Presentation on Principles of Distributed Embedded Systems (Embedded Systems - ECE)​​ I -M.TECH I -Semester (AUTONOMOUS-R18) Prepared by, Dr. S. Vinoth Associate Professor UNIT - I REAL TIME ENVIRONMENT 2 UNIT - I REAL-TIME ENVIRONMENT .Real-time computer system requirements .classification of real time systems .simplicity, global time .internal and external clock synchronization .real time model. Real time communication .temporal relations, dependability .power and energy awareness .real time communication .event triggered .rate constrained .time triggered. 3 What is an Embedded system? 4 What is a real-time system? . A real-time system is any information processing system which has to respond to externally generated input stimuli within a finite and specified period –the correctness depends not only on the logical result but also the time it was delivered –failure to respond is as bad as the wrong response! . The computer is a component in a larger engineering system => EMBEDDED COMPUTER SYSTEM 99% of all processors are for the embedded systems market 5 Terminology • Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E.g. Flight control systems. • Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E.g. Data acquisition system. • Real real-time — systems which are hard real-time and which the response times are very short. E.g. Missile guidance system. • Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service. A single system may have all hard, soft and real real-time subsystems In reality many systems will have a cost function associated with missing each deadline. 6 Characteristics of a RTS • Large and complex — vary from a few hundred lines of assembler or C to 20 million lines of Ada estimated for the Space Station Freedom • Concurrent control of separate system components — devices operate in parallel in the real-world; better to model this parallelism by concurrent entities in the program • Facilities to interact with special purpose hardware — need to be able to program devices in a reliable and abstract way 7 Characteristics of a RTS cont…… . Extreme reliability and safe — embedded systems typically control the environment in which they operate; failure to control can result in loss of life, damage to environment or economic loss . Guaranteed response times — we need to be able to predict with confidence the worst case response times for systems; efficiency is important but predictability is essential 8 Embedded computing systems are becoming pervasive in our society (more than 109 units/year): . Robotics . Flight control systems . Plant control . Automotive . Consumer electronics . Multimedia systems . Sensor/Actor 9 Criticality QoS management High performance Safety critical firm hard Timing contraints 10 Common features .In these diversified domains some shared features can be identified: .Dedicated function (vs general-purpose computers) .Reactive / Interactive .Real-time .Constraints on several metrics: cost, power, performance, noise, weight, size, flexibility, maintainabilit y, correctness, safety, time-to-market 11 Embedded systems overview .Embedded computing systems .Computing systems embedded within Computers are in here... electronic devices and here... .Hard to define. Nearly any computing system and even here... other than a desktop computer .Billions of units produced yearly, versus millions of desktop units Lots more of .Perhaps 50 per household and per these, though they automobile cost a lot less each. 12 A “short list” of embedded systems Anti-lock brakes Modems Auto-focus cameras MPEG decoders Automatic teller Network cards machines Network Automatic toll systems switches/routers Automatic transmission On-board navigation Avionic systems Pagers Battery chargers Photocopiers Camcorders Point-of-sale systems Cell phones Portable video games Cell-phone base Printers stations Satellite phones Cordless phones Scanners Cruise control Smart Curbside check-in ovens/dishwashers systems Speech recognizers Digital cameras Stereo systems Disk drives Teleconferencing Electronic card readers systems Electronic instruments Televisions Electronic toys/games Temperature Factory control controllers Fax machines Theft tracking systems Fingerprint identifiers TV set-top boxes And the list Home security systems VCR’s, DVD players Life-support systems Video game consoles goes on and on 13 Common characteristics of Embedded systems Some common characteristics of embedded systems .Single-functioned –Executes a single program, repeatedly .Tightly-constrained –Low cost, low power, small, fast, etc. .Reactive and real-time –Continually reacts to changes in the system’s environment –Must compute certain results in real-time without delay 14 An embedded system example – Digital camera .Single-functioned -- always a digital camera .Tightly-constrained -- Low cost, low power, small, fast .Reactive and real-time -- only to a small extent 15 What is real-time? What is real-time? Is there any other kind? .A real-time computer system is a computer system where the correctness of the system behavior depends not only on the logical results of the computations, but also on the physical time when these results are produced. .By system behavior we mean the sequence of outputs in time of a system. 