Software engineering for real-time systems

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SOFTWARE ENGINEERING for REAL-TIME SYSTEMS (© J.E.Cooling 2003) Introduction to real-time systems - slide 1

Software engineering for real-time systems

Section 1 Introduction to real-time systems

Objectives

To:

• Outline the differences between general-purpose

computer applications and real-time systems.

• Give an overview of practical real-time system

structures.

• Describe some key environmental and performance

requirements of embedded real-time computers.

• Describe the structures of modern microprocessors

and microcomputers.

Introduction

Cray

supercomputer

Fibre-optic

communication unit

Fibre-optic

communication unit

Fibre-optic

data link

Remote site

Local site

A modern batch system

Local multiplexer Site 1 Local multiplexer Site 2 Central (mainframe) computer Remote site

Typical interactive on-line computer system

Control panel

Control

computer

Power unit

Vehicle

sensors

A real-time computer system

SLOW

FAST

SOFT

Machinery condition monitoring

Man-Machine Interfacing

HARD

Missile point defence system

Airbag control system

• Two major categorization factors: criticality and speed.

• Criticality:

• Hard systems - deadlines (responsiveness) is critical. Failure to meet these

have severe consequences (e.g. injury, damage or death).

• Soft systems - deadlines are less critical; in many cases significant

tolerance can be permitted.

• Speed:

– Fast systems - responses in the microseconds to hundreds of milliseconds.

– Slow systems - responses in the range seconds to days.

– Arbitrary boundary: 1 second (chosen because the problems shift from

individual computing issues to overall system behaviour and interaction at

around this point).

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• Major attributes of real-time systems.

• For hard systems emphasis is placed on the need to meet deadlines.

• Hard-fast systems tend to have low software complexity.

• For soft-fast systems emphasis is put on computation performance.

• In general soft systems tend to be the more complex ones (from a software

perspective).

Execution

time

Deadlines

Software

size

Software

complexity

Hard - Fast

zzzz

z

Hard - Slow

z

zzzz

z

Æ

zzz

z

Æ

zzzz

Soft - Fast

zzzz

zz

z

Æ

zzz

z

Æ

zzz

Soft - Slow

zz

z

Æ

zzzz

z

Æ

zzzz

Attribute rating

z

Low

zzzz

high

Attributes of real-time systems

SOFTWARE ENGINEERING for REAL-TIME SYSTEMS (© J.E.Cooling 2003) Introduction to real-time systems - slide 8 Remote plant 1 Data collection Control functions Communication interface Microprocessor-based outstation Sensors Actuators Land line communication link (10 miles) Wireless communication link (100 miles) Front-end communication interface Main control computer Hot standby control computer Operator display and controls Data logging computer Management information computer Local area network

Central control room

Remote plant 2

Telemetry control system

Sea Skua missile system

Submarine control console

Typical avionic platforms

(3)

Consumer communications

Embedded systems

platforms

Bespoke designs

Commercial off the shelf

(COTS) equipment

Board-based

designs

PC-based

designs

VME

PC/104

Embedded PC

Desktop

PC

Ruggedized

PC

Vendor

specific

Embedded systems platforms

Embedded systems

characteristics

Defined by

Environment

Performance

Interfacing

Physical

Electricial

Operational

Speed of

response

Failure

modes

Embedded systems characteristics

Degrees centigrade

-55 0 +50 +85 +125 +200

Military range

Industrial

range

Oil exploration

Space exploration

Extended industrial

range

Typical temperature specifications for real-time systems

Computer loading - single synchronous (periodic) task

Execute

event task

(E)

processor idle (I)

T

e

T

e

T

e

(I)

Execute

event task

(E)

Execute

event task

(E)

Time

(I)

Event

signal

Event

signal

Event

signal

(4)

Computer loading - multiple asynchronous (aperiodic) tasks

(I)

(E

1

)

(E

1

)

(I)

(E

2

)

(E

1

)

(I)

(E

1

)

(E

2

)

