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Grundlagen FlexRay BasicsV 1.1 32
2.11 Communication structure
The communication structure of a FlexRay bus is organized in Cycles. The duration of a
Cycle is constant, and is defined by the parameter gdMacroPerCycle. The current active cycle
is saved in vCycleCounter, it can be 0 to 63 ≡ cCycleCountMax.
2.11.1 FlexRay Cycle
Figure 2.28 shows FlexRay Cycles. Every Cycle is divided in four segments, which each have
different properties.
Static Segment:
- Time-triggered transmission
- Bus access using TDMA
Dynamic Segment (optional):
- Event-triggered Transmission
- Bus access using FTDMA
Symbol Window (optional):
- Test symbol per Cycle for the use with a Bus Guardian
NIT – Network Idle Time:
- Segment for synchronization of the nodes
Figure 2.28 Segments in a FlexRay cycle [VeSc11]
The static segment is used for time-triggered data transfer, and is divided into slots, which are
available and have to be used in every cycle. As bus access method TDMA is used, and
therefore the defined access of the ECU to their slot(s) is guaranteed.
In the dynamic segment, an event-triggered data transfer is possible. The length of the
dynamic Slots may vary, and is implemented using the FTMDA method.
The Symbol Window can be used to transmit test symbols, e.g. for the use with a bus
guardian.
The Network Idle Time is used by the nodes to perform the necessary time synchronization.
Only the static segment and the Network Idle time are needed for a valid communication. The
dynamic segment and the Symbol Window are optional. Therefore, there are four different
possibilities for bus access.
2.11.2 Bus access
Depending on the application, the bus access can be configured flexibly. A FlexRay cluster
can use either one of the modes shown in Figure 2.29.
Bäurle 12.10.2012