AP29000
Connecting C166 and C500 Microcontroller to CAN
Introduction
Application Note
7
V 1.0, 2004-02
2
Introduction
Ever increasing quantities of electronic devices are fitted to modern motor vehicles.
Examples of such devices include engine management systems, active suspension,
ABS, gear control, lighting control, air conditioning, airbags and central locking. All this
means more safety and more comfort for the driver and of course a reduction of fuel
consumption and exhaust emissions. To improve the behaviour of the vehicle even
further, it is necessary for the different control systems (and their sensors) to exchange
information. At present, this is usually done by discrete interconnection of the different
systems (i.e. point to point wiring). The requirement for information exchange has now
grown to such an extent that a cable network with a length of up to several kilometers
and many connectors is required. This produces growing problems concerning
material cost, production time and reliability.
The solution to this problem is the connection of the control systems via a serial bus
system. This bus has to fulfill some special requirements due to its usage in a vehicle:
Data Rate
Much of the data that is exchanged by the control systems (or by sensors) has to be
processed in real time which requires very fast transmission. Data items may differ in
transmission priority (e.g. Airbag data is likely to be high priority, Air Conditioning data
is likely to be low priority). For very high priority data the latency period between the
transmission request and the actual start of the transmission is very important and
must be minimized. The serial bus therefore must provide very fast transmission, short
message length and bus access prioritisation.
Data Integrity
The bus must have a high resistance to Electromagnetic Interference. Erroneous
messages must be detected and repeated. To ensure valid data across the sytem,
every bus node has to be informed about an error. Bus communication must not be
disturbed if one (or more) bus nodes malfunction. Faulty nodes must withdraw from
bus communication on their own. The bus must therefore have a linear structure with
equal bus nodes (multimaster concept).
Data Sharing
All control systems needing a common data item should be able to simultaneously
receive this data item from the bus (e.g. vehicle speed might be used by the engine
management system, active suspension ABS and gear control) The bus must
therefore have multicast capability.