Grundlagen FlexRay - Institut fÃ¼r Automatisierungs- und ...

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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
Grundlagen FlexRay BasicsV 1.1 31 ( ) (4) Table 2.3 Timing Parameter [VeSc11], example From the selected bus data rate of 10 Mbit/s results the bit duration gdBit = 0.1µs (1), by multiplying the number of samples per bit cSamplePerBit = 8 with the sample duration gdSamplesClockPeriod. As for choosing the duration of a Microtick pdMicroTick, there are three different configurations to choose from, depending on the ECU. For example, with an oscillator frequency of 80MHz the number of samples per Microtick equals pSamplesPerMicroTick = 1 an using (2) , the duration of a Microtick equals pdMicroTick = 0.0125µs. By selecting the length of a Macrotick gdMacroTick it is possible to create a standardized proportion pMicroPerMacroNom between Microtick und Macrotick (3). Using the cycle length gdCycle we are able to calculate the number of Macroticks per Cycle gMacroPerCycle (4). Even more details can be found in the FlexRay Specification [FRPS05]. Bäurle 12.10.2012
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