Real Time System
A real time system is a software system where the correct functioning of the system depends on the results produced by the system and the time at which these results are produced.
- Time is critical so real-time systems must respond within specified times.
- Systems which monitor and control their environment.
- Inevitably associated with hardware devices:
- Sensors: Collect data from the system environment
- Actuators: Change the system’s environment in some way
The real time system can be classified as:
1.Soft real time system:
A soft real-time system is a system whose operation is degraded if results are not produced according to the specified timing requirements.
2. Hard real time system:
A hard real-time system is a system whose operation is incorrect if results are not produced according to the timing specification.
Fig: An embedded real time system model
- Sensor control processes: Collect information from sensors. May buffer information collected in response to a sensor stimulus.
- Data processor: Carries out processing of collected information and computes the system response.
- Actuator control processes: Generates control signals for the actuators.
Fig: Sensor and actuator processes
- System Design comprises of both the hardware and the software associated with system.
- Design decisions should be made on the basis on non-functional system requirements.
- Hardware delivers better performance but potentially longer development and less scope for change.
1 Real Time System Design Process
- Identify the stimuli to be processed and the required responses to these stimuli.
- For each stimulus and response, identify the timing constraints.
- Aggregate the stimulus and response processing into concurrent processes. A process may be associated with each class of stimulus and response.
- Design algorithms to process each class of stimulus and response. These must meet the given timing requirements.
- Design a scheduling system which will ensure that processes are started in time to meet their deadlines.
- Integrate using a real-time operating system.
2 Timing Constraints
- May require extensive simulation and experiment to ensure that these are met by the system.
- May mean that certain design strategies such as object-oriented design cannot be used because of the additional overhead involved.
- May mean that low-level programming language features have to be used for performance reasons.
3 Real Time system Modeling
- The effect of a stimulus in a real-time system may trigger a transition from one state to another.
- Finite state machines can be used for modeling real-time systems.
- However, FSM models lack structure. Even simple systems can have a complex model.
- The UML includes notations for defining state machine models.
Fig: Petrol pump state model
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