CAN Basic Architectures - CAN System Engineering - page 33

Fig. 1.15 Synchronization
of local time by reference
message (Level I)

reference messages, while in case of simultaneous transmissions the one with the
highest priority will win. When the standard time transmitter recovers, this one
will try to take over again the time transmitter function when the next basic cycle
is to start.
This technique guaranties a fast and well-organized system start up: The first
one of the potential time transmitters being operational will start the basic cycles.
The other higher priority potential time transmitters becoming operational over time
then will take over, while at the end of this start-up phase the standard time transmit-
ter will finally control the communication process.

1.3.8 Event-Synchronized Cycle

In non-event-synchronized TTCAN systems, the reference message is transmitted
periodically in equidistant steps.
However, TTCAN systems also allow us to interrupt this periodic process and to
start basic cycles on an event in a time transmitter. The event may be derived from
a software trigger or from a timer elapsed or it may come from an external signal
source.
For synchronization, the currently running cycle will be halted at the end of the
basic cycle while the next_is_gap bit is set in the reference message. This halts the
next reference message until the predefined event will start a new cycle by transmit-
ting the reference message.
This technique enables synchronization of communication on, e.g. crank shaft
synchronous tasks, synchronization of multiple subnets, or smoothing cycle drifts
caused by corrupted reference messages.
Figure 1.17 shows pure cyclic and event synchronized cyclic message commu-
nication.