CAN Introduction 2014(2)

Lesson : Controller Area Network (CAN)

Semester 3 – Power Systems in hybrid (HEVs) and electric (EV) vehicles

Lecturer: F.F.Linera

EECPS/STEPS Universidad de Oviedo

2

Controller Area Network (CAN)

• 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. 8 Bytes per message

data rates 100KBPS to 1MBPS

short bus length, physical length depends on data rate

self-synchronised bit coding technology

Robust EMC - behaviour

build in fault tolerance

3

Main Features :

• CAN is a priority-based-on-message bus with multi-master access.

• It allows to provide a bounded communication delay for each

message priority.

• The MAC (Medium Access Control) protocol of CAN uses

CSMA/CA (Carrier Sense Multiple Access with Colission

Avoidance)

• The bus arbitration is non-destructive and is realized over a part of

the frame called the ID (Identifier).

• The identifier is coded using 11 bits (CAN2.0A) or 29 bits

(CAN2.0B) and it also serves as priority.

• Short message length. Up to 8 Bytes of data per message can be

carried by one CAN frame.

• A CRC (Cyclic Redundancy Code) is added as part of the frame for

transmission error detection.

• Robust EMC - behaviour


4

Main Features :

• CAN uses a NRZ bit encoding scheme for making possible the bit

by bit arbitration with a logical bit arbitration

• But, the bit-by-bit arbitration limits the bit rate of CAN as the bit

time must be long enough to cover the propagation delay on the

whole network

• A maximum of 1Mbps is specified to a CAN bus not exceeding

25m. Physical length depends on data rate.

• Recommended Guidelines for Cable length according to CANopen

DS 102 V2.0 Physical Layer for Industrial applications (appendix):


5

Main Features :

• Maximum message transmission time include the worst-case bit

stuffing number and is:

Tframemax = 44 + 8 DLC +34 + 8 DLC

4 𝑥 𝑇𝑏𝑖𝑡,

𝑤ℎ𝑒𝑟𝑒 𝐷𝐿𝐶 𝑖𝑠 𝑑𝑎𝑡𝑎 𝑙𝑒𝑛𝑔𝑡ℎ & 𝑇𝑏𝑖𝑡 𝑖𝑠 𝑡ℎ𝑒 𝑏𝑖𝑡 𝑡𝑖𝑚𝑒

The fraction represent the overhead due to the bit stuffing

• Bit stuffing is a technique implemented by CAN for bit

synchronization which consists in inserting an opposite bit every

time five consecutive bits of the same polarity are encountered.


6

The Standardization of CAN

• CAN is an open system and has been standardized by

ISO

CAN follows the ISO - OSI seven layer model for open

system interconnections

CAN implements layer 1, 2 and 7 only

However, Layer 7 is not standardised

Physical Layer Type Europe

www.iso.org

North America

www.sae.org

Single – Wire CAN n/a SAE J2411

Single Wire CAN for Vehicle

Applications

Low-Speed Fault Tolerant

CAN

ISO 11519 - 2

ISO 11898 - 3

n/a.

High-Speed CAN ISO 11898 SAEJ2284

7

Open Systems Interconnection (OSI)

ISO Reference Model:

Layer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data LInk Layer

Physical Layer

void

void

void

void

Layer 1: transmission line(s)

differential two-wire-line, twisted

pair with/without shield

Transceiver Integrated Circuit

Optional: fibre optical lines (passive

coupled star, carbon )

Optional: Coding as PWM, NRZ,

Manchester Code

ISO 11898

Layer 2: Data Link Layer

message format and transmission

protocol

ISO 11898

CSMA/CA access protocol

Layer 7: Application Layer

different standards in industry, not

standardized in automotive


8

• ISO11898 is the standarized version of the CAN protocol as it

was originally defined by Bosch

• It only defines the layers1 (physical) and 2 (Data Link) of the

ISO/OSI model.

• Layer 2 (Data Link) is the most complete defined.