S

1

S

1

S

1

S

1

S

2

(W)

S

2

(W) = waiting time for task to

complete

(I)

Time

(E1) = Event task 1 execution

(E2) = Event task 2 execution

(S1) = Event signal, task1

(S2) = Event signal, task 2

(I) = Idle (spare) time

Computing elements

Microprocessors Microcomputers

Specialized computing General purpose computing

General purpose microprocessors Highly integrated microprocessors Single chip microcomputers Single chip microcontrollers Digital signal processors (DSPs) Mixed signal processors Bespoke system-on-chip designs

The computing elements of real-time systems

Elements of a microcomputer system

Timing in hardware

Real-time clock

Programmable timer

Address

bus

Chip

select

line

Control

bus

To

interrupt

Address

decoder

Retriggerable timer

(monostable)

I/P

O/P

To non-maskable interrupt

External alarm (optional)

Write

control

Address

bus

O/P

I/P

Time

Timeout

The watchdog timer

Fig.1.21 DMA operation

ROM RAM DMA controller Peripheral interface DMA request/ack (B) DMA mode of operation Micro ROM RAM DMA controller Peripheral interface Processor hold/ack DMA request/ack

(a) Normal mode of operation

(5)

Intelligent I/O processing

Main

processor

I/O processor

Disk store

Local

memory

Primary

bus

Secondary

bus

Programmable command

register

Data bus buffer

Address and control interface I/O port I/O port I/O port 8 lines 8 lines 8 lines Address and control bus Data bus

I/O interface peripheral

PowerPC 603e microprocessor core (including floating point

maths unit) Central control unit Data path controller Memory controller

PCI bus interface unit Interrupt controller and timers DMA controller Serial comms controller (I2C) Intelligent message controller (I2O) PCI bus arbiter Bus request (5) Bus grant (5) PCI bus I2C bus Data bus Memory bus Interrupts

Highly integrated processor -Motorola MPC8240

Processor

Clock

Program store

(8kByte ROM)

Data store

(256 Bytes RAM)

Hardware timers

(3)

Interrupt

controller

Serial

communication

controller

I/O ports (4)

Security ROM

External bus

Single chip microcomputer - Philips Semiconductor 8052

SOFTWARE ENGINEERING for REAL-TIME SYSTEMS (© J.E.Cooling 2003) Introduction to real-time systems - slide 29 LED driver DAC (2) Stepper motor control I2C serial interface Sound generator GP Serial I/O ADC (16) Interrupt interface (8) Real-time clock Hardware timers (6) Pulse generator Digital I/0 y CPU y Watchdog y Interrupt controller

y Data RAM (16 kByte)

y DMA

y Cache memory (1 kByte)

Processor unit

Boot ROM (2 kByte)

Bus interface CAN interface (3) Flash (512 kByte) External bus

Single chip microcontroller - Fujitsu MB91F361

(6)

Flash

(256 bytes)

RAM

(256 bytes)

CPU

Watchdog

timer

Debug

interface

Power-on

reset

Ports with

interrupt

facility (2)

PWM

Timer

Analogue

comparator

Clock

Mixed signal processor structure - Texas MSP430

ROM

RAM

Processor

core

Custom I/O

interface

Custom

power-management

unit

Custom

MMU

Custom

peripherals

and debug

interface

Custom

co-processor

On-chip databus (OCB)

External

bus

Example system-on-a-chip (SOC) design

Review of section

You should now:

• Clearly understand the important features of real-time systems.

• Know what sets them apart from batch and interactive application.

• See how real-time systems may be categorized in terms of speed and criticality.

• Have a general understanding of the range of real-time (and especially embedded)

applications.

• Realize that environmental and performance factors are key drivers in real-time

systems design.

• Know the basic component parts of real-time computer units.

• Appreciate the essential differences between microprocessors, microcomputers and

microcontrollers.

• Realize why there is a large market for specialized processors.

END OF SECTION ‘Introduction to real-time systems’