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Controller Area Network (CAN)

Controller Area Network (CAN)
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Developed by Robert Bosch GmbH, Germany in 1987
Products available from all microcontroller manufacturers
International Standards: ISO11898 (Europe), SAE J2284 (US) for “high – speed” CAN; ISO 11519-2 for
“fault-tolerant low speed” CAN
Backbone serial bus system for automotive applications, but also used in industrial automation & control
Event triggered Serial Bus System; Self-Synchronisation
More Features :
 multi master bus access
 random access with collision avoidance (CSMA / CA )
 short message length , at max
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0A specification published
• 1992 : Mercedes-Benz used CAN network
• 1993 : ISO 11898 standard

• 1995 : ISO 11898 amendment
• Present : The majority of vehicles use CAN bus
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FlexRay
• Time Triggered Protocol for fail safe applications;
• 10 Mbit/s; dual channel redundancy

Classification of CAN
Implementation

Basic CAN

• Close coupled MCU and
CAN
• Only one transmit buffer
• Only two receive buffer
• Only one filter for
incoming messages
• Software routines are
needed to select between
incoming messages

Full - CAN

• Provide a message server
• Extensive acceptance filtering
on incoming messages
• User configurable Mailboxes
• Advanced error recognition
• Mailboxes memory area and
size of mailbox areas depends
on manufacturer

Data Format

Standard

• CAN Version 2
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0B
• Messages with 29-bit
identifiers

CAN Network and Nodes
A CAN network consists of a number of CAN nodes
which are linked via a physical transmission medium
(CAN bus) In practice, the CAN network is usually
based on a line topology with a linear bus to which a
number of electronic control units are each connected
via a CAN interface
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An unshielded twisted two-wire line is the physical
transmission medium used most frequently in
applications (Unshielded Twisted Pair — UTP),
over which symmetrical signal transmission occurs
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34 mm2 and 0
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Line resistance should be
less than 60 mΩ
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This comprises a CAN controller and a
CAN transceiver
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CAN Bus

Physical signal transmission in a CAN network is based on
transmission of differential voltages (differential signal
transmission)
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Consequently, the transmission medium (CAN
bus) consists of two lines: CAN high line (CANH) and CAN
low line (CANL)
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Terminating the ends of the communication channel
using termination resistors (simulation of the electrical
properties of the transmission medium) prevents reflections in a
high-speed CAN network
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The specific differential
voltages depend on the bus interface that is used
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ISO 11898-2 assigns logical “1” to a differential voltage of 0
Volt
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Highspeed CAN transceivers interpret a differential voltage of more
than 0
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Below 0
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A hysteresis circuit increases immunity to
interference voltages
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CAN Bus Logic

A basic prerequisite for smooth communication in a CAN network — especially for bus access, fault indication and
acknowledgement — is clear distinctions between dominant and recessive bus levels
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The recessive bus level corresponds to logical “1”
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When different CAN nodes send dominant and recessive
bus levels simultaneously, the CAN bus assumes the dominant bus level
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In terms of logic, such behavior is AND-logic
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Practice with the interactive figure “Bus Logic” to learn about the wired-AND bus logic upon which a CAN
network is based
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The transmission of CAN
messages does not follow any
predetermined time sequence, rather it is
event-driven
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In principle,
real-time data transmissions on the
magnitude of milliseconds are no
problem in a CAN network, because of
the ability to quickly react to
asynchronous events and very high data
rates up to 1 MBit/s
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0 A and CAN 2
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0 B)
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The distinction between CAN base frame format and CAN extended frame format is made
by using the IDE bit, which is transmitted as dominant in case of an 11-bit frame, and transmitted as recessive in case of a 29bit frame
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All frames begin with a start-of-frame (SOF) bit that denotes the start of the frame transmission
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A data frame is made up of
many different components
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Tasks to be
performed are: Initiate and maintain synchronization between
communication partners, establish the communication
relationships defined in the communication matrix and transmit
and protect the user data
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e
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Nonetheless, these frames are hardly ever
used in automotive applications, since data transmission is not
based on requesting, rather it is primarily based on the selfinitiative of information producers
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Error Frame and Overload Frame
The error frame consists of two different fields:
The first field is given by the superposition of ERROR FLAGS
(6–12 dominant/recessive bits) contributed from different
stations
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If a node needs more time to process the message at the protocol
level it has the ability to transmit out up to two Overload
frames before the next data or remote is present on the bus
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References
[1] Kvaser CAN Bus User Manual
[2] Vector Can Bus User Manual
[3] Texas Instrument Tutorial and CAN Module
[4] MicroChip Can Module Tutorial
[5] Bosch Can Bus 2
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0B



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