It gives the good explanation about the role of CAN BUS in modern day automotives

1. BY
ARAVINDKUMAR B

2. Distributed Systems in Vehicles
 CAN
 LIN
 MOST
 Byteflight
 D2B
 K-line
 FlexRay

3. Introduction to CANBUS
CANBUS or CAN bus – Controller Area Network bus
An automotive serial bus system developed to satisfy the following
requirements:
 Network multiple microcontrollers with 1 pair of wires.
 Allow microcontrollers communicate with each other.
 High speed, real-time communication.
 Provide noise immunity in an electrically noisy environment.
 Low cost

4. Who uses CANBUS?
 Designed specifically for automotive applications
 Today - industrial automation / medical equipment
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Automotive Medical / Industrial
Markets
CANBUS Market Distribution

5. CANBUS History
 First idea - The idea of CAN was first conceived
by engineers at Robert Bosch Gmbh in Germany
in the early 1980s.
 Early focus - develop a communication system
between a number of ECUs (electronic control
units).
 New standard - none of the communication
protocols at that time met the specific
requirements for speed and reliability so the
engineers developed their own standard.

6. CANBUS Timeline
1983 : First CANBUS project at Bosch
1986 : CAN protocol introduced
1987 : First CAN controller chips sold
1991 : CAN 2.0A specification published
1992 : Mercedes-Benz used CAN network
1993 : ISO 11898 standard
1995 : ISO 11898 amendment
Present : The majority of vehicles use CAN bus.

7. CANBUS and the OSI Model

8. Application Areas of Distributed Systems in
Vehicles – Power Train
 Communicating units
 engine, brake, gear, ABS/ESP
 steering wheel position, steering booster
 light control, damper …
 High-speed and reliability are required
 running only when the ignition is on
 in future the technologies for X by wire will be applied
 Today standards
 CAN (high-speed)
 Byteflight
 Future: the FlexRay standard

9. Application Areas of Distributed Systems
in Vehicles – Comfort Functions
 Communicating units
 seat position control, mirrors control, windows control
 air condition, vehicle top,
 tires pressure control, parking assistant
 wiper control, door control …
 Lower-speed is enough
 Low-power mode is required
 units wake-up by data transmission
 running also when ignition is off
 Today standards
 CAN (low-speed)
 LIN

10. Application Areas of Distributed Systems in
Vehicles – Infotainment and Telematics
 Communicating units
 sound system, CD player, changer, tuner
 TV set, mobile phone, navigation
 inter-vehicle communication, traffic info reception …
 Different communication speeds
 low speed for control transfers
 high speed for user data transfers (audio, video)
 Low power mode required
 unit wake-up by data communication
 running if ignition is off
 Today standards
 CAN (low-speed)
 MOST

11. Application Areas of Distributed Systems in
Vehicles – Diagnostics
 Communicating units
 all (use their native interface) …
 Different interfaces
 today often so called K-line with diagnostics
protocols
 gateway unit often translates diagnostics protocols
of particular ECUs
 in future the wireless diagnostics is expected
 using bluetooth ??? …
 security is the issue

12. CAN and ISO-OSI Model
 Physical layer
 transmission line
parameters, signaling
levels, transmission speed,
…
 Link Layer = CAN
 medium access control
 frame coding and decoding
 addressing
 data security
 error states behavior

13. CAN and ISO-OSI Model
 Application layer
 defines the data content of link
layers frames
 defines when (under which
conditions) the frames are
transmitted
 in automotive industry there
are only company standards
 standards do exist for
diagnostics
 Application protocols are
defined e.g. in industrial
automation field (CANopen)
 effort to use them in vehicles
too

14. CAN and ISO-OSI Model
 Inter-layer communication
 each protocol layer adds some information that
allows the layer protocols to provide the required
service for the layer above
P1 P1
P2 P2
P3 P3
data
data
data
data
layer L1
layer L2
layer L3
layer L1
layer L2
layer L3

15. CAN – Physical Layer Requirements
 The basic requirement for the physical layer is to
provide so called wired OR functionality
 Two basic signaling levels
 recessive
 dominant
 available e.g. in fiber optics
Vcc Vcc
1 2 3
Bus

16. CAN – Link Layer, MAC and LLC
 MAC – Medium Access Control
 provides the physical channel access for the units, prevents
destructive collisions
 provides priority transmission
 implements the channel coding
 provides the data security by means of CRC check
 solves high error rate problem for particular nodes in network
 provides mechanism for acknowledgement of correctly
received frames
 LLC – Logical Link Control
 allows filtering of received frames
 solves the overload condition

