1. Main Features:
o CAN stands for Controller Area Network
o Serial communication protocol
o Supports real time control
o High level of error handling.
o Multi-Master Multi-Slave communication.
o Message based protocol .
2. CAN Application:
o Motor vehicles
o Utility vehicles
o Industrial automation.
o Other applications for CAN are:
trains, medical equipment, building automation,
household appliances, and office automation.
Automotive
Industrial
3. o Initially developed for motor
vehicles by Robert Bosch
GmbH, Germany, starting
1983, also holding the CAN
license.
o CAN is internationally
standardized by ISO and
SAE in 1986 .
CAN History:
CAN
Spec
ISO/OSI
SAE
Physical Layer
Data Link Layer
Application Layer
...
CAL, CANopen (CiA),
DeviceNet (ODVA),
SDS (Honeywell)
7
2
1 }CAN
4. o The original CAN specifications (Versions 1.0, 1.2 and 2.0A) specify
an 11 bit message identifier.
o 16 messages with the lowest priority (2032-2047) are reserved.
CAN History:
Identifier
11 bits
CRC
Field
Data Field
(0..8 Bytes)
Control
Field
ACK
Field
End
of Frame
Start of
Frame
Standard CAN
5. Extended CAN
o CAN specification Version 2.0B updated to meet the SAE J1939
standard .
o 29-bit identifier is made up of the 11-bit identifier ("Base ID") and the
18-bit Extended Identifier ("ID Extension").
o CAN v2.0B still allows message identifier lengths of 11 bits to be
used.
CAN History:
Identifier
29 bits
CRC
Field
Data Field
(0..8 Bytes)
Control
Field
ACK
Field
End
of Frame
Start of
Frame
6. CAN Specification :
o 2.0A :
Considers 29 bit ID as an error
o 2.0B Passive:
Ignores 29 bit ID.
o 2.0B Active :
Handles both 11 and 29 bit ID.
V2.0B
Active CAN
Tx/Rx OK Tx/Rx OK
Frame with
11 bit ID
V2.0B
Passive CAN
V2.0A CAN
Frame with
29 bit ID
ToleratedTx/Rx OK
Tx/Rx OK
Bus
ERROR
Three Types of CAN Modules :
Care when
mixing
protocol
versions !
7. CAN properties :
CAN-Bus
(logical)
CAN
Nodes
e.g.
ABS
e.g.
EMS
Node A Node n
o Multi Master Concept
o Number of nodes not limited by protocol
o No node addressing, Message identifier
specifies contents & priority
o Easy connection/ disconnection of nodes
o Broadcast capability
o Sophisticated error-detection and error
handling mechanisms
o Non-Return-to-Zero bit coding
Signal to
transmit a“0”
Signal to
transmit a“1”
HIGH
LOW
HIGH
LOW
8. CAN properties :
o High data transfer rate of 1000 kbps at a
maximum bus length of 40 meters
o Twisted wire pair is the bus medium
used for CAN.
o Multiple Access/Collision Detection
with Non-Destructive Arbitration.
9. CAN properties :
o The bus can have one of two complementary
logical:
o Dominant(„0‟)
o Recessive(„1‟).
o During simultaneous transmission of
‟dominant‟ and ‟recessive ‟bits, the resulting
bus value will be ‟dominant‟.
A B C BUS
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1
10. CAN properties :
A B C BUS
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1
Node A Node B Node C
5V
Bus State: recessive (High)
R T
recessive
R T
recessive
R T
recessive
1 1 1
11. CAN properties :
A B C BUS
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1
Node A
CAN-Bus
(single logical
bus line)
Node B Node C
5V
Bus State: dominant
R T
dominant
R T
recessive
R T
recessive
0 1 1
12. Bus Arbitration :
o CAN protocol handles bus accesses according to :
“Carrier Sense Multiple Access with Arbitration on Message Priority”
(CSMA/CD w/AMP).
o It avoids collisions of transmit messages simultaneously
o Arbitration priories the massages using the identifier .
o It isn‟t permitted for different nodes to send messages with the same
identifier as arbitration could fail leading to collisions and errors.
CAN properties :
13. Node X
Node A
Node B
Node C
Transmit
Request
Rest of
Frame
Arbitration
phase
Node A
Node B
Node C
Node B loses Arbitration Node C loses Arbitration
Start
Bit
CAN Bus
Identifier Field
Bus Arbitration :
CAN properties :
15. o NRZ if there are no edges, receivers
lose track of bits.
o Periodic edges allow receiver to
resynchronize to sender clock
o Bit stuffing is used to ensure
synchronization of all bus nodes.
