Roger Johansson

Embedded Real time systems - an Introduction 1

Embedded Real time systems – an Introduction

What’s a ”Real time system”? The concept of control. Embedded real time control – some examples Some terminology ”Scheduling” – determination of actions ”Execution time analysis” – a prerequisite for scheduling

Roger Johansson

Embedded systems

Embedded Real time systems - an Introduction 2

Wikipedia...

” An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints.

It is usually embedded as part of a complete device including hardware and mechanical parts.”

More distinct...

” An embedded system is a special-purpose digital system designed to perform dedicated functions with real-time computing constraints.

The digital system hardware is embedded as part of a complete device which might include other hardware such as mechanical, pneumatic or hydraulical parts.”

Roger Johansson

Embedded Real time systems - an Introduction 3

Digital systems

Server applications

Personal computing

Embedded systems

Real-Time Control systems

Safety-critical R-T control

Secure/RobustR-T control

Roger Johansson

Embedded Real time systems - an Introduction 4

In Control ? Swedish fighter JAS 39 GRIPEN

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Embedded Real time systems - an Introduction 5

In Control ? but there is control ...

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Embedded Real time systems - an Introduction 6

In Control ?

A control system is a feedback system

Time properties (temporal aspects) is fundamental for a well designed control system.

A computer designed for control has to handle REAL-TIME as desired by the controlled object.

Control requires feedback

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Embedded Real time systems - an Introduction 7

traffic control

local control

train (consist) control

((( )))

((( )))

wayside control

traffic control

local control

train (consist) control

((( )))

((( )))

wayside control

Real time control applications

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Embedded Real time systems - an Introduction 8

The “F-8 Digital Fly-By-Wire (DFBW)” validated fundamental concepts still used today. The first “by-wire” flight was in the 25’th of May in 1972. The project lasted for 13 years.

hydraulic information

carrier

electronic information

carrier

steer ”by-wire”

Roger Johansson

Embedded Real time systems - an Introduction 9

Computers in space…

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Embedded Real time systems - an Introduction 10

..and within marine applications.

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Embedded Real time systems - an Introduction 11

..development of Automotive electronics

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Embedded Real time systems - an Introduction 12

control characteristics in a car

simple reactive (ON/OFF) …climate, windows …

reactive, autonomous …engine- and gear- control,

anti-brake-lock, anti-spin, etc.

safety criticalairbag ...

others..navigation, audio-video ...

Roger Johansson

correct time

computer time

”computer clock”

”perfect clock”

”Correct time” is an convention

A real time clock, basic terminology

13Embedded Real time systems - an Introduction

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Cs(t)

t

Ideal : C(t)=t

Cs(t)>t

Cs(t)<t

computer time

correct time

A real time clock, basic terminology

14Embedded Real time systems - an Introduction

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)()( tCtCs

1

)(1

)()(

0

lim

dt

tCd

t

tCttC

t

DefinitionsCorrectness

Deviation

)()( 1212 tCtCtt Monotoni

15Embedded Real time systems - an Introduction

Roger Johansson

rt0 rt1

rt

ct

Δt

Clock adjustment

16Embedded Real time systems - an Introduction

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1 21

2

oscillator

timer

Mechanisms

17Embedded Real time systems - an Introduction

)Hz(p

1)s(p

Roger Johansson

Timer interrupt

interval

0 IRQ IRQ IRQ t

Y = X / interval

18Embedded Real time systems - an Introduction

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timer interrupt ”TICK”

”TIMESLICE” ”TIMESLICE”

”Timer interrupt routine” – invoked approx each ms, requires low overhead

”TIMESLICE” – systemcheck, task switch, more overhead

100 000 ”TIMESLICE” – clock adjustment

Real time clock

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SetTimeGetTimeSetTimeoutSetPeriod......

