Tta protocolsfinalppt-140305235749-phpapp02
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The document provides an overview of time-triggered architecture (TTA) and communication protocols. TTA treats physical time as fundamental and provides a fault-tolerant global time base. It decomposes applications into clusters, nodes, and their interfaces. Communication is specified via global time and time-triggered protocols like TTP/C and FlexRay are used. TTA architecture consists of nodes with host and communication subsystems connected via a time-triggered bus.
Tta protocolsfinalppt-140305235749-phpapp02
- 1. Hrudya The Time-Triggered Architecture And Protocols
- 2. Outline of the talk Overview of TTA Architecture Model Real Time Image and Real Time Entity Communication Sub system Host Subsystem Transducer Node Global Timing Concepts Communication Protocols TTP/C Flex Ray Advantages .
- 3. Treatment of physical time as a first-order quantity. Provides fault-tolerant global time base. Decomposes a large application into: Clusters Nodes Combination of both Use global time to specify interfaces between nodes. Communication and agreement protocols. Overview Of TTA
- 4. Architectural Model Decomposes a large application into: Clusters Nodes Combination of both Fault Tolerant Units Interfaces Use global time to specify interfaces between nodes. Communication and agreement protocols.
- 5. The TTA architecture consist of a set of electronic modules that are connected to a channel. Communication between electronic modules is performed using the time triggered protocols. Each electronic module is called a node . A node consist of a host sub system and a communication sub system. Host sub system - executes the application software . Communication sub system - consist of TTA communication controller and Communication Interface.
- 6. The communication sub system decides autonomously when to transmit a message. whether a particular message is relevant for the particular electronic module or not. Such an electronic module is 'Smallest Replaceable Unit ' or SRU. TTP network and its associated modules is called a cluster . One electronic control unit is called a ttp node . Several nodes connected by cables compose of TTP
- 7. At the core of the TT-model is the notion of the temporal accuracy of real-time data. Real-time data loses its validity as time progresses. In order to refine this notion, the concepts of a real time (RT) entity and of a real- time (RT) image.
- 8. Real Time Image And Real Time Entity Real Time Entity changes its state as a function of time. If we freeze time, we can describe the current state of the object by recording the values of its state variables at that moment. The information about the state of an RT entity at a particular instant is captured by the notion of an observation. An observation is an atomic data structure Real Time Image A real-time (RT) image is a current picture of an RT entity. A real-time (RT) image at instant t, is accurate if the duration between the time of observation and the instant t is smaller than the accuracy interval dacc, which is an application specific parameter associated with the given RT entity. The validity of an RT image is time-dependent and is likely to be invalidated by the progression of real-time
- 9. Interfaces A boundary between two sub systems. Consists of a memory element that is shared between two interfacing subsystems. It also contains valid RT images of the relevant RT entities. Can be viewed as a dual-ported memory, where the information-producing subsystem updates the RT image periodically in order to ensure that the RT image in the interface is valid. The information-consuming subsystem reads this temporally accurate information whenever needed.
- 10. Communication Sub System Connects interfaces Transports data elements from one interface to another within a priori (known deterministic time bounds). The semantics of the transported data is state- message semantics,. The points in time when messages are send and delivered are stored in dispatching tables. The contents of these dispatching tables are designed before run time and are common knowledge to all communicating partners.
- 11. Host Sub System A host computer is an encapsulated computational machine Consists of one or more processing units, a memory, system software and application software. The host computer reads the input data from one or more interfaces writes the output data into one or more interfaces. Priori instants when input data to the host computer are delivered at the interfaces when output data from the host computer are fetched from the interfaces If a host computer has to react to a specific state change (event) it has to periodically sample the respective interface data items and trigger some action if the anticipated state change has been observed.
- 12. Transducer A transducer translates the environment's representation of an RT entity to the digital format of the corresponding RT image that is stored in the associated interface and vice versa. The time delay between sampling the RT entity in the environment and writing the RT image into the interface is constant and known a priori. Transducers are introduced to model the input/output system of a real-time system.
- 13. Node Figure depicts the structure of a standard TTA node. A standard node consists of two subsystems, a host computer and a time- triggered communication protocol (TTP) controller with a controller internal data structure (the message descriptor list MEDL) that determines when a message must be sent or received.
- 14. The host computer of the node corresponds with the host computer in the TT model. It is a self contained computer with its own operating system and the application software. It interfaces to the communication controller via the communication network interface CNI. The CNI is the concrete implementation of the interface building block of the TT model.
- 15. Global Timing concepts The model of time of the TTA is based on Newtonian physics. Real time progresses along a dense timeline, consisting of an infinite set of instants, from the past to the future. A duration (or interval) is a section of the timeline, delimited by two instants. A happening that occurs at an instant (i.e., a cut of the timeline) is called an event. An observation of the state of the world is thus an event. The time-stamp of an event is established by assigning the state of the node-local global time to the event immediately after the event occurrence.
