Introduction

Real-Time Systems

the notion of a "system" is central to software engineering
a system for engineers is an assembly of things interacting together according to a precise relationship for transforming a set of functions called inputs into a set of functions called outputs
there is no system without interactions, and there is no system with outputs happening before the inputs are manifested
every real world entity, whether organic or synthetic, can be modeled as a system the condition of transforming inputs in outputs
in computing systems, the inputs represent digital data collected from hardware devices called sensors
the application has modules which acquire and process the data produced by the input hardware, and deliver it to output devices called actuators
the actuators deliver the output to the process

Example:

The objective: building a controlled Robot
requires the design of a computer application whose server, runs on a multi threaded CPU under an RTOS (Linux);
collects data from the robot sensors and stores it (some apps use a database), and at the same time calculates the next step to be made (by the Robot).
Sensors:
- two rotation speed sensors,
- a GPS sensor
- two collision sensors (one in the front and one at the back of the robot)
- a real-time streaming camera which sends images to the CPU of the robot

A block diagram, and the block itself is a conventional representation of a system, system components, and system elements.

Every real world entity can be modeled as a system.

Input/Output (I/O) devices usually have some analog or non-electronic component.

Digital logic is used to interact with the CPU.

Devices have different registers to store information.

Status register: read-only register holding information about current state of the I/O device
Data register: Read/write register with most recent data passed between CPU and I/O device

It may be necessary to interact with a device frequently to retrieve or send information.

Most basic ways to use devices is busy-wait I/O.

This is necessary when devices are slower than CPU. To avoid inefficient use of processor time, interrupts are used as an alternative.

inputs which occur at periodic time intervals
ex. timed events and synchronous communication, a temp. sensor may be polled 10 times per second, consequence: the computer or the OS has to trigger time events in order to poll the transducers

inputs which occur at unpredictable times
ex. asynchronous communication, interrupts and exceptions. A system power failure may trigger an interrupt which must be processed by the system, consequence: the computer or the OS has to react to event and trigger tasks for processing these events.

Signal (or event) used to get immediate attention of processor
Handled by an interrupt service handler implemented for the specific event that occurred (and potentially others)

consists of an operator (human or machine), computer system, and an external object (plant) being controlled
instrumentation interface: mechanisms used by the system to control the plant
man machine interface: mechanisms used by the user to interact with the system

Many different definitions with similar descriptions

A computer (or processor) based information processing system with the following attributes

Must respond to externally generated input within a finite and specified period of time
Responses are computer generated signals or actions which commands the external device(s) connected to the computer
The correctness of a response depends on the logical result and the time it was delivered a time constraint which not fulfilled leads to severe consequences, including failure.

Failure for a real-time system to respond on time is as bad as not responding at all

Special purpose computer system
Completely encapsulated by the device it controls
Specific requirements and performs pre-defined tasks (unlike general purpose PC)
Programmed hardware device
Combination of hardware and software
- Facilitates mass production and variety of applications
Significant growth market

discipline where computers are used to control external processes
RTS automate processes using computer systems
Processes controlled in timely manner
Computers act as control devices instead of humans
Controlling an external process is the feature which distinguished a general computer from a Real-Time or Embedded Processors

Real-time systems are used to control external processes (also called a plant or controlled object)
Closed-loop (feedback) operation is common with real-time systems
The system has to keep a response (output) signal as close as possible to a desired value (input or prescribed value)
the system output is brought back to the input, compared with the input and the result (the error signal or simple the error) is used to make a decision about the next appropriate action. This action is called the feedback operation
The control system will often use the error signal to make a decision about how to manipulate the output
the error signal is a measure of the difference between the system response as determined by a sensor or transducer (the feedback signal) and the desired response