Bit Stuffing :
CAN properties :
16. o When five consecutive bits of the
same polarity transmitted:
Transmitter insert one additional bit of
the opposite polarity.
o The receiver checks the number of
bits with the same polarity
Removes the stuff bits again from the
bit stream. "destuffing".
Bit Stuffing :
CAN properties :
17. o Hard Synchronization:
• CAN synchronize the message transfer
for each node with the leading edge of
SOF bit.
o Re-synchronize
• With each recessive to dominant edge.
SOF ID10 ID9 ID8 ID7
All nodes synchronize on
leading edge of SOF bit
(Hard Synchronization)
Re-
sync
Re-
sync
Re-
sync
Re-
sync
Synchronization:
CAN properties :
19. 11 1
Standard Data Frame Inter Frame Space
dominant
recessive
1 1 1 4 0...64 15
StartofFrame
IdentifierField
DataField
DataLenghtCode
CRCSequence
CRCDelimiter
ACKDelimiter
ACKSlot
EndofFrame
Intermission
BusIdle
Arbitration
Field
Control
Field
CRC
Field
Acknowledge
Field
IDEBit(D)
RTRBit(D)
(reserved(D))
1 7 311
CAN Frames
Standard Data Frame
RTR: Remote Transmission Request
IDE :Identifier Extension
CRC :Cyclic Redundancy
20. CAN FramesCAN Frames
Standard Data Frame
o SRR (Substitute Remote Request) :replace for the RTR .
o r0 and r1: reserve for future applications.
21. Standard Data Frame Inter Frame Space
dominant
recessive
1 11 1 1 1 4 0...64 15 1 1 1 7 3
Standard Remote Frame Inter Frame Space
dominant
recessive
1 11 1 1 1 4 15 1 1 1 7 3
CAN Frames
Remote Frame
22. CAN Frames
Data and Remote Frame scenario
Data Frame; Identifier 'oil_tmp';
contains desired information ~~~~
~~~~~
Remote Frame; Identifier 'oil_tmp'Node A
Node B
(oil temp.-
sensor)
How hot is the oil ?
115°C
115 °C !
23. CAN Frames
Error Frame
Overload Frame
o Generated due to internal conditions of
a receiver, which requires a delay of
the next DATA or REMOTE frame.
Inter-Frame space
24. o There are 5 error detection mechanisms:
CAN Error Detection:
ACK
Error
Form
Error
Stuff
Error
Bit
Error
CRC
Error
25. o Calculated and received CRC checksum must match.
CAN Error Detection:
CRC
CAN_H
Node A Node B
CAN_L
Received
CRC Checksum:
Calculated
CRC Checksum:
Transmitted
CRC Checksum:
Calculated
CRC Checksum:
1234h
Idle
Receive
Transmit
Idle
Receive
Transmit
CRC Sequence
Data Frame
1234h
1234h 1234h match
26. o Otherwise Frame was not received correctly (CRC Error)
CAN Error Detection:
CRC
mismatch
CAN_H
Node A Node B
CAN_L
Received
CRC Checksum:
Calculated
CRC Checksum:
Transmitted
CRC Checksum:
Calculated
CRC Checksum:
1234h
Idle
Receive
Transmit
Idle
Receive
Transmit
Error Frame
1235h
1234h 1234h
27. o No Dominant Bits allowed in:
• –CRC Delimiter
• –ACK Delimiter
• –End of Frame
• –Inter-frame space
o Otherwise Form Error is generated
CAN Error Detection:
Frame Check
28. o Each Node transmits a bit must be
able to read back it correctly.
o Dominant bits are allowed to
overwrite recessive bits only in the
arbitration phase and ACK slot.
o Otherwise, a bit error should be
issued.
CAN Error Detection:
Bit Monitoring
29. o 6 consecutive bits with same
polarity are not allowed between
Start Of Frame and CRC Delimiter
o Otherwise Bit Stuffing Error.
CAN Error Detection:
Bit Stuffing
30. o A frame must be acknowledged
by at least one other node.
o Otherwise ACK Error.
CAN Error Detection:
Acknowledgment (ACK)
31. o Detected errors are made public to all other nodes via Error Frames.
o The transmission of the erroneous message is aborted and the frame is
repeated as soon as possible.
CAN Error Handling:
CAN_H
Node A Node CNode B
Error Frame
Error
detected
Error
reco-
gnized
Error
reco-
gnized
Data Frame
Idle
Receive
Transmit
Idle
Receive
Transmit
Idle
Receive
Transmit
CAN_L
32. o CAN node has three error states
• Error active
• Error passive
• Bus off
o State is according to the value of
their internal error counters.
o the error counters are updated according specific rules:
• For each error on reception or transmission,
Error counters are incremented.