17. CAN – Communication Principle
 All nodes within the systems are equal (from the
communication point of view) – peer to peer
 Frames, (sometimes called messages) are broadcasted into
the network and received by all nodes simultaneously
 There is no node oriented addressing
 frame always starts with identifier, which must provide a
unique frame content identification
 In case the frame is received correctly by receiving nodes,
the acknowledge is sent to the transmitting node
 In case there is an error detected during the transmission,
the error identification sequence is sent and frame has to
be transmitted again

18. Message Oriented Transmission Protocol
 Each node – receiver & transmitter
 A sender of information transmits to all devices on the bus
 All nodes read message, then decide if it is relevant to them
 All nodes verify reception was error-free
 All nodes acknowledge reception

19. Message Format
 Each message has an ID, Data and overhead.
 Data –8 bytes max
 Overhead – start, end, CRC, ACK

20. CAN – Medium Access Control
 Any node can start transmitting only if the bus idle state is
detected
 In case that more than one node start transmitting
simultaneously
 there is no physical contention on the bus, as the dominant bit
transmission „beats“ the recessive one
 Each node receives back the transmitted bit value
 if a node transmitting the recessive bit value receives back the
dominant bus state, it stops transmitting immediately
 This method is called CSMA/CR
 Carrier Sense Multiple Access with Collision Resolution
 sometimes it is also less correctly called CSMA/CA (…Collision
Avoidance)

21. CAN – Medium Access Control
 3 nodes start transmitting simultaneously
 Start of Frame (SOF) bit is always dominant
 11 bits of identifier follow
 identifier must be unique within the system

22. CAN – Frame Identifier
 Identifies the frame content
 It is not the sender nor the receiver address
 usually one node transmits frames with different
identifiers
 each node receives frames with identifiers it is interested
in
 Identifier must be unique within the system
 two different nodes are not allowed to transmit frame with
the same identifier (because of arbitration)
 In case the information source redundancy is required,
identifiers usually differ in low significant bit
 Identifier is transmitted from the most significant bit
 log. 0 is transmitted as a dominant state, log. 1 as a
recessive
 the lower identifier value, the higher frame priority

23. CAN – Frame Format
 Current standard version is CAN 2.0
 Bosch, 1990
 it defines only the link layer protocol
 there are two parts (variants) A and B
 CAN2.0A is backward compatible with older CAN
versions, it uses 11-bit identifier
 CAN2.0B defines two data frame types – standard and
extended
 standard frame offers 11-bit identifier
 extended frame offers 29-bit identifier
 Accepted as ISO11898-1 standard
 next standard parts define the physical layer
protocols too

24. CAN – Frame Format
 4 frame types are defined
 data frame
 used for data transfer
 variable length (0 – 8 data bytes)
 remote request frame
 used to request the data frame with the same identifier
 it contains no data
 error frame
 consists of six consequent dominant or recessive bits
 it is transmitted to indicate the error
 overload frame
 the same format like the error frame
 nodes transmit it to delay the next data frame transmission

25. CAN 2.0A – Data Frame Format
 bus idle – recessive state
 SOF – start of frame
 arbitration field (identifier + RTR bit)
 control field (dedicated bits + data length)
 Both dedicated bits have a dominant value
 data (0 – 8 bytes)
 CRC (15-bit CRC, 1 recessive bit as a delimiter)
 acknowledge (1 bit acknowledge, 1 bit delimiter)
 end of frame (7 recessive bits)
 inter-frame space (3 recessive bits)
frame
identifier
R
T
R
data
length
0 - 8 data bytes
Control
field CRC
Bus
idle
CRC
15 bits
E
R
C
S
O
F
Length: 1 11 1 1 41 0 - 64 15 1 1 1 7
end of
frame
A
C
K
A
C
D
3
R
0
Arbitration field Data field
R
1
Acknowledge
inter-frame
space

26. CAN 2.0B – Standard Data Frame Format
 In fact the same like CAN 2.0A frame format
 Only the formal difference
 bit r1 name changed to IDE (identifier extended)
 The IDE bit is always dominant in a standard data
frame
Arbitration
field Data field
S
O
F
frame
identifier
11 bits
R
T
R
Control
field
I
D
E
R
0
data
length
Bus
idle

27. CAN 2.0B – Extended Data Frame Format
 Allows higher number of frames in particular system
 RTR bit is replaced by SRR bit (substitute remote request)
 always recessive
 IDE bit is always recessive
 standard frame with the same first 11 bits of identifier has
higher priority
 Following 18 identifier bits are used for arbitration among
extended frames only
 CAN controllers provide either active or passive
compatibility with CAN2.0B
R
0
I
D
E
S
O
F
R
1
R
T
R
S
R
R
Data field
R
0
I
D
E
data
length
frame
identifier
11 bits
frame identifier
18 bits
R
1
R
T
R
S
R
R
Arbitration field
Control
field
Bus
idle

