Christer Norström

Timed automata as task models for event-driven systems

We extend the classic model of timed automata with a notion of real-time tasks. The main idea is to associate each discrete transition in a timed automaton with a task (an executable program). Intuitively, a discrete transition in an extended timed automaton denotes an event releasing a task and the guard on the transition specifies all the possible arrival times of the event (instead of the so-called minimal inter-arrival time). This yields a general model for hard real-time systems in which tasks may be periodic or non-periodic. We show that the schedulability problem for the extended model can be transformed into a reachability problem for standard timed automata, and thus it is decidable. This allows us to apply model-checking tools for timed automata to schedulability analysis for event-driven systems. In addition, based on the same model of a system, we may use the tools to verify other properties of the system (e.g. safety and functionality). This unifies schedulability analysis and formal verification in one framework. We present an example where the model-checker UPPAAL is applied to check the schedulability and safety properties of a control program for a turning lathe.

Response time analysis under errors for CAN

The Controller Area Network (CAN) is extensively used for timely communication in automotive and other applications. As timing analysis of CAN messages is emerging into industrial practice, and as the use of CAN in safety-critical applications is increasing, there is an apparent need to include the effects of transmission errors in the analysis of message latencies. The authors provide a general fault model and extend the timing analysis of CAN messages to cater for the effects of transmission errors on message latencies. This fault model helps us in composing the effects of interferences from multiple sources and to account for them in the calculation of message response times. We illustrate our model by applying it to derive the worst case latencies of a subset of messages selected from the frequently used SAE case study. We also discuss and illustrate the implications on message latencies for some realistic fault scenarios.

Probabilistic worst-case response-time analysis for the controller area network

This paper presents a novel approach for calculating a probabilistic worst-case response-time for messages in the Controller Area Network (CAN). CAN uses a bit-stuffing mechanism to exclude forbidden bit-patterns within a message frame. The added bits eliminate the forbidden patterns but cause an increase in frame length. How much the length is increased depends on the bit-pattern of the original message frame. Traditional response-time analysis methods assume that all frames have a worst-case number of stuff-bits. This introduces pessimism in the analysis. In this paper we introduce an analysis approach based on using probability distributions to model the number of stuff-bits. The new analysis additionally opens tip for making trade-offs between reliability and timeliness, in the sense that the analysis will provide a certain probability for missing deadlines, which in the reliability analysis can be treated as a probability of failure. We evaluate the performance of our method using a subset of the SAE/sup 1/ benchmark.

Minimizing CAN response-time jitter by message manipulation

Delay variations (jitter) in computations and communications cause degradation of performance in applications such as control. There are many sources of jitter, including variations in execution time and bus contention. This paper presents methods to reduce the variation of frame (message) transmission time caused by the bit-stuffing mechanism in a controller area network (CAN). By introducing restrictions, such as a small reduction of available frame priorities, we are able to reduce the number of stuff-bits in the worst case. We also combine this with some of our previous work that reduces the number of stuff-bits in the data part of the frame. We show the actual penalty introduced by forbidding priorities, and show the overall improvement obtained by using these techniques together in a small case study.

Data management issues in vehicle control systems: a case study

We present a case study of a class of embedded hard real-time control applications in the vehicular industry which, in addition to meeting transaction and task deadlines, emphasize data validity requirements. We elaborate on how a database could be integrated into the studied application and how the database management system (DBMS) could be designed to suit this particular class of systems.

A Metaheuristic Approach for Best Effort Timing Analysis Targeting Complex Legacy Real-Time Systems

Many companies developing real-time systems today have today no means for response time analysis, as their systems violate the assumptions of traditional analytical methods for response-time analysis and are too complex for exhaustive analysis using model checking. This paper presents a novel approach for best effort response time analysis targeting such systems, where probabilistic simulation is guided by a search algorithm of metaheuristic type, similar to genetic algorithms. The best effort approach means that the result is not guaranteed to be the worst-case response time, but also that the method scales to large industrial systems. The proposed method should be regarded as a form of testing, focusing on timing properties. An evaluation is presented which indicates that the proposed approach is significantly more efficient than traditional probabilistic simulation in finding extreme task response times. The paper also presents a method for finding good parameters for the search algorithm, in order to improve its efficiency.