SetRealTime

GetRealTime

init_timerstart_timerstop_timertimer_interrupt

”software” clock

hardware clock

Application task

Clock services

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Embedded Real time systems - an Introduction 21

A control system

A/D

A/D

Computationscontrol laws

D/A

actuatorsensor object

r(t)

y(t) u(t)

yk

rk

uk

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Embedded Real time systems - an Introduction 22

Time triggered implementations

object

initiate

input

calculate

output

Pros

Easy to understand, easy to implement.

Cons

Time synchronization is tied to underlying hardware.

Difficult to anticipate and handle unordered events.

Software complexity depends on object .

Roger Johansson

Embedded Real time systems - an Introduction 23

activity..

initiate

object

wait...

Event triggered implementations

Pros

Easy to understand.

Short response times (good feedback performance)

Cons

Exteremely complex software when object becomes complex.

Roger Johansson

Embedded Real time systems - an Introduction 24

A software task

actuator

actuator

sensor

micro computer

sample input from

sensor

update actuators

compute new values

(control laws)

t

task 1 task 2 task 3

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Embedded Real time systems - an Introduction 25

Scheduling strategies

task 1 task 2 task 3

task switch

t

Non-preemptive

(serial)

task 1

task 2

task 3

task switch

t

Preemptive

(parallel)

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Embedded Real time systems - an Introduction 26

Measurements, program under test is executed a number of times. A Worst Case Execution Time is estimated.

Code analysis, determine the most time consuming path through the program and determine execution time for this path.

Task Execution time analysis

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Embedded Real time systems - an Introduction 27

// Example

t_start = C(t);P();t_finish = C(t);

t_exec = t_finish – t_start;

Execution time measure

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Embedded Real time systems - an Introduction 28

BB 1BB2

BB3

start

stop

label:..instr 1..instr 2..instr 3

...

..instr n

bra ...

label:..instr 1..instr 2..instr 3

...

..instr n

bra ...

Execution time analysis, basic blocks

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Embedded Real time systems - an Introduction 29

BB 1

BB 2

BB 3

start

stop

A Control Flow Graph represents all possible execution paths in a program. Contributions from all basic blocks are summed.

A Control Flow Graph represents all possible execution paths in a program. Contributions from all basic blocks are summed.

Σ BB

start

stop

Execution time analysis, Control Flow Graphs

Roger Johansson

* MC68HCS12BB:movw _b,_amovw _lb,_la

movw 2+_lb,2+_la

int a,b;long int la,lb;a = b;la = lb;

Each instruction takes 6 processor cycles (from Datasheet).

So, for the block:6+6+6=18 cycles

Execution time:18 * 1/fClock

Basic blocks, assignments

30Embedded Real time systems - an Introduction

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At the end of the block the ackumulator D should hold 1 (if the expression is true) and 0 otherwise. (Compiler convention)

At the end of the block the ackumulator D should hold 1 (if the expression is true) and 0 otherwise. (Compiler convention)

* MC68HCS12* a == b *BB:

ldd _a cpd _b bne _1 ldd #1 bra _2_1: ldd #0_2:

BB= [min,max]

Basic blocks, test evaluation

31Embedded Real time systems - an Introduction

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A program flow graph for:if( c == a )

b = 1;else

b = d+e+f+g;

A program flow graph for:if( c == a )

b = 1;else

b = d+e+f+g;

test

S1 S2

Code for ’test’ is always executed.Exactly one of statements ’S1’ or ’S2’ is always executed.

Combining basic blocks, if/else

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Inspect the code, identify and annotate the basic blocks: BB0: if( c == a )BB1: b = 1;

elseBB2: b = d+e+f+g;

From the program flow graph we conclude that:BB(min) = BB0(min) + min ( BB1(min) , BB2(min) ) andBB(max) = BB0(max) + max ( BB1(max) , BB2(max) )

Combining basic blocks, if/else

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Embedded Real time systems - an Introduction 34

Summary

we have got a brief introduction towe have got a brief introduction to

The concept of control. Embedded real time control Real Time Systems terminology Task Scheduling Execution time analysis