• Layer 2 defines message framing, arbitration, acknowledgement,

error detection and error signalling in the Medium Access

Control (MAC) sublayer.

• Layer 2 defines how message filtering, overload notification or

the error recovery procedure must be realized.

• Layer 2 can not be modified by any given protocol based on

CAN.


9

• Layer 1 is not completely defined by ISO11898

• The Bosch Reference document describes only the

detailed bit representation (Physical Signalling

Sublayer).

• But the driver/receiver characteristics of the

physical layer are not defined and so, the transport

medium and the signal levels can be optimized for

any given application.


10

So, Layer 1 allows wire media of all kinds:

• Single wire, simple and asymmetrical.

• Two-wire in differential mode, with/without galvanic

isolation using optocouplers or transformers.

• Optical media using optical fibers

• Electromagnetic media: using ASK and FSK

modulated radio-frequency waves or infrared

waves.

Single wire and two-wires in differential mode are used

in the automotive sector


11

Practical implementations of CAN Practical implementations of differential wires CAN

• Two differential wires medium is very widespread in

industry and so, ISO has standarized the use of CAN in

this particular medium.

• Two main ISO Standards:

• ISO11898-2 – High Speed CAN (125Kbps to 1Mbps)

• ISO11898-3 – Low Speed CAN (10Kbps to 125Kbps)

• ISO defines the operation and electrical characteristics of

the transmission media, i.e. the electrical levels, but do not

cover the mechanical aspects (forms of connector) or any

of the problems of the other layers.

12

Practical implementations of CAN

• It is defined for use in applications with bit rates from

125Kbps to 1Mbps

• It is implemented using a simple differential wire pair

• It is the most widespread implementation used in vehicle

(power train domain) and industry applications

• The following figure shows the electrical levels adopted for

the dominant (nom.2V) and recessive bits (nom. 0V)

High Speed CAN – ISO11898-2

13


Real HS CAN Waveform: CANH, CANL and the difference

High Speed CAN – ISO11898-2 – Real Frame Waveform

14

Practical implementations of CAN High Speed CAN – ISO11898-2

Typical High Speed node implementation

• Normally the uC has an integrated CAN

controller

• ISO defines a characteristic line

impedance of 60 ohm.

• This impedance can be obtained as it is

shown in the figure. This implementation

reduces EMC pollution.

• Other impedance implementations are:

2x120 ohmios, 3x180ohmios.

• Normally these resistances must be

situated at the end of the lines and can be

measured with a polymeter.

• Normally the termination resistances are

included as part of the CAN devices with

the possibility to connect it between CANH

and CANL using a jumper or a switch

15

Practical implementations of CAN High Speed CAN – ISO11898-2 – Typical Driver ICs

PCA82C250/251 from NXP TJA 1050 from NXP

This resistance

reduces slew-rate

in transition an so,

EMC pollution.

Depending of the

speed, a double

choke to reduce

EMC can be

neccesary.

16

Practical implementations of CAN High Speed CAN – ISO11898-2 – Typical Driver ICs

TJA 1040 from NXP

TJA1040 has a

split voltage Vcc/2

drawn from the IC,

to avoid some

error reads during

wake-ups for a

sleep-node

17

Practical implementations of CAN High Speed CAN with diagnosis of the physical layer –

ISO11898-5 – Typical Driver ICs

TJA 1041/43 from NXP

Pin compatible

with ISO11898-

2 drivers

The driver detects some type of failures on the CAN bus

and send this information to the microcontroller using

the ERR signal.

Normally, the microcontroller disconnects the node from

the CAN bus.

Typical failures are: TXD dominant (low level) clamping

detection, RXD recessive (high level) clamping

detection, TXD-to-RXD short circuit detection, Bus

dominant clamping detection or overtemperature

detection

Typical application

with 3V

microcontroller

18

Practical implementations of CAN Partial Network systems

• In automotive systems, some nodes have to remain live for functional

reasons while the contact is off (clamp-30).

