TC1796
Peripheral Units (Vol. 2 of 2)
Controller Area Network (MultiCAN) Controller
User’s Manual 22-11 V2.0, 2007-07
MultiCAN, V2.0
22.1.6 TTCAN Basics
In general, a CAN network is event-oriented due to the error handling and bus arbitration.
The exact time for a message to be transferred is difficult to determine. In order to allow
deterministic message transfers, the event-driven mechanism has been changed into a
time-driven mechanism by a TTCAN extension, with “TT” standing for Time-Triggered.
TTCAN is a higher-layer protocol above the standard CAN protocol that synchronizes
the communication schedules of all CAN nodes in a CAN network, and that provides a
global system time. When CAN nodes are synchronized, messages can be transmitted
at a specific time slot, without competing with other messages for the bus. Thus, the loss
of arbitration is avoided, and the latency time becomes predictable. Apart from the
synchronized communication schedule, TTCAN-compliant nodes operate according to
the standard CAN protocol as defined by ISO 11898-4.
22.1.6.1 Time Reference
In TTCAN networks, a global time reference makes it possible to synchronize the
activities of all CAN nodes. Each CAN node has its own local time represented by a
counter that is incremented with each network time unit (NTU). The NTU is derived from
the node’s local clock and local Time Unit Ratio (TUR).
In TTCAN, synchronization is achieved by a periodic transmission of a reference
message. This reference message (transmitted by a time master) restarts the cycle time
in each node. The reference message can be easily recognized by its identifier. Within
TTCAN level 1, the reference message holds control information in one byte of the
message Data Field. In TTCAN level 2, the reference message provides additional
control information in four bytes of the message Data Field (e.g. the global time
information of the current TTCAN time master) with downwards compatibility.
22.1.6.2 Basic Cycle
A reference message starts a new basic cycle and resets the cycle time. The cycle time
counts the time since the start of the basic cycle. A basic cycle consists of several non-
overlapping time windows of different sizes. The sequence of the time windows is
described by time marks that determine when a time window begins. Time marks are
compared to the cycle time, and particular actions are triggered when the cycle time
matches the time mark.
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