• For each successful transaction,
Error counters are decremented .
CAN Error Handling:
Node Error State
33. o The error-active Node state:
• State after reset.
• Receive and transmit messages.
• Transmit active Error Frames ,
• Both error counters are smaller than or equal
to 127.
Reset
REC<=127
and
TEC<=127
Error
active
Error Handling:
Node Error State
34. o the error-passive node state/;
• Either the receive or the transmit error counter reach to128.
• Messages can still be received and transmitted.
• After transmission ,the node must suspend transmission.
Must wait 8 bit times longer than error-active nodes before
it may transmit another message.
• Passive Error Frames can be transmitted
• If both error counters go below 128 again
Node switches back to the error-active state.
Error
passive
REC>127
or
TEC>127
Error Handling:
Node Error State
35. o The bus-off state
• Transmit error counter exceeds the value
of 255.
• All bus activities are stopped which
makes it temporarily.
• Messages can be neither received nor
transmitted.
• To return to the error active state and to
reset error counter :
CAN node has to be reinitialized.
Error Handling:
Node Error State
Reset
TEC>255
Re-Initialization only
Bus
off
36. o Error Frame consists of 2 fields
• Error Flag field
• Error Delimiter field.
o Error Delimiter :
• Consists of 8 recessive bits and allows the bus nodes to restart bus
communications cleanly after an error.
o There are two forms of Error Flag fields.
o Form of the Error Flag field depends on the “error status” of the node
that detects the error.
Error Handling:
Error Frame:
37. o If “error-active” node detects a bus error :
• Node interrupts transmission by generating an “active error flag”.
• “Active error flag” is composed of six consecutive dominant bits.
• This bit sequence actively violates the bit stuffing rule.
• All other stations recognize the resulting bit stuffing error and in turn generate
Error Frames themselves.
• Error Flag field therefore consists of between six and twelve consecutive
dominant bits (generated by one or more nodes).
Error Handling:
Error Frame:
38. o If a “error passive” node detects a bus error:
• Node transmits an “passive Error Flag”.
• “Passive Error Flag” consists of six consecutive recessive bits,
• Error Frame consists of 14 recessive .
• Unless the bus error is detected by master node, the transmission of an Error
Frame by an “error passive” node will not affect any other node on the network.
• If the bus master node generates an “error passive flag” then this may cause
other nodes to generate error frames due to the resulting bit stuffing violation.
Error Handling:
Error Frame:
39. o CAN chips Dividing into:
o Basic-CAN devices
o Full-CAN devices.
o It‟s possible to use both Basic-CAN and Full-CAN devices in the
same network.
CAN Devices
40. o Only basic functions concerning the filtering and management of
CAN messages are implemented in hardware.
o Provides one transmit buffer and one or two receive.
o CAN receiver accept message which filtering certain CAN identifiers
are stored in the receive buffer.
CAN Devices
Basic-CAN devices:
41. o Only two buffers for the reception of messages the host controller is
quite busy reading and storing the incoming messages before they get
overwritten by the following ones which results in a quite high CPU
load.
o The answering of Remote Frames with the corresponding Data Frame
has to be handled by the host controller.
o Basic-CAN devices should only be used at low baudrates and low bus
loads with only a few different messages.
CAN Devices
Basic-CAN devices:
42. CAN Devices
Full-CAN devices
Host CPU
CPU load
low high
Status/Control
Registers
Message
Object 1
CAN
Protocol
Controller
Acceptance
Filtering
Host
Inter-
face
Message
Object 2
Message
Object n
...
Receive
Buffer(s)
CAN
Bus
BusInterface
43. o Provide the whole hardware for convenient acceptance filtering and
message management.
o For each message to be transmitted or received these devices have so
called message objects which contains all information regarding the
message (e.g. identifier, data bytes etc.).
o During the initialization, the host CPU defines which messages are to
be sent and which are to be received.
CAN Devices
Full-CAN devices
44. o Only if the CAN controller receives a message whose identifier
matches with one of the identifiers of the programmed (receive-)
message objects the message is stored and the host CPU is informed
by interrupt.
o Remote Frames can be answered automatically by the Full-CAN
controller with the corresponding Data Frame.
o the CPU load is strongly reduced compared to the Basic-CAN
solution.
o Using Full CAN devices, high baudrates and high bus loads with
many messages can be handled.
CAN Devices
Full-CAN devices