• Others are deliberately disconnected from the battery when the contact

is off because they need to save power (clamp-15).

• Others can also be put in standby mode or sleep mode By the

microcontroller

• The key point is that nodes which are in sleep mode or switched off do

not disturb the communication between the ones which remain awake

19

Practical implementations of CAN High Speed CAN – ISO11898-2 &ISO11898-5 Typical Driver ICs

High Speed CAN NXP transceivers High Speed CAN TI transceivers


• It is defined for use in applications with bit rates up to 125Kbps

• It is implemented using a simple differential wire pair

• It is the most widespread implementation used in vehicle (body/confort

domain) and industry applications

• The transmission is yet maintained, in degraded mode, even in sever

conditions of physical disturbances

FT Low Speed CAN – ISO11898-3 -

20

21

Practical implementations of CAN FT Low Speed CAN – ISO11898-3 –

Electrical levels of the FT Low Speed physical layer

Main characteristics

• Maximum speed: 125 Kbps

• From 2 to 20 nodes

• Differential (twisted) pair with groun

return

• A distributes termination network

through the bus.

• Low EMC radiation

Nominal Values Recessive Dominant

VCANH 0V 3.6V VCANL 5V 1.4V VCANH – VCANL -5V 2.2V

3.6V

1.4V

22


• The main advantage to use a slower speed is that the communication distance

can be increased without problems of EMC pollution.

• Some applications do not require long distances and the communication

speed can be below 125 Kbps. In this case, this low speed CAN network can

be implemented using the same ISO11898-2 drivers shown in the previous

slides. The main difference with ISO 11898-3 is the electrical levels on the

physical layers.

• It will found that as the length of the network increases, it is more likely that it

will suffer of various kinds of electromechanical disturbances due to short

circuits and/or open circuits (wires isolated or cut).

• If a CAN communication must be reliable, it must be capable of being

maintained even in severe conditions of physical disturbances, continuing to

transmit, even in “degraded mode” and signalling and/or indicating any faults

that ocurre.

• This is the reason to define a Fault Tolerant Low Speed CAN (FT LS CAN).

• This CAN network is not very usual in medium-end cars. It is more usual in

high-end cars used in body applications like roof, seat, windows where a short-

circuit or cut wires can be more likely.

23


• Principle of FT LS CAN: If there are any faults on the bus, they must be

detected, signalled and repaired as well as possible.

24


• All nodes continue communicating at least with reduced signal and noise ratio.

25


Typical implementation of a FT-CAN node

• It is based in a distributed termination

resistance philosophy to avoid

communication problems if some

wire/node is broken

• The device provides differential transmit

capability but will switch in error

conditions to a single-wire transmitter

and/or receiver.

• In the supported failure cases all nodes

continue communication with reduced

signal-to-noise ratio. Once the wire

corruption is removed, the fault-tolerant

transceiver automatically resumes

differential signal transmission

26

Practical implementations of CAN FT Low Speed CAN – ISO11898-3 – Typical Driver ICs

FT LS CAN NXP transceivers

27

Practical implementations of CAN Single-Wire CAN – SAE J2411

• Leaving aside any standarization of the physical layer for differential wires, a single-wire

solution (plus ground, of course) was developed several years ago for economical

reasons.

• Drivers for this implementation only use one pin, normally called CANH. High Level on

this pin can be achieved with 5V (normal mode)/12V (wake-up mode) with respect to

GND.

• Due to a high RF radiation, this bus is only used at low speeds. Typical values are 33,3

Kbps (normal/wake-up mode) and 83,3Kbps (high speed mode)

• Failures of the short circuit and opencircuit types are theoretically insoluble and so, it is

only used for non-critical solutions intended to use and comply with the CAN protocol,

benefit from its advantages and the cost must be reduced.

• Examples of application: GMLAN /SAE J2411 specification by OPEL/GM for body

system.

