Grundlagen FlexRay - Institut fÃ¼r Automatisierungs- und ...

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Grundlagen FlexRay BasicsV 1.1 12 A detailed overview of the requirements can be found in the FlexRay Requirements Specification V2.1 [FRRS05]. Because of these properties, the FlexRay system is suited as a backbone, and for use in realtime and safety-critical applications. FlexRay is currently used in the BMW X5, BMW 7, Audi A8, and the new S-Class by Daimler [VeSc11]. 2 Technical basics 2.1 OSI-Model Figure 2.1 OSI-Model fort he bus system and protocols The general structure of a communication system is described by the OSI-Model. As seen in Figure 2.1, the model consists of layers. Every layer is assigned a specific function, and is based on the layers below itself. A FlexRay system defines OSI-Layers 1 and 2, the other layers are not defined by FlexRay. The physical layer (OSI-Layer 1) is defined in the FlexRay Communications System Electrical Physical Layer Specification [FREPLS06]. The data link layer (OSI-Layer 2) specifies the actual communication protocol, and is defined in the FlexRay Communications System Protocol Specification [FRPS05]. As mentioned before, the higher layers are not defined by FlexRay. In the automotive sector, this role is taken by the AUTOSAR specification. The application layer (OSI-Layer 7) enables the applications to access the hardware, and provides data structures and protocols. 2.2 Structure of a communication node A FlexRay network consists of several communication nodes and communication channels. A FlexRay node (see Figure 2.2) consists of a Microcontroller (µC, also called host), a Communication Controller (CC), an optional Bus Guardian (BG), and one or two Bus Driver (BD). The Bus Driver isn’t used in practical applications, since there are no available components on the market. In the future, a second Communication Controller will be used, which renders the Bus Driver useless [VeSc11]. Bäurle 12.10.2012
Grundlagen FlexRay BasicsV 1.1 13 Figure 2.2 Structure of a FlexRay node [VeSc11] Every component has specific tasks. There are different types of microcontrollers, with or without an integrated Communication Controller. The Bus Driver provides the physical connection to the respective channel. The Bus Guardian was designed as a security system between Communication Controller and Bus Driver, but isn’t used as discussed before. The Communication Controller implements the FlexRay protocol. It controls the bus access (Media Access Control (MAC)), verifies the timing specified by the scheduling and the syntax of the frames (Frame and Symbol Processing (FSP)), performs the transformation of payload data into a bit stream (Coding and Decoding Process (CODEC)) and the synchronization with the global time base (Clock Synchronization Process (CSP)). An indepth description of these processes can be found in the FlexRay Communications System Protocol Specification [FRPS05], where they are described in the Specification and Description Language (SDL). The host runs the actual application, which controls the Communication Controller via the Controller Host Interface (CHI) and the Protocol Operation Control (POC)). Bäurle 12.10.2012
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