- 16. Working Of TTA Access to the bus is controlled by a cyclic time division multiple access (TDMA) scheme derived from the global notion of time Every active electronic module owns a TDMA slot The sequence of TDMA slots in which each electronic module sends its frames forms a TDMA round . After a tdma round is completed the next tdma round with the same temporal access pattern , but possibly different frames is started . The number of different TDMA rounds determines the length of the cluster cycle . After a cluster cycle is finished the transmission pattern starts over again with the start of next cluster cycle .
- 17. Two types of message frames are sent on the bus N Frames which contains user data and. I frames which are system messages needed for reconfiguration.
- 18. Communication Protocols The time triggered communication protocol TTP provides the clock synchronization service that is needed for the implementation of the TT-model. The TTP communication controller contains Its own data structure The message descriptor list MEDL-- specifies the global interaction pattern among the nodes of a cluster and The temporal and value parameters of the communication network interface (CNI).
- 19. TTP/C In TTP/C communication is organized into TDMA rounds as depicted in Figure . A TDMA round is divided into slots. Each node in the communication system has its sending slot and must send frames in every round. The frame size allocated to a node can vary from 2 to 240 bytes in length, each frame usually carrying several messages. The cluster cycle is a recurring sequence of TDMA rounds; in different rounds different messages can be transmitted in the frames, but in each cluster cycle the complete set of state messages is repeated.
- 20. The data is protected by a 24 bit CRC (Cyclic Redundancy Check). The schedule is stored in the message descriptor list (MEDL) within the communication controller.
- 21. Membership Consistency Check: Due to the strict round-robin scheme of the TDMA round, each node sees and checks the membership lists of all other nodes in one TDMA round. Each sender with a different membership list is assumed as incorrect. This ensures a consistent view of all nodes, which accept each other in the membership, i.e., they communicate successfully with one another.
- 22. A TTP/C frame can carry up to 240 bytes. Part of the frame can be used for event messages. The event-triggered messages are piggy-backed on TTP/C frames. This partitioning of each slot in state data, event data, and spare bandwidth for future extensions is depicted in Figure 4.
- 23. FlexRay Flex Ray is a serial communication technology that is used in particular for data communication in very safety-critical use areas in the automobile. A Flex Ray communication system (Flex Ray Cluster) is made up of a number of Flex Ray nodes and a physical transmission medium (Flex Ray Bus) interconnecting all of
- 24. The FlexRay cluster is based on a time-triggered communication architecture, whose core property is static, time defined triggering of actions in the distributed system. The principle of time control not only enables deterministic data communication, but also simple composability of a communication system and implementation of concepts building upon it, such as fault tolerance that is attained by integrating redundancies and synchronous triggering of actions. To implement time-triggered control, the TDMA method (Time Division Multiple Access) is used, which means that the FlexRay nodes may not access the bus in an uncontrolled manner in response to application-related events as in CAN.
- 25. Flex Ray nodes must conform to a precisely defined communication schedule, which assigns a specific time slot to each Flex Ray message per communication cycle and thereby prescribes the send times of all Flex Ray messages. An illustrative communication schedule and the communicative flow on the communication medium are shown in the figure “TDMA Principle”. The communication schedule is based on a communication system consisting of four bus nodes, where each bus node must transmit two messages at specific times
- 26. Flex Ray Node A Flex Ray node is an electronic control unit (ECU), which is connected to a Flex Ray bus via a Flex Ray interface. The Flex Ray interface is made up of a communication controller and one or two bus drivers, depending on the number of channels. The communication controller is referred to as a Flex Ray controller. The bus driver is referred to as a Flex Ray transceiver. The Flex Ray controller executes the communication protocol defined in the Flex Ray specification
- 27. In a FlexRay cluster, the FlexRay nodes are granted access to the communication medium in two different ways: TDMA method (Time Division Multiple Access) FTDMA method (Flexible Time Division Multiple Access), whose core consists of the TDMA method. The TDMA method is based on a communication schedule, which is organized into a number of time slots (static slots) of equal length, each assigned to a FlexRay node. During communicative operation a FlexRay node is granted access to the communication medium (bus) according to this schedule.
- 28. From the first to the last static slot, the Flex Ray nodes assigned to the static slots obtain exclusive access to the bus to transmit the messages assigned to the static slots. The communication schedule is executed periodically by all Flex Ray nodes during communicative operation; all static messages are transmitted with a specified period, i.e. deterministically. The communication schedule defines nothing other than a communication cycle, or stated more precisely, the Flex Ray communication cycle.
- 29. Data transmission in a FlexRay cluster is executed using a uniform message frame (FlexRay message). Each FlexRay message is composed of three parts: header, payload Trailer. The header consists of 40 bits. Of these, eleven bits belong to the identifier (ID). This identifies a message and corresponds to a slot. A maximum of 254 user bytes (payload) can be transported by one message.
- 30. The payload length parameter shows the size of the payload in words. The system designer must define this value during the configuration phase. Since dynamic messages are not restricted to a fixed payload size, the payload length may assume different values for such messages. To protect the payload, the CRC method (CRC: Cyclic Redundancy Check) is used.
- 32. END!