Grundlagen FlexRay - Institut fÃ¼r Automatisierungs- und ...

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Grundlagen FlexRay BasicsV 1.1 24 2. Scenario – single Coldstart Node in the cluster After the Wakeup of a Coldstart node, it enters POC:startup state. As a next step, it checks weather there is already an active communication on the channels. In that case, it synchronizes itself to the cycle, and enters POC:normal active state. If there is no communication, the node starts the Startup procedure by sending a Collision Avoidance Symbol (CAS). This identifies the node als Leading Coldstart Node. After that, the node sends 4 Startup Sync Frames (SyF), see Figure 2.20 - (1). If no other node answers (like in this scenario), the Coldstart Node stops its activity after a defined number of retries, and waits for another node to take the role of Leading Coldstart Node. 3. Scenario – two or more Coldstart Node in the cluster After “racing” for the role of Leading Startup Node, the winning node commences the Startup Procedure as described in scenario 2. All other Coldstart Nodes are called Following Coldstart Nodes. These commence scheduling after receiving the first two Sync Frames (SyF), and start synchronizing in the following cycle. In the fourth cycle, the Following Coldstart Node start sending Sync Frames on their own, the so called Coldstart Join (see Figure 2.19 and Figure 2.20 - (2)). All other nodes start scheduling at the 0 th cycle, followed by the synchronization. After the 7 th cycle, they start transmitting data (see Figure 2.20 - (3)). As with the 8th cycle, there is a normal communication on the bus, all nodes are in POC: normal active state. Figure 2.19 Startup sequence This behavior can be recorded and verified in CANoe, see Figure 2.20. In this case, two Coldstart Nodes have ben simulated by the software. The control unit under test joins the communication as expected. Bäurle 12.10.2012
Grundlagen FlexRay BasicsV 1.1 25 Figure 2.20 FlexRay Startup CANoe log file 2.8 Synchronization A precondition for using the TDMA method is a synchronized bus. For this, FlexRay uses a mechanism for distributed clock synchronization, i.e. as shown in Figure 2.21 , every node derives the cycle clock from its own clock Figure 2.21 Synchronization of the ECUs [VeSc11] For adjusting the local timer, a combination of offset- and frequency-adjustment is used, see Figure 2.22 and Figure 2.23. By combining these methods it is ensured, that the local times of all nodes are almost identical at one point of time, and don’t diverge while time progresses. Bäurle 12.10.2012
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