Image Processing Reference
In-Depth Information
FIGURE .
TTP/A node.
14.5.3 TTCAN
The Time-Triggered CAN (XCTL) [TBW + ] protocol is an example for time-triggered services,
which are layered on top of an event-triggered protocol. XCTL is based on the CAN data link layer
as specified in ISO  [Int].
14.5.3.1 Clock Synchronization
XCTL is a master-slave protocol that uses a time master for initiating communication rounds. XCTL
employs multiple time masters for supporting fault-tolerance. he time master periodically sends a
reference message, which is recognized by clients via its identiier. he period between two succes-
sive reference messages is called “basic cycle.” he reference message contains information about the
current time of the time master as well as the number of the cycle.
To improve the precision of the global time base, slave nodes measure the duration of the basic
cycle and compare this measured duration with the values contained in the reference messages. he
difference between these measured duration and the nominal duration (as indicated by the time
master) is used for drift correction.
14.5.3.2 Periodic Exchange of State Messages
The basic cycle can consist of three types of windows, namely, exclusive time windows, free time
windows, and arbitrating time windows.
. An exclusive time window is dedicated to a single periodic message. A statically defined
node sends the periodic message within an exclusive time window. An offline design tool
ensures that no collisions occur.
. The arbitrating time windows are dedicated to spontaneous messages. Multiple nodes
can compete for transmitting a message within an arbitrating time window. he bitwise
arbitration mechanism of CAN decides which message actually succeeds.
. Free time windows are reserved for further extensions. When required, free time windows
can be reconfigured as exclusive or arbitrating time windows.
Retransmissions are prohibited in all three types of windows.
 
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