28. CAN 2.0A – RTR Frame Format
 RTR bit is always recessive
 RTR frame identifier is the same like the data frame
identifier which transmission is requested
 RTR frame has lower priority than the data frame with the
same identifier
 do you know WHY ???
 Data length is always 0
 Similarly it exists an extended RTR frame format according
to the CAN 2.0B
frame
identifier
R
T
R
data
length
Control
field CRC
Bus
idle
CRC
15 bits
E
R
C
S
O
F
Length: 1 11 1 1 41 15 1 1 1 7
end of
frame
A
C
K
A
C
D
3
R
0
Arbitration
field
R
1
Acknowledge field
inter-frame
space

29. CAN – Error Frame Format
 Error frame consists of six dominant or recessive bits
 it depends on error state of the node which transmits it
 It is transmitted by the node (-s) that detect (-s) any
communication error
 it result in an immediate transmission stop and its later
repetition
 this is all controlled by the controller (implemented in
silicon), not by application software
Error flag
Superpozition of error flags
Data frame or
error delimiter or
overload delimiter
Error frame
Error
delimiter
Inter-frame space
or
overload frame

30. CAN – Overload Frame Format
 Overload frame consists of six dominant bits
 Its transmission is requested by the receiver in order to
 delay the transmission of a next data frame
 indicate a detection of a dominant value in a last bit of the end
of frame field or in first two bits of inter-frame space
 Its occurrence does not mean an error
 previous frame is not retransmitted
 Today controllers do not use it to delay the next
transmission – they are fast enough
Overload flag
Superpozition of overload flags
Overload frame
Overload
frame delimiter
Inter-frame space
or
overload frame
Data frame or
error delimiter or
overload delimiter

31. CAN – Error Detection
Several simultaneously used mechanisms:
 Monitoring
 transmitter receives back the bus state and if it detects a
different value, its behavior is:
 in case it detects a dominant bus state within the arbitration
field while transmitting the recessive one, it stops
transmitting
 in case it detects a recessive bus state within the arbitration
field while transmitting the dominant one, or if it detects
anywhere else (excluding the ACK bit) opposite bus state
than that one it is currently transmitting, it sends the error
frame
 CRC (Cyclic Redundancy Check)
 in case the locally evaluated CRC is different from the received
one, the error frame is transmitted

32. CAN – Error Detection
 Bit stuffing
 transmitter transmits particular bits using NRZ (not
return to zero) coding
 if there is a sequence of 5 consecutive bits of the
same level, one bit of the opposite level is inserted
 during the reception an inverse process takes place,
it means after five received bits of the same level the
next bit must be of the opposite level (check) and it
is known it is an inserted bit – it is thus removed

33. CAN – Bit Stuffing
 Error frame transmission violates bit stuffing rule
 all nodes thus detect the error
 it is thus ensured that the frame is received either by all nodes
or by no node
 data consistency

34. CAN – Error Detection
 Frame format check
 some bits have predefined level
 CRC or ACK delimiters are always recessive
 end of frame field is whole recessive
 if there is a dominant bit detected, an error frame is sent
 data length field can contain value higher than 8
 length of 8 is expected
 error frame is not sent
 Frame receive acknowledge
 by the dominant level in the ACK bit
 if there is no acknowledge, transmitting node sends an error
frame
 data frame transmission is repeated

35. CAN – Node Error States
 One node encountering communication problems could
block complete communication within the system
 it detects an error in each received frame and transmits an
error frame
 it is necessary to limit this possibility
 There are two so called error counters in each CAN
controller (and thus in each network node)
 one for errors during transmission, one during reception
 at the beginning they are reset
 if there is an error during transmission, transmission error
counter value is increased, the same is tru for reception
 If the transmission or reception have passed without error,
the respective value is decremented (up to zero)
 According to the error counters values particular node is in
one of three error states

36. CAN – Node Error States
 Error active
 value of each of both error counters has to be lower than
128
 in case the error is detected during the frame
transmission or reception, an active error flag (6
consecutive dominant bits) is generated
 it breaks communication and all other nodes within the
system detect error as well
 the number the respective error counter is increased (0, 1
or 8) depends on the error context, it means the
situation and conditions of error detection
 if any of error counters reaches the value higher than 127,
the respective node goes into the error passive state

37. CAN – Node Error States
 Error passive
 the value of at least one error counter is higher than 127
 if the node detects communication error, it generates a
passive error flag (6 consecutive recessive bits)
 if the error passive node is the only one who detects error
(probably incorrectly), the communication is not broken
and can be finished and acknowledged by other nodes
 in case of successful reception the respective counter is
either decremented (if its value was lower than 128) or set
between 119 and 127 (if its value was higher 127)
 if the values of both counters fall under 128, the node goes
back to error active state
 if the value of transmission error counter gets over 255, the
controller enters bus-off state

