Real Time & Embedded Systems

Lecture 1: Real Time Systems Software (RTSS)

• RTSS: the software half of the Real Time & Embedded Systems unit• What do we mean by RTSS

– Two aspects:– A: RTS(S), i.e. the applications Software for Real Time Systems (RTS)

• The design of complex RTS(S) is a special subject, not covered in this unit. We will look at simple examples where design tools are not needed.

– B: RT(SS), i.e. the Systems Software (operating systems or other) to support A, the applications.

Real Time & Embedded Systems

• RTS(S) – What? Firstly, what is time– Real time is:

• measured by real clocks (atomic clocks, Big Ben, GMT, UST), in sidereal years, seconds.

• one of the 4 co-ordinates describing the progress of events in the universe (World lines, basic laws of physics)

• inherent in the inputs of embedded systems (sensors usually measure things which are changing in the real world, i.e., in real time according to the laws of Physics)

– Logical time is:• an abstract, philosophical idea that doesn’t involve any special time units.• to do with the ordering of events – including the concepts of ‘now’, ‘before’ and

‘after’. • essential to understanding any procedural computer algorithm.

– Algorithms involve sequences, loops and alternatives, all of which are based on the idea of a CPU executing the ‘next’ instruction at the ‘next’ tick of its clock. But the correct logical outcome of a ‘normal’ program doesn’t depend on how many real seconds there are in each tick of the CPU clock.

– 2 (input), 4 (input) >>> average algorithm >>> 3 (output) is entirely logically determined, not depending on real time at all.

– A computer could even be built without fixed intervals between its clock ticks and its logical performance would still be fine (this happens when your laptop gets too hot). Asynchronous computers are being designed which don’t have clocks at all (‘my computer is faster than yours, it has a clock speed of 0Hz!’).

Real Time & Embedded Systems

• RTS – What? Then secondly, what is a Real Time System (RTS)

– One that has Real Time (RT) in its specification, so RTS(S) has RT in its specification.

• RT specs can be absolute (Mars Lander, DVD recorder, GPS software)• or relative (Anti-lock brakes, engine management system, weather

forecast)– A correct RTS(S) meets both its logical and temporal Functional

Specifications– RTS’s are not to do with speed

Computer: Digital Electronic Engine with memory and a logical clock +access to real clock

0’s & 1’s in 0’s & 1’s out

Logical specification

+ time specs

output time related to inputtime

Program (0’s & 1’s)

Real Time events Real time responses

Real Time & Embedded Systems

A chess program – RT or not?

• Yes, partly, if it is playing to professional rules• NB

– Not all the program has time specifications – only parts need to be synchronised with a UST clock. This is typical of most RTS.

– If this was an elaborate chess playing machine (actually moving pieces around, robot style, then there would probably be additional time specs associated with the HCI)

Chess

Set-up ReviewPlay

Make move

Next turn *

Invite move

until game over

Real Time & Embedded Systems

Hard and Soft Real Time Systems

• Hard Real Time Systems– fail if any part of the time specifications are not not met, e.g. for an airbag– many safety critical systems will be hard real time systems – just think about

air traffic control, rail signalling, or intensive care life support systems

• Soft Real Time systems– Some time specifications can be missed without compromising the long

term performance of the system, e.g. a car engine management system.• Too many missed time specs will turn a soft RTS into a hard one• Soft real time systems can be hard to classify, e.g. HCI’s

• Think about ATM’s and games – hard or soft?

Real Time & Embedded Systems

Real Time Embedded Systems (RTES)

• RTES is the main answer to the Why? question for RTS(S)• What is an Embedded System?

– Systems with computers (CPU’s, programs and memory) whose purpose is not computation – they have another purpose.

• What is computation?– Logical transformation of input data (0’s & 1’s0 into more useful

output data (0’s & 1’s), some examples: Numerical calculations of all kinds (inc spreadsheets), Word processing, Imaging programs

– Other purposes usually involve real things in the real world:• Xmas cards, Smart cards, Washing machines and most ‘white’ goods,

Modern airplanes.

• Jean Bacon views RTS as just a sub-set of Concurrent Systems

Real Time Embedded System

Multiple concurrent events, sensors,Measurements, coming from the real world

Multiple activators