• SW-CAN and LIN physical layers are very similar. SW-CAN has the main advantage of

maintaining consistency and performance of the CAN protocol throughout the network,

thus avoiding the need of gateways between protocols or between a protocol and a

“sub-protocol”, such as LIN.

28


29


• There are not many single-wire transceiver manufaturer in the market: NXP AU5790,

Infineon TLE6255, Melexis TH8055/56, Freescale 33897 and ONSemi NCV7356.

GMLAN operation

During Wake-up mode (Mode0 = 0 & Mode1 =

1) the nominal CAN bus electrical levels used

are 12V (high) and 0V (low).

After wake-up, normal mode is activated

(Mode0 = 1 & Mode1 = 1) the nominal CAN

bus electrical levels used are 4V (high) and 0V

(low)

30

Practical implementations of CAN Galvanic Isolated CAN wire links

By optical coupling Every optocoupler delays the signal 20 ms

(equivalent to 200ns)

By isolation transformer Every optocoupler delays the signal 20 ms

(equivalent to 200ns)

31

Connectors


RJ-45 Connector

32

CAN Frame Structure

32

Courtesy of Vector Informatik GmbH

• All fields in a CAN frame have a bit length defined with the exception of the arbitration

field.

• The arbitration field has a length of 12 bits if the CAN frame corresponds to CAN-

Version 2.0A (Standard), where the messages uses a 11-bit standard identifier and no-

stuffing bits must be added.

• The arbitration field has a length of 32 bits if the CAN frame corresponds to CAN-

Version 2.0B (Extended), where the messages uses a 29-bit standard identifier and no-

stuffing bits must be added.

• If stuffing bits must be added, the arbitration field length can be different to 12 or 32 bits.

• CAN-Standard 2.0A messages and CAN-Extended 2.0B messages can share the same

CAN physical network

33

CAN-Version 2.0A

messages with 11-bit -identifiers

CAN-Version 2.0B

messages with 29-bit-identifiers

==> Suitably configured, each implementation ( BASIC or FULL)

can handle both standard and extended data formats.

Standard

Extended

CAN Frame Structure

34

CAN Frame Structure

34

ISO11898 distinguishes four types of frames in CAN networks:

• Data Frame: Data is transmitted form a transmitter to one or several receivers upon the

initiative of the source (transmitter). Normally, the frame has all fields, but the data field

is optional.

• Remote Frame: A bus node (receiver) can request the transmissión of a data frame of

the same frame identifier by a source. It has not data field.

• Error Frame: It is used to signal an error detected by a bus node (transmitter or receiver)

and to destroy the frame.

• Overload Frame: It is used to provide for an extra delay between a preceding and a

succeeding data or remote request frame or to signal a specific error condition.

REMOTE FRAME ESCENARIO:

35

CAN Access Procedure: CSMA/CA

CSMA/ CA: “Carrier Sense Multiple Access with Collision

Avoidance” • access-control with non

destructive bit-wide

arbitration

• if there is a collision , the

“winner” continues

• the message with higher

priority is not delayed!

• real-time capability for high

prioritised messages

• the lower the identifier, the

higher the priority

• Controllers losing

arbitration will wait until

bus-idle before reattempting

transmission

CAN Frame Structure

36

CSMA / CA =

"bit - wide arbitration during transmission with simultaneous

receiving and comparing of the transmitted message"

means :

• if there is a collision within the arbitration-field, only the

nodes with lower priorities cancel transmission.

• The node with the highest priority continues with the

transmission of the message.

node 1 node 2 node 3

high : reccessive

low : dominant

node 1 node 2 node 3 bus

high high high high high low high low low low high low

Vcc

R

CAN Frame Structure

37

(1) arbitration-field :

denote the priority of the message

logical address of the message (identifier)

Standard frame, CAN 2.0A: 11 bit-identifier

Extended frame, CAN 2.0B: 29 bit-identifier

CAN Frame Structure

38

CAN Frame Structure

Courtesy of Vector Informatik GmbH

• Because dominant (logic “0”) wins over recessive (logic “1”), low-valued identifiers always

have higher priority to access the bus.

