TC1784
FlexRay™ Protocol Controller (E-Ray)
User´s Manual
20-195
V1.1, 2011-05
E-Ray, V3.13
20.6.2
Communication Modes
The FlexRay™ Protocol Specification v2.1 defines the Time-Triggered Distributed (TT-
D) mode.
Time-triggered Distributed (TT-D)
In TT-D mode the following configurations are possible:
•
Pure static
: minimum 2 static slots + symbol window (optional)
•
Mixed static/dynamic
: minimum 2 static slots + dynamic s symbol window
(optional)
A minimum of two coldstart nodes need to be configured for distributed time-triggered
operation. Two fault-free coldstart nodes are necessary for the cluster startup. Each
Startup Frame must be a SYNC Frame, therefore all coldstart nodes are sync nodes.
20.6.3
Clock Synchronization
In TT-D mode a distributed clock synchronization is used. Each node individually
synchronizes itself to the cluster by observing the timing of received SYNC Frames from
other nodes.
20.6.3.1 Global Time
Activities in a FlexRay™ node, including communication, are based on the concept of a
global time, even though each individual node maintains its own view of it. It is the clock
synchronization mechanism that differentiates the FlexRay™ cluster from other node
collections with independent clock mechanisms. The global time is a vector of two
values; the cycle (cycle counter) and the cycle time (Macrotick counter).
Cluster specific:
•
Macrotick = basic unit of time measurement in a FlexRay™ network, a Macrotick
consists of an integer number of Microticks
•
Cycle length = duration of a communication cycle in units of Macroticks
20.6.3.2 Local Time
Internally, nodes time their behavior with Microtick resolution. Microticks are time units
derived from the oscillator clock tick of the specific node. Therefore Microticks are
controller-specific units. They may have different duration in different controllers. The
precision of a node’s local time difference measurements is a Microtick.
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