Grundlagen FlexRay - Institut fÃ¼r Automatisierungs- und ...

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Grundlagen FlexRay BasicsV 1.1 34 Figure 2.30 Static and dynamic FlexRay segment [VeSc11] If an ECU sends data in the dynamic segment, several minislots are replaced by a dynamic slot, see Figure 2.30. The length of a dynamic slot can be chosen, unlike the length of static frames. If it is still possible to send the frame can be determined using pLatestTx (0 to 7980 Minislots). This parameter shows if there is enough space for the dynamic frame in the current dynamic segment. To determine the current Slot ID, the counter of each node is incremented with every Minislot or dynamic slot. It is only possible to assign Slot IDs that have not been used in the static segment, the maximum Slot ID is cSlotIDMax = 2047. The lowest Slot ID has the highest priority. The length of a dynamic frame depends on the amount and length of data in the payload, and is therefore variable. Header, trailer and Channel Idle Delimiter are identical to the ones in a static frame, but the length of the payload can differ between cycles. The Dynamic Trailing Sequence acts as a buffer, so the frame length can be adjusted to the length of a Minislot. 2.11.5 Symbol Window Figure 2.31 Symbol Window and Network Idle Time [VeSc11] Bäurle 12.10.2012
Grundlagen FlexRay BasicsV 1.1 35 Figure 2.31 shows that the optional Symbol Window follows the dynamic segment. It can be used to send one Symbol per Cycle. The length gdSymbolWindow of the Symbol Windows can be defined between 0 and 142 Macroticks. The Media Access Test Symbol (MTS) is used to check the Bus Guardian and is the only Symbol currently used. The length cdCAS of the Media Access Test Symbols is 30 low bits, just like the one of the Collision Avoidance Symbol. The Collision Avoidance Symbol isn’t transmitted in the Symbol Window, but during the Startup oft he cluster. 2.11.6 Network Idle Time Every communication cycle ends with the Network Idle Time. In this segment, the bus is silent, there is no communication. The Network Idle Time is used to synchronize the ECUS, and is defined by the parameter gdNIT (2 to 805 Macroticks). As seen in Figure 2.31, the Network Idle is divided in two parts. During Channel Idle (CI) the synchronization calculation is performed, and is adjusted in the Offset Correction Segment. 2.12 Frame format A frame consists of three parts, header, payload, and trailer, see Figure 2.32. Figure 2.32 Structure of a Frames 2.12.1 Header The first 40 bits of a frame are called header. It contains information which is necessary for the protocol. • Status Bits – at the start of a frame, the Reserved Bit, Payload Preamble Indicator, Null Frame Indicator, Sync Frame Indicator and Startup Frame Indicator are transmitted. The bits describe the type of payload transmitted. A detailed description is available in [FRPS05]. Bäurle 12.10.2012
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