• If Std IDs and Ext IDs are involved, the Std IDs compete with the 11 most significant bits

(b28…b18) of the Ext IDs.

• If all 11 first bits are identical, the Std ID wins if it is a data frame because of the

dominant RTR bit (SRR is always recessive)

• If all 11 first bits are identical, the Std ID wins if it is a remote frame because of the

dominant IDE bit (the IDE bit belongs to the arbitration field in the Extended frame and

belongs to the control field in the standard frame)

39

(3) control-field :

Transmit the number of bytes in the data-field (DLC – From 0 to 8)

CAN Frame Structure

40

(3) data field :

up to 8 bytes per message ,

a 0 byte message is also permitted

CAN Frame Structure

41

(4) CRC field:

cyclic redundancy check ; contains a checksum generated by a CRC-

polynomial

CAN Frame Structure

42

(5) ACK field:

• The ACK Field is two bits long and contains the ACK Slot and the ACK Delimiter (always

recessive).

• The transmitter of a frame transmits both bits fo the ACK Field recessive.

• Every CAN node that receives any error-free transfer (from SOF to CRC) reports this to

the transmitter by sending a dominant bit during the ACK slot – regardless of whether the

message was intended for that node.

• If the transmitter detects a positive acknowledge, that is a dominant ACK slot, the

transmitter knows that in minimum one node has got his message.

• If no one receives the message correctly the ACK Slot stays recessive.

CAN Frame Structure

43

(6) End Of Frame Field:

• Seven Recessive bits

If a dominant level occurs in bit number:

• [1..6] A form error is detected by the receiver and the transmitter. The frame is discarded

and an error flag is started. The transmitter repeats the transmission.

• [7] The frame is valid for the receiver but not for the transmitter. The tranmitter repeats

the transmission, that frame is received twice. A higher layer protocol must handle this

situation.

CAN Frame Structure

44

EOF + IFS

10 bits

ACK

2 bits

CRC

15 bits

data

0...8 byte

DLC

4 bits

r0

1 bit

IDE

1 bit

RTR

1bit

Identifier

11 bits

start

1 bit

Identifier

11 bits

start

1 bitEOF + IFS

10 bits

ACK

2 bits

CRC

15 bits

data

0...8 byte

DLC

4 bits

r0

1 bit

r1

1bit

RTR

1bitSRR

1bit

IDE

1bit

Identifier

18bit

DATA-Frame CAN 2.0A (11-bit-identifier)

DATA-Frame CAN 2.0B (29-bit-identifier)

CAN Frame Structure

45

start bit (1 bit - dominant): beginning of a message; after idle-time falling-edge to

synchronize all transmitters

identifier (11 bit): mark the name of the message and its priority ;the lower the value

the higher the priority

RTR (1 bit): remote transmission request; if RTR=1 (recessive) no valid data inside

the frame - it is a request for receivers to send their messages

IDE (1 bit): Identifier Extension; if IDE=1 then extended CAN-frame

r0 (1 bit): reserved

CDL (4 bit): data length code in byte (0...8)

data (0...8 byte): the data of the message

CRC (15 bit): cyclic redundancy code for error detection, no correction; hamming-

distance 6 (up to 6 single bit errors can be detected)

ACK (2 bit): acknowledge; if a receiving node has received a valid message, it

must transmit an dominant acknowledge – bit

EOF (7 bit = 1, recessive): end of frame; intentional violation of the bit-stuff-rule ;

normally after five recessive bits one stuff-bit follows automatically

IFS (3 bit = 1, recessive): inter frame space; time space to copy a received

message from bus-handler into buffer

Extended Frame only : SRR (1 bit = recessive): substitute remote request ; substitution of the RTR-bit in

standard frames r1 (1 bit ): reserved

CAN Frame Structure

46

Remote Frames

RTR bit = 1 No Data Field

CAN Frame Structure

47

INTERFRAME SPACE: SEPARATES A FRAME (of whatever type) FROM A DATA O REMOTE FRAME:

• Data frames and remote frames are separated from preceding frames whatever type

they are (Data, remote, error or overload) by a field called INTERFRAME SPACE.

