Image Processing Reference
In-Depth Information
CAN standard
arbitration
TDMA
Basic cycle
Reference
message
Exclusive
window
Exclusive
window
Arbitration
window
Free
window
Exclusive
window
Reference
message
Exclusive
window
...
Master node
transmission
Time windows for
messages
FIGURE .
Example of a TTCAN basic cycle. (From Navet, N., Song, Y.Q., Simonot-Lion, F., and Wilwert, C.,
Proc. IEEE
., (), , . With permission.)
controllers have the possibility to disable automatic retransmission of frames upon transmission
errors and to provide the upper layers with the point in time at which the first bit of a frame was sent
or received []. The bus topology of the network, the characteristics of the transmission support,
the frame format, and the maximum data rate— Mbit/s—are imposed by CAN protocol. Channel
redundancy is possible (see Ref. [] for a proposal), but not standardized and no bus guardian is
implemented in the node. he key idea is to propose, as with FlexRay, a flexible TT/event-triggered
protocol. As illustrated in Figure ., TTCAN defines a basic cycle (the equivalent of the FlexRay
communication cycle) as the concatenation of one or several TT (or “exclusive”) windows and one
event-triggered (or “arbitrating”) window. Exclusive windows are devoted to TT transmissions (i.e.,
periodic messages) while the arbitrating window is ruled by the standard CAN protocol: transmis-
sions are dynamic and bus access is granted according to the priority of the frames. Several basic
cycles, that differ by their organization in exclusive and arbitrating windows and by the messages
sent inside exclusive windows, can be deined. he list of successive basic cycles is called the system
matrix, which is executed in loops. Interestingly, the protocol enables the master node (i.e., the node
that initiates the basic cycle through the transmission of the “reference message”) to stop functioning
in TTCAN mode and to resume in standard CAN mode. Later, the master node can switch back to
TTCAN mode by sending a reference message.
TTCAN is built on a well-mastered and low-cost technology, CAN, but, as defined by the standard,
does not provide important dependability services such as the bus guardian, membership service, and
reliable acknowledgment. It is, of course, possible to implement some of these mechanisms at the
application or MW level but with reduced efficiency. Some years ago, it was thought that carmakers
could be interested in using TTCAN during a transition period until FlexRay technology is fully
maturebutthiswasnotreallythecaseanditseemsthatthefutureofTTCANinproductioncarsis
rather unsure.
13.2.3 Low-Cost Automotive Networks
Several fieldbus networks have been developed to fulfill the need for low-speed/low-cost commu-
nication inside mechatronic-based subsystems generally made of an ECU and its set of sensors
and actuators. Two representatives of such networks are LIN and TTP/A. The low-cost objective
is achieved not only because of the simplicity of the communication controllers, but also because
the requirements set on the microcontrollers driving the communication are reduced (i.e., low