Extracting Simulation Models from Complex Embedded Real-Time Systems

A modeling process is presented for extracting timing-accurate simulation models from complex embedded real-time systems. The process is supported by two complementary methods for tool-supported model extraction, Model Synthesis and Hybrid Model Extraction. The generated models enable impact analysis for complex real-time systems with respect to dynamic system properties, such as timing and resource usage. This can make software mainetnance more predictable with respect to time-to-market and development costs, since timing errors can be identified early and avoided. The contribution of the paper is the modeling process, the Hybrid Model Extraction method and an interactive modeling tool, MASS, designed to support Hybrid Model Extraction of large implementations in C.

Real world influences on software architecture - interviews with industrial system experts

Industrial systems are examples of complex and often long-lived systems in which software is playing an increasingly important role. Their architectures play a crucial role in maintaining the properties of such systems during their entire life cycle. In this paper, we present the results of a case study based on a series of interviews and a workshop with key personnel from research and development groups of successful international companies in their Swedish locations. The main goal of the investigation was to find the significant factors which influence system and software architectures and to find similarities and differences between the architecture-determining decisions and the architectures of these systems. The role of the architect was an important subject of the investigation. Our findings result in recommendations relating to the design and evolution of system architectures and suggestions regarding areas in which future research would be beneficial.

Towards Aspectual Component-Based Development of Real-Time Systems

Increasing complexity of real-time systems, and demands for enabling their configurability and tailorability are strong motivations for applying new software engineering principles, such as aspect-oriented and component-based development. In this paper we introduce a novel concept of aspectual component-based real-time system development. The concept is based on a design method that assumes decomposition of real-time systems into components and aspects, and provides a real-time component model that supports the notion of time and temporal constraints, space and resource management constraints, and composability. We anticipate that the successful applications of the proposed concept should have a positive impact on real-time system development in enabling efficient configuration of real-time systems, improved reusability and flexibility of real-time software, and modularization of crosscutting concerns. We provide arguments for this assumption by presenting an application of the proposed concept on the design and development of a configurable embedded real-time database, called COMET. Furthermore, using the COMET system as an example, we introduce a novel way of handling concurrency in a real-time database system, where concurrency is modeled as an aspect crosscutting the system.

Managing complex temporal requirements in real-time control systems

Design and implementation of motion control applications includes the transition from control design to real-time system implementation. To make this transition smooth, the specification model for the real-time system should allow also for temporal requirements other than deadlines, e.g., deviation from nominal period time of an activity, end-to-end timing constraints, temporal correlation between different sampling tasks and constraints on temporal variations in output. Many real-time systems in industry today are based on pre-emptive priority based run-time systems, and hence, the temporal requirements should be fulfilled by correctly assigning attributes such as priorities and offsets to the tasks executing in such systems. Assigning priorities and offsets in order to fulfill complex temporal requirements originating from control design and computer system design is a hard task that should be supported by powerful methods and tools. In this paper we propose a method which, by assigning priorities and offsets to tasks, guarantees that complex timing constraints can be met. In addition to the complex timing constraints, the method supports sporadic tasks, shared resources, and varying execution times of tasks. We present the idea and implementation, which is based on a genetic algorithm, and illustrate the method by an example.Design and implementation of motion control applications includes the transition from control design to real-time system implementation. To make this transition smooth, the specification model for the real-time system should allow also for temporal requirements other than deadlines, e.g., deviation from nominal period time of an activity, end-to-end timing constraints, temporal correlation between different sampling tasks and constraints on temporal variations in output. Many real-time systems in industry today are based on pre-emptive priority based run-time systems, and hence, the temporal requirements should be fulfilled by correctly assigning attributes such as priorities and offsets to the tasks executing in such systems. Assigning priorities and offsets in order to fulfill complex temporal requirements originating from control design and computer system design is a hard task that should be supported by powerful methods and tools. In this paper we propose a method which, by assigning priorities and offsets to tasks, guarantees that complex timing constraints can be met. In addition to the complex timing constraints, the method supports sporadic tasks, shared resources, and varying execution times of tasks. We present the idea and implementation, which is based on a genetic algorithm, and illustrate the method by an example.