• In contrast, Overload and error frames are not preceded by an Interframe space and also

if multiple Overload frames has to be transmitted.

• Interframe Space constains two parts: INTERMISSION and BUS IDLE.

• INTERMISSION consists of 3 recessive bits. During Intermission the only action to be

taken is signalling an OVERLOAD condition and no station is allowed to actively start a

transmission of a data or remote frame

• BUS IDLE may be of arbitrary length. The bus is recognized to be free and any node

having something to trnsmit can access the bus. Even if a message is pending of

transmission during a previous one, it will be started in the first bit following

INTERMISSION.

CAN Frame Structure

48

Overload Frames

USED TO DELAY NEXT CAN MESSAGE:

A node can generate a maximum of 2 sequential overload frames to delay the start of the next data or remote frame

CAN Frame Structure

49

CAN Error Frame

• any node that detects a bus error generates an error - frame

• an error frame is transmitted as soon as an error has been

detected, e.g. inside a data frame

• consists of two fields: Error Flag Field; Error Delimiter Field

• Error Delimiter Field:

• 8 recessive bits

• allow bus nodes to restart bus communication after an error

• Error Flag Field:

Type depends on the error-state of the node:

• error active: 6 consecutive dominant error bits; all other

nodes will respond to this violation with their own error

frames Error Flag Field = 6…12 dominant bits

• error passive: 6 consecutive recessive bits plus 8 error

delimiter bits = 14 recessive bits • receiver: does not corrupt the message • transmitter: other nodes may respond with active error frames

CAN Frame Structure

50

data error frame Inter frame space

6 bit error flag

6..12 bit error overlay 8 bit error delimiter

Active error frame

Example1: Active error frame

CAN Frame Structure

CAN Error Frame

51


6 bit error flag 8 bit error delimiter

passive error frame from a receiver


6 bit passive error flag

6 bit active error overlay

from another active node

8 bit error delimiter

passive error frame from a transmitter

CAN Frame Structure

CAN Error Frame

52

data

Transmitter X

CAN - Tx

Example: active error frame

Receiver Y

CAN - Tx

Receiver Z

CAN - Tx

CAN

Bus - level

1

2

3

4

4

5

6 6 8 3

6

CAN Frame Structure

CAN Error Frame

53

1 Node X sees a bit error

2 Node X generates an active error flag field

3 Nodes Y, Z realize a stuff bit error after bit 6 of the active error flag field

(note: if the corrupted data frame had dominant bits, the stuff bit error is

detected earlier)

4 Nodes Y,Z transmit their own active error flag field of 6 dominant bits

5 All nodes transmit the recessive error delimiter field. Node Y and Z see

no difference @ bus level, but node X detects a delay of 6 bits

between bus level and its own output First node to message error

6 After the last 8 recessive error delimiter bits @ CAN-bus and 3 bit of

inter frame space a new arbitration is entered by node X, e.g. it has to

compete again with other nodes

CAN Frame Structure

CAN Error Frame

54

1. Bit-Error

the transmitted bit doesn’t read back with the same digital

level (except arbitration and acknowledge- slot ) 2. Bit-Stuff-Error

more than 5 continuous bits read back with the same digital

level (except ‘end of frame’-part of the message ) 3. CRC-Error

the received CRC-sum doesn’t match with the calculated sum 4. Format-Error

Violation of the data-format of the message , e.g.: CRC-

delimiter is not recessive or violation of the ‘end -of-frame’-

field 5. Acknowledgement-Error

transmitter receives no dominant bit during the

acknowledgement slot, i.e. the message was not received by

another node.

CAN Frame Structure

CAN Error Frame

55

error

handling

error

detection

error

managing

error

limitation

error

active

error

passive

bus

off

Purpose: avoid persistent disturbances

of the CAN by switching off defective

nodes three Error States :

Error Active: normal mode, messages will be received and

transmitted. In case of error an active error frame will be transmitted.