38. CAN – Node Error States
 Bus-off state
 transmission error counter value is higher than 255
 reception error counter value has no influence on entering
into the bus-off state
 bus-off node is completely disconnected from the network
(logical not physical disconnect)
 it is not possible to transmit frames nor to influence the bus
communication by any way (no ACK, error frame..)
 reception is possible (depends on implementation)
 from the point of view of other nodes the bus-off node
disappears from the network (like switched off)
 to leave the bus-off state only the controller hardware or
software reset is available
 then after the detection of 128 recessive 11-bit sequences the
controller enters an error active state
 incorrect software implementation can make global
problems in communication

39. CAN – Error States Servicing
CAN controller Status register usually contains:
 Error warning flag
 set if any error counter reaches some limit
 usually 96
 sometimes this limit can be preset
 controller can generate an interrupt service request
 application software (node firmware) may or may not service
this event
 Error passive flag
 set by entering the error passive state
 interrupt service request possible
 Application software should také into account possible data
inconsistency within the local node and the rest of a network
 Bus-off flag
 controller reinitialization is necessary

40. CAN – Persisting communication problems
 It always depends on the error type, direction of
communication (reception, transmission) where it takes
place as well as the physical layer protocol version
 e.g. tolerance to some shorts
 Fatal errors (e.g. Short connection of both CAN lines to
ground) always finishes in bus-off state
 only in case the node tries to transmits
 The node which is alone in the network (or disconnected by
cable interrupt), enters after 16 unacknowledged frame
transmissions into an error passive state
 If the software service of error states is not correctly
implemented, serious communication problems may occur
 Under the standard conditions all nodes within the system
are in the error active state and there are no error frames
generated

41. CAN – Physical Layer Standards
 CAN standard (Bosch) defines the link layer protocol only
 It is also standardized by ISO as ISO11898-1
 Physical layers are defined in standards ISO11898-2
 high-speed CAN
 up to 1 Mbit/s
 and ISO11898-3
 low-speed CAN
 up to 125 kbit/s
 particular fault tolerance
 Both these physical standards are widely used in today
vehicles
 For trucks there are othe standards available with higher
degree of immunity
 They are all defined as SAE standards too

42. CAN – ISO11898-2 Physical Layer
 Bus structure with terminators
 Line impedance of 120 ohm
 Communication speed up to 1 Mbit/s
 Differential signaling (logical level defined by voltage
difference)
 CAN_H and CAN_L lines

43. CAN – ISO11898-2 Physical Layer
 The recessive level is provided by terminators and „wake“
voltage sources in transceivers
 CAN_H – CAN_L difference is near 0
 The dominant state is driven by the transceiver
 CAN_H – CAN_L difference is about 2 volts

44. CAN – ISO11898-2 Physical Layer
 The transceiver contains the temperature, short circuit and ESD
protection
 Low load in power off state
 Some offer low power states

45. CAN – ISO11898-3 Physical Layer
 Bus structure
 Sleep mode with remote wake-up by CAN communication
 Communication speed up to 125 kbit/s
 Differential signaling

46. CAN – ISO11898-3 Physical Layer
 Particular fault tolerance, transceiver enters so
called single wire mode
 CAN_H or CAN_L wire broken
 short connection of CAN_H or CAN_L to ground
 short connection of CAN_H or CAN_L to +5V
 short connection of CAN_H or CAN_L to +12V
 short connection of CAN_H to CAN_L
 Single wire mode is indicated by the transceiver
output
 after the fault is removed transceiver automatically
enters standard two wire mode

47. CAN – ISO11898-3 Physical Layer
 RTH and RTL resistors provide termination
 their values depend on number of nodes in system
 Transceiver supports node wake-up when the CAN activity is
examined, or by he local signal
 Sleep mode is controlled by the local microprocessor
 if the whole bus is in sleep (low power) mode, CAN_H voltage
is near 0 and CAN_L voltage is near the battery one

48. CAN – Immunity to External Disturbances
 Information is transferred by the voltage difference
 if both wires are close the induced disturbance is the same,
difference stays the same
 the absolute value of the induced disturbance can be further
decrease with utilization of twisted pair wire

49. CAN – Transmission Timing
 všechny uzly v síti musí mít nastavenu shodnou
nominální přenosovou rychlost
 skutečné rychlosti se mírně liší (tolerance
oscilátorů)
 vzhledem k faktu, že v průběhu arbitráže může
vysílat více uzlů najednou, že arbitráž probíhá bit
po bitu a že šíření informace z jednoho uzlu do
druhého je zatíženo zpožděním (budič – vedení –
přijímač), je třeba:
 kompenzace statických zpoždění
 průběžné synchronizace
 kvůli odchylkám oscilátorů

50. THANK YOU