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Grundlagen FlexRay BasicsV 1.1 30
2.10 Timing
In addition to the signal levels, timing plays an important role in the communication of a
FlexRay bus, since without timing none of the protocol mechanisms would work. In Figure
2.27, the hierarchical structure of the different time units is shown.
Figure 2.27 FlexRay timing hierarchy
The so-called Microtick is derived from the Sample Clock. The duration of a Microtick may
differ between ECUs, since it depends on component tolerances, temperature and time drift of
the oscillator of each ECU.
Macroticks (MT) are composed from local Microticks, and are global synchronous time units.
Since the duration of a Macrotick is the same throughout the whole cluster, the number of
Microticks to a Macrotick may differ from ECU to ECU.
A Communication Cycle consists of a constant number of Macroticks, and has the same
duration for every node.
The duration of a bit in FlexRay is determined by the Sample Clock, i.e. is independent of
Macroticks and Microticks. Since the duration of a bit has to be equal for all ECUs, they get
re-synchronized on bit-level.
With the following formulas [FRPS05] and the system parameters in Table 2.3, the following
example values can be calculated:
(1)
(2)
( ) (3)
Bäurle 12.10.2012