real time embedded systems-introduction

8/8/2019 Real Time Embedded Systems-Introduction

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REAL TIME EMBEDDED SYSTEMS


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Introduction

Real Time Systems:

A system is a mapping of a set of inputs into a set

of outputs.


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Typical real-time control

system including inputs from

sensors and imaging devicesand producing control signals

and display information

A classic representation of a

real-time system as a

sequence of jobs to be

scheduled.


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Response Time : The time between the

presentation of a set of inputs to a system

(stimulus) and the realization of the required

behavior (response), including the availability

of all associated outputs, is called the

response time of the system. Real-time System: A system that must satisfy

explicit (bounded) response-time constraints

or risk severe consequences, including failure. Failure : a system that cannot satisfy one or

more of the requirements stipulated in the

formal system specification.


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Or

A real-time system is one whose logical

correctness is based on both the correctness ofthe outputs and their timeliness.

Real-time systems are often reactive orembedded systems.

Reactive systems are those in which scheduling isdriven by ongoing interaction with theirenvironment;

When Is a System Real-Time? it must respond within a certain time or there

could be an academic or financial disaster.


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Soft Real Time: A soft real-time system is one inwhich performance is degraded but not

destroyed by failure to meet response-timeconstraints.

Hard Real-time Systems : A hard real-time systemis one in which failure to meet a single deadline

may lead to complete and catastrophic systemfailure.

A firm real-time system :is one in which a fewmissed deadlines will not lead to total failure, but

missing more than a few may lead to completeand catastrophic system failure.


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Events and Determinism:

A change in state results in a change in theflow-of-control of the computer program.

The decision block represented by the

diamond suggests that the stream of program

instructions, can take one of two paths,

depending on the response in question.

If-then, goto, and case statements in any

language represent a possible change in

flow-of-control.


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A simple program flowchart showing a branch as a change in flow-of-control, represented by the diamond icon.


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Any occurrence that causes the program

counter to change non-sequentially is

considered a change of flow-of-control, andthus an event.

In scheduling theory, the release time of a

job is similar to an event. The release time is the time at which an

instance of a scheduled task is ready to run,

and is generally associated with an interrupt. Events are slightly different from jobs in that

events can be caused by interrupts as well as

conditional and unconditional branches.


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Synchronous and Asynchronous Events:

Synchronous events are those that occur at

predictable times in the flow-of-control, such asthat represented by the decision box in theflowchart.

The change in flow-of-control, represented by a

conditional branch instruction, or by theoccurrence of an internal trap interrupt,

can be anticipated.

Asynchronous events occur at unpredictable

points in the flow-of-control and are usuallycaused by external sources.

Events that do not occur at regular intervals (orperiods) are called aperiodic or sporadic.


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A periodic but synchronous event is onerepresented by a sequence of invocation of tasks

in a repeated, circular fashion, otherwise knownas cyclic code.

A branch instruction that happens infrequently,say, on the detection of some exceptional

condition, is both sporadic and synchronous.

Interrupts that are generated irregularly

(randomly) by an external device are classified aseither asynchronous aperiodic or sporadic,depending on whether the interrupt is generatedfrequently or not with respect to the systemclock.


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Determinism:

Software control of any real-time system and

associated hardware is maintained when the nextstate of the system, given the current state and aset of inputs, is predictable.

In other words, the goal is to anticipate how a

system will behave in all possible circumstances. Or:

A system is deterministic if, for each possible

state and each set of inputs, a unique set of

outputs and next state of the system can

be determined.


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if in a deterministic system the response time

for each set of outputs is known, then, the

system also exhibits temporal determinism.

A side benefit of designing deterministic

systems is that guarantees can be given that

the system will be able to respond at anytime, and in the case oftemporally

deterministic systems, when they will

respond. This reinforces the association ofcontrol with real-time systems.


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CPUUtilization:

The (CPU) utilization or time-loading factor, U, isa measure of the percentage of non-idleprocessing.

A system is said to be time-overloaded if

U>100%.

Systems that are too highly utilized areundesirable because changes or additions cannotbe made to the system without risk of time-overloading.

Systems that are not sufficiently utilized are notnecessarily good, because this implies that thesystem was over-engineered and that costs canbe reduced with less expensive hardware.


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Suppose a system has n 1 periodic tasks, each

with an execution period ofpi, and hence,

execution frequency, fi= 1/pi

If task i is known to have (or has been estimated

to have) a maximum (worst case) execution time

ofei, then the utilization factor, ui, for task eiis:ui= ei/pi (1.1)

Then the overall system utilization is

(1.2)in

i

i

n

i

i peuU /11

!!

!!


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The deadline for the periodic task i, di is thenext cycle or the time period, which is a

critical design factor constrained by ei , whichis difficult to determine in many cases.

For aperiodic and sporidic tasks ui is calculated

by assuming some worst case delay betweenthe event occurrences, which can inflate theutilization factor.

The utilization factor is the number of micro

instructions per second that can be processedbased on some predetermined instructionmix.


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Real Time System Design Issues

Real time systems are strongly influenced bycontrol theory, software engineering andoperations research.

The figure shows some of the disciplines ofcomputer science and electrical engineering thataffect the design and analysis of real timesystems, which makes it a highly specialized area.

The design and implementation of real timesystems requires attention to the numerousproblems:


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The selection of hardware and software andevaluation of the trade-off needed for cost

effective solution including dealing withdistributed computing systems and issues ofparallelism and synchronization.

Specification and design of real time systemsand correct representation of the temporalbehavior.

Understanding the nuances of the

programming language(s) and the real-timeimplications resulting from their translationinto machine code.


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Maximizing of system fault tolerance and

reliability through careful design.

The design and administration of tests, and the

selection of test and development equipment.

Taking advantage of open systems technology

and interoperability with open system standards,such as the CORBA real-time standard.

Measuring and predicting response time and

reducing it, performing a schedulability analysis,

for determining and guaranteeing deadline

satisfaction, a priori, of most of scheduling

theory.


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Sample systems


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The inertial measurement system of an

aircraft:

The software specifications states that the

software to receive x , y and z accelerometer

pulses at a 10 millisecond rate from special

hardware, determining the accelerations in eachdirection and the roll, pitch and yaw of the

aircraft.


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The software will also receive information such astemperature at a 1-second rate.

The task of the software is to compute actual velocityvector based on the orientations, accelerometerreadings and various compensation factors at a 40-millisecond rate.

The system is to output true acceleration, velocity and

position vectors to a pilots display every 40-millisecond rate, but using a different clock.

These tasks execute at four different rates in theinertial measurement system and need tocommunicate and synchronize.

The accelerometer readings must be time-relative orcorrelated; i.e., it is undesirable to mix anxaccelerometerpulse from t with z and y pulses fromtime t + 1.

These are critical design issues for this system.

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