Image Processing Reference
In-Depth Information
Pure static configuration
Static segment (min. 2 static slots)
Symbol window
Optional
Mixed configuration
Static segment (min. 2 static slots)
Dynamic segment
Symbol window
Optional
Pure dynamic configuration
Dynamic segment
Symbol window
Degraded static segment (min. 2 static slots)
Optional
FIGURE .
FlexRay configurations.
Considering the most likely application domains, mixed conigurations will be dominant. Depending
on the actual configuration, FlexRay can achieve a best-case bandwidth utilization of about % with
the average utilization ranging around %.
16.3.2 Clock Synchronization
he media access scheme of FlexRay relies on a common and consistent view about time that is shared
between all CCs in the network. It is the task of the clock synchronization service to generate such a
time view locally inside each CC. For the detailed description of the clock synchronization, first the
representation of time inside a CC is described. he physical basis for each time representation is the
tick of the local controller oscillator. his clock signal is divided by an integer to form a clock signal
called microtick. This is often done by a frequency divider implemented in hardware. An integer
number of microticks form a time unit called macrotick. Minislots and static slots are set up as integer
multiple of macroticks. he number of microticks that constitute a macrotick is statically configured.
However, for adjustment of the local time, the clock synchronization service can temporarily adjust
this ratio in order to accelerate or decelerate the macrotick clock. If, for instance, a clock is too fast,
microticks are added to a macrotick in order to slow down the counting.
The clock synchronization service is a distributed control system that produces local macroticks
with a defined precision in relation to the local macroticks of the other controllers of a network.
The control system takes some globally visible reference events that represent the global time ticks,
measures the deviation of the local time ticks to these global ticks, and computes the local adjustments
in order to minimize the deviation of the local clock from the global ticks. Due to the distributed
nature of the FlexRay system, no explicit global reference event exists. he only event, which is glob-
ally observable, is a frame transmission on the communication channel. he start of a transmission is
triggered by the local time base of the sending controller. Each controller can collect these reference
events to form a virtual global time base by computing a fault-tolerant mean value of the deviation
betweenthelocaltimeandtheperceivedevents.
The reasoning behind this approach is based on the assumption that a majority of local clocks in
the network is correct. Correctness of a local clock is given, when the local clock does not deviate
from every other by more than the precision value. A controller with locally correct clock sees only
deviation values within the precision value. By computing the median value of deviations, the actual
