realtimeembeddedsystem.ppt

8/20/2019 RealTimeEmbeddedSystem.ppt

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EE5900 Advanced Embedded

System For Smart Infrastructure

Introduction to Real-Time Embedded

Systems


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Definition

Real-time embedded systems are defined as

those systems in which the correctness of thesystem depends not only on the logical resultof computation, but also on the time at whichthe results are produced.

!ard real-time systems "e.g., #$ionic control%.

&irm real-time systems "e.g., 'an(ing%.

Soft real-time systems "e.g., )ideo ondemand%.


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Deadline

•Hard deadline: penalty due to missing deadline is a higher

order of magnitude than the reward in meeting thedeadline

•Firm deadline: penalty and reward are in the same orderof magnitude

•Soft deadline: penalty often lesser magnitude thanreward


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A typical real-time embedded system


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Car example

Mission: Reaching the destination safely.

Controlled System ar.

!peratin" environment Road conditions.

Controllin" System

- Human driver: Sensors - Eyes and Ears of the dri$er.

- Computer: Sensors - ameras, Infrared recei$er, andaser telemeter.

Controls #ccelerator, Steering wheel, 'rea(-pedal.

Actuators /heels, Engines, and 'ra(es.


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Car example #contd$

Critical tas%s Steering and brea(ing.

&on-critical tas%s Turning on radio.

Cost of fulfilling the mission Efficient solution.

'eliability of the dri$er &ault-tolerance needs to beconsidered.


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'eal-(ime (as%s

)eriodic tas%s

- Time-dri$en. haracteristics are (nown a priori

- Tas( T i is characteri3ed by " pi , c i %

E.g.: Tas( monitoring temperature of a patient.

Aperiodic tas%s

- E$ent-dri$en. haracteristics are not (nown a priori

- Tas( T i is characteri3ed by "ai , r i , c i , d i %E.g.: Tas( acti$ated upon detecting change in patient4s condition.

Sporadic (as%s 5 #periodic tas(s with (nown minimum inter-arri$al time.

pi 6 tas( period ai 6 arri$al time r i 6 ready time d i 6 deadline c i 6 worst case e7ecution time.


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(as% constraints

9eadline constraint

Resource constraints 5 Shared access 5 E7clusi$e access

:recedence constraints 5 T1 T26 Tas( T2 can start e7ecuting only after T1

finishes its e7ecution

&ault-tolerant re;uirements 5 To achie$e higher reliability for tas( e7ecution 5 Redundancy in e7ecution


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Computin" systems

Uniprocessor, multiprocessor, distributed system


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&otion of )redictability

The most common denominator that is e7pected from a real-timesystem is predictability .

– The behavior of the real-time system must be predictable which means that with certainassumptions about workload and failures, itshould be possible to show at design time thatall the timing constraints of the application

will be met.

&or static systems, 1==> guarantees can be gi$en at design time. &or dynamic systems, 1==> guarantee cannot be gi$en since the

characteristics of tas(s are not (nown a priori.

In dynamic systems, predictability means that once a tas( isadmitted into the system, its guarantee should ne$er be $iolated aslong as the assumptions under which the tas( was admitted hold.


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Common *isconceptions

Real-time computing is e;ui$alent to fast

computing. Real-time programming is assembly coding,

priority interrupt programming, and writing

de$ice dri$ers.

Real-time systems operate in a staticen$ironment.

The problems in real-time system design ha$ebeen sol$ed in other areas of computerscience and engineering.


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*a+or c,allen"e

The main challenge in real time embedded

system design is real time scheduling ?any e7isting scheduling techni;ues

5 Round robin scheduling

5 :lanning scheduling

5 :riority scheduling

5 Static scheduling


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)reemptive vs &on-preemptive sc,edulin"

)reemptive Sc,edulin"

5 Tas( e7ecution is preempted and resumed later 5 :reemption occurs to e7ecute higher priority tas(.

5 @ffers higher schedulability

5 In$ol$es higher scheduling o$erhead due to conte7t

switching

&on-preemptive Sc,edulin"

5 @nce a tas( starts e7ecuting, it completes its full

e7ecution

5 @ffers lower schedulability

5 ess o$erhead due to less conte7t switching


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!ptimal sc,edulin" -- definition

# static sc,edulin" algorithm is said to be optimal if,

for any set of tas(s, it always produces a feasibleschedule "i.e., a schedule that satisfies the constraintsof the tas(s% whene$er any other algorithm can do so.

# dynamic sc,edulin" algorithm is said to be optimal

if it always produces a feasible schedule whene$er astatic algorithm with complete prior (nowledge of allthe possible tas(s can do so.

Static scheduling is used for scheduling periodic tas(s,whereas dynamic scheduling is used to schedule bothperiodic and aperiodic tas(s.


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Summary

Real-time embedded systems re;uire logical correctness andtimeliness.

Real-time embedded system consists of a controlling system,controlled system, and the en$ironment.

Real-time systems are classified as6 hard, firm, and soft real timesystems.

Tas(s are periodic, aperiodic, sparodic.

The notion of predictability is important in real-time systems andthe maAor challenge is real time scheduling.

realtimeembeddedsystem.ppt

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