Error Passive: after detection of a certain number of errors, the node

reaches this state. Messages will be received and transmitted but in

case of an error the node sends a passive error frame.

Bus Off: the node is separated from CAN, neither transmission nor

receive of messages is allowed and the node is no longer able to

transmit error frames.

CAN Frame Structure

CAN Error STATUS

56

error passive

error active

bus off

REC <127

and

TEC <=127

REC >127 or

127<TEC<255

TEC > 255

'reset' or 'init

node'

• transitions will be carried out automatically by

the CAN-chip

• states are managed by 2 Error Counters :

Receive Error Counter (REC)

Transmit Error Counter (TEC)

• Possible situations :

a) a transmitter recognises an error:

TEC:=TEC + 8

b) a receiver sees an error : REC:=REC + 1

c) a receiver sees an error, after transmitting an

error frame: REC:=REC + 8

d) if an ‘error active’-node find’s a bit-stuff-error

during transmission of an error frame:

TEC:=TEC+ 1

e) successful transmission:

TEC:=TEC - 1

f) successful receive:

REC:=REC - 1

State - Diagram:

CAN Frame Structure

CAN Error Counter

57

• The data transmission via CAN network is based on the principle of message

broadcasting.

• This means that all messages transmitted on the bus are accessible to all nodes and

received by the CAN controllers.

• In many cases a certain node will only be interested in a few messages intended for it.

For this reason it is very appropriate to implement an additional message filtering

mechanism which ensures that a host controller is only informed of the reception of a

new message is actually relevant for it

• For the implementation of this filtering mechanism, the CAN controllers has several

internal registers normally called: mask registers and filter register

• The mask registers are formed, at least, by the same number of bits than an identifier in

a CAN frame (29). Every bit in this register indicates if the corresponding bit in the

received frame is relevant or not to inform to the controller core about its reception.

• The filter register are also formed, at least, by the same number of bits than an identifier

(29). A received message is relevant for this node if the corresponding identifier bit has

the same value in the filter register.

• Normally, a CAN controller has several filter register and several mask register. The

same CAN register can be combined with several filter register

• The filter and/or mask registers has normally extra bits in order to define if the filtering

process is only applicable to standard messages, extended messages or both of them

Message Filtering

58

• To explain the filtering main principle, we are going to suppose that the identifiers are

formed by only 4 bits

• Mask bit = 1 means that this bit is relevant for recepetion

• Mask bit = 0 means that this bit is not relevant for reception. It can be 1 or 0.

Message Filtering

Transmitter ID: 1001b

Receiver1 Mask: 1001b Filter: 1111b




X

Main Principle

• Receiver1 relevant messages: ID=1xx1b Receiver2 relevant messages: ID=xxxxb (all)

• Receiver 3 relevant messages: ID=1xxxb Receiver4 relevant messages: ID=0xxxb

• Relevant means that the microcontroller must expend some time to extract the message

from the CAN buffer and decode the data

59

Message Filtering






X

Main Principle

X






X

X

60

CAN Useful Links

Videos

• Youtube - Controller Area Network(CAN)]-Introduction CAN Bus

Technology.flv by SaidyNguyen. http://www.youtube.com/watch?v=YKWrlsu2IXc

• Youtube – CAN Seminar Part 1/2/3/4 of 4 by Wilfried Voss (esd-electronics) http://www.youtube.com/watch?v=m8HTVlEC7Lg

Web Resources

• CAN-CiA Organization. http://www.can-cia.org/index.php?id=can

http://www.youtube.com/watch?v=YKWrlsu2IXc

http://www.youtube.com/watch?v=m8HTVlEC7Lg

http://www.youtube.com/watch?v=m8HTVlEC7Lg

http://www.can-cia.org/index.php?id=can




Date post:	18-Jul-2016
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CAN Introduction 2014(2)

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