% of reservation requests the ticket should be processed and printed within 20 sec. In case of delay (missing the deadline) in response, the system does not fail but the utility of the results falls slowly. URL (Uniform Resource Locator): when an URL is clicked the corresponding web page is displayed within a couple of seconds on the average. When the requested page is delayed for minutes we don’t consider the system to have failed but just the performance is degraded. Classification of Real Time Systems (cont.)

16. Note: It is not necessary that all the tasks of a real time application belong to the same category (hard, firm, and soft)  It is possible that real time tasks of a real time system can belong to different categories (hard, firm, and soft). Classification of Real Time Systems (cont.)

17. Real Time and High Performance The performance is an indication of the processing capacity of how much processing can be carried out in a certain period of time. On the other hand, real time is an indication of completing the processing before the deadline. For example, Anti-Lock Braking System (ABS) has the suitable hardware and software to complete all required calculations within a certain time interval to meet its deadline and hence it is a real time system and there is no need of a higher performance. The performance of a Supercomputer which simulates complex models is much higher than ABS and however it is not considered a real time system. A chess playing system can be considered real time or non real time based on the situation. If the game is limited with a certain time interval for each move, then the computer in this case has a deadline must be respected or the game will be lost. In this case it is a real time system. On the other hand, if there is no time constraint for each move then it is a non real time system.

18. More Examples of Real Time Systems 1.Distributed Control system (DCS): Industrial plants are controlled by DCS which collects the data from sensors and slave peripherals, then process this data and delivers outputs at each program cycle. This cycle is monitored by a watch dog module. The period of this cycle is predetermined according to the application. If a cycle exceeded the maximum allowed time interval the system is transferred to a stop mode safely.

19. More Examples of Real Time Systems (cont.) 2. An aircraft must process accelerometer data within a certain period that depends on the specifications of the aircraft; for example, every 10 milliseconds. Failure to do so could result in a false position or velocity indication and cause the aircraft to go off-course at best or crash at worst.

20. More Examples of Real Time Systems (cont.) 3. Printers There are microprocessors embedded in printers. These processors receive the commands from computers and monitors different tasks such as: sensing the papers, papers jamming…..etc. In addition, these processors control the printing process to produce fonts graphs as required. Time constraint in the order of milliseconds.

21. More Examples of Real Time Systems (cont.) 4. Multi-Point Fuel Injection (MPFI) MPFI is an automotive control system. This system monitors the operation of the engine and commands from the driver and control the fuel rate to achieve the an optimal performance in terms of fuel consumption and pollution reduction by reducing partial combustion.

22. More Examples of Real Time Systems (cont.) 5. Video Conference Video and audio signals are sampled at the sending end and compressed using the proper codecs and sent to the receiving end where the information are decompressed and played back with time constraint based on the required frames per seconds.

23. More Examples of Real Time Systems (cont.) 6. A simple Self Balancing Robot

24. Thanks