16 Real-time means reactive .A real-time computer system must react to stimuli from its environment .The instant when a result must be produced is called a deadline. .If a result has utility even after the deadline has passed, the deadline is classified as soft, otherwise it is firm. .If severe consequences could result if a firm deadline is missed, the deadline is called hard. Example: Consider a traffic signal at a road before a railway crossing. If the traffic signal does not change to red before the train arrives, an accident could result. 17 Reliability • The Reliability R(t) of a system is the probability that a system will provide the specified service until time t, given that the system was operational at the beginning (t-t0). • The probability that a system will fail in a given interval of time is expressed by the failure rate, measured in FITs (Failure In Time). • A failure rate of 1 FIT means that the mean time to a failure (MTTF) of a device is 10^9 h, i.e., one failure occurs in about 115,000 years. • If a system has a constant failure rate of λ failures/h, then the reliability at time t is given by, R(t)= exp(-λ(t-to)) MTTF = 1/λ 18 Computer System Organization Computer-system operation • One or more CPUs, device controllers connect through common bus providing access to shared memory • Concurrent execution of CPUs and devices competing for memory cycles 19 Distributed Systems • Computation is distributed among several processors. In contrast to tightly-coupled systems, the processors do not share a clock or memory. Each has its own local memory. • Communication is via a network. These systems are termed loosely- coupled or distributed systems. The processors vary in size and function and are called nodes. Advantages of distributed systems: • Reliability: If one node fails, the remaining nodes can continue operating. So by building enough redundancy, the system will not fail if one or more nodes fail (e.g. redundant web servers). • Computation speedup: Computation can be distributed among various nodes to run concurrently (e.g. load balanced web servers). 20 Distributed Systems (contd.) • Resource Sharing: Software, data, and hardware resources can be shared. E.g. data files in node A can be accessed by a user at node B. Files can be printed at a shared laser printer. • Communication: Processes at various nodes can exchange information. 21 Special Purpose Systems 22 WHEN IS A COMPUTER SYSTEM REAL-TIME? • A real-time computer system is a computer system in which the correctness of the system behavior depends not only on the logical results of the computations, but also on the physical instant at which these results are produced. • A real-time computer system is always part of a larger system–this larger system is called a real-time system. • A real-time system changes its state as a function of physical time, e.g., a chemical reaction continues to change its state even after its controlling computer system has stopped. • It is reasonable to decompose a real-time system into a set of sub- systems called clusters. 23 Real- Time system cont….. • We refer to the controlled object and the operator collectively as the environment of the real-time computer system. • If the real-time computer system is distributed, it consists of a set of computer) nodes interconnected by a real-time communication network 24 Real- Time system cont….. • The interface between the human operator and the real-time computer system is called the man-machine interface, and the interface between the controlled object and the real-time computer system is called the instrumentation interface. • The man-machine interface consists of input devices (e.g., keyboard) and output devices (e.g., display) that interface to the human operator. • The instrumentation interface consists of the sensors and actuators that transform the physical signals (e.g., voltages, currents) in the controlled object into a digital form and vice versa. • A node with an instrumentation interface is called an interface node. 25 Real- Time system cont….. • A real-time computer system must react to stimuli from the controlled object (or the operator) within time intervals dictated by its environment. • The instant at which a result must be produced is called a deadline. If a result has utility even after the deadline has passed, the deadline is classified as soft, otherwise it is firm. If a catastrophe could result if a firm deadline is missed, the deadline is called hard. 26 Real- Time system cont….. Consider a railway crossing a road with a traffic signal. • If the traffic signal does not change to "red" before the train arrives, a catastrophe could result.