AN1077 View Datasheet(PDF) - STMicroelectronics
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'AN1077' PDF : 14 Pages View PDF
OVERVIEW OF ENHANCED CAN CONTROLLERS FOR ST7 AND ST9 MCUS
1.2 IMPACT ON CAN TRAFFIC
These changes have an impact in terms of CAN message traffic.
Application Messages
Even if the information transmitted by each ECU has not increased significantly, the growing
number of ECUs in body applications has led to a drastic increase in CAN traffic. Sometimes
the same ECU may be used in several locations in the car - for instance for the right and for
the left door module – so a node must be able to handle more message identifiers than the ap-
plication itself would really require. A similar effect occurs when the same ECU is planned to
be implemented in different car models or versions, this means the number of identifiers the
node has to handle will increase.
Network Management Messages
In addition to the application messages, management messages like OSEK Network Manage-
ment are needed to configure the network at start-up time, to control the bus when the nodes
enter and exit sleep mode, to monitor the node while running and to handle bus-off conditions.
This distributed NM requires the implementation of the OSEK-NM software on each ECU,
which has to monitor the NM messages of all the other nodes. This NM is based on the token
ring method and each node has its own address. The source and destination addresses are
coded in the identifier of the CAN message. This means that there are as many identifiers
needed for the NM as nodes in the network.
Diagnosis Messages For diagnosis purposes each ECU must be accessible to external diag-
nosis tools. According to ISO 15765-1 and 2 these diagnosis services are now implemented
via CAN. These services require their own CAN messages.
1.3 IMPACT ON CAN CONTROLLERS
Just as the introduction of networks has modified automotive application implementations in a
major way, modern network architectures have also changed the requirements imposed on
CAN controllers. As mentioned earlier, the resource requirements have increased especially
on the receiver side:
s A huge number of identifiers on the bus
s A high number of identifiers to be handled
s Various types of messages
All Quiet in the 8-bit CAN World?
While many CAN controllers for 16- and 32-bit microcontrollers have been recently developed
in order to satisfy these new requirements, most of the CAN controllers implemented on 8-bit
microcontrollers do not provide the required hardware to support these new functions effi-
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