controller area network_dgk.pptx

7/27/2019 Controller Area Network_DGK.pptx

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Controller Area Network

CAN

ByKanade D G


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Controller Area Network Invented by Robert Bosch GmbH in 1980 for automotive

applications

Asynchronous Serial Bus

Simple 2-wire differential bus Absence of node addressing

Message identifier specifies contents and priority

Lowest message identifier has highest priority

Non-destructive arbitration system by CSMA with collisiondetection

Multi-master / Broadcasting concept

Sophisticated error detection & handling system

What is CAN ?


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The CAN is an ISO standard (ISO 11898) for serialcommunication

Today CAN has gained widespread use:

Industrial Automation

Automotive, etc.

The CAN standard includes:

Physical layer

Data-link layer

Some message types

Arbitration rules for bus access

Methods for fault detection and fault confinement

What is CAN ?


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Why CAN?

Mature Standard

CAN protocol more than 25 years

Numerous CAN products and tools in the market

Hardware implementation of the protocol

Combination of error handling and fault confinement with hightransmission speed (up to 1Mb/s)

Simple Transmission Medium

Twisted pair of wires is the standard, but also just one wire willwork

Other links works, too: Opto - or radio links

Excellent Error Handling

CRC error detection mechanism

Fault Confinement

Built-in feature to prevent faulty node to block system

Most used protocol in industrial and automotive world


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Example Application of CAN


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Introduction

Originally designed for automotive industry

Is a specification for serial network

ISO standard

Used in Embedded Systems

Network established among micro-controllers.

It is two wire, half duplex, high speed network

system .

Well suited for high speed applications using

short messages


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Applications

Today CAN has gained widespread use:

Industrial Automation

Automotive

Home Area Networks , etc.

The CAN standard includes

Physical Layer

Data-link layer


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7. Application Layer

5. Session Layer

6. Presentation Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer

CAN Protocol

HLPs: CANopen, DeviceNet, OSEK/VDX

Partially Implemented by

High Layer Protocol (HLP)

CANL

ayers

ISO-OSI Reference Model

ISO-OSI Reference Model


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Basics of CAN

Transmitter

A node which originates the message.

Remains the transmitter until the bus becomes

idle or loses arbitration.

Any node can start transmitting if the bus is free.

Receiver

A node is called receiver if it is not sending the

message , and bus is not idle.


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Basic Concepts

Message: Information on the bus sent in fixed

format message of different but limited length

Message Routing: Each message is named by

an IDENTIFIER

The IDENTIFIER does not indicate the destination

of the message, but describes the meaning of the

data, so that all nodes in the network are able todecide by MESSAGE FILTERING whether the data is

to be acted upon by them or not.


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Concepts

Multicast: As a consequence of the concept of

MESSAGE FILTERING any number of nodes can

receive and simultaneously act upon the same

message.

Priorities: The IDENTIFIER defines a static

message priority during bus access.

Remote Data Request: A node requiring data

may request another node to send the

corresponding DATA FRAME.


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Arbitration

Arbitration: If two or more units start

transmitting message at the same time, the

bus access conflict is resolved by bitwise

arbitration using the IDENTIFIER.

The mechanism of arbitration guarantees that

neither information nor time is lost.


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Physical Layer

CAN transmits signals on the CAN bus whichconsists of two wires, a CAN-High and a CAN-Low.

Two wires operate in differential mode (carrying

inverted voltages) to decrease noise interference

Optical fibres are also used.

Must support bit dominance.

The low bits are always dominant

If one node tries to send a low and another node tries to

send a high, the result on the bus will be low


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Controller CAN

Controller

CAN

Transceiver

CANB

us

CAN_H

CAN_L

TXD

RXD

CAN Bus is a simple 2-wire differential serial bus

CAN Bus is terminated on each side by a 120 Ohm resistor

Typical CAN Node


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CAN Physical Layer

Differential

Bus Driver

Optical

Transmitter

Receiver

CAN_Txd

CAN_Rxd

Physical CAN Bus Differential e.g.

Twisted Pair

CAN_Txd

CAN_Rxd

Optical

Fiber

120 ohm Rresistor

120 ohm Rresistor


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Node 1 Node 2 Node 3 Bus

D D D D

D D r D

D r D D

D r r D

r D D D

r D r D

r r D D

r r r r

1 Recessive (r)

0 Dominant (D)

Two logic states on the CAN bus

Bus in dominant state

Wired-AND Function

Bus in recessive state

or idle

CAN Bus Logic


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Bit Encoding

Non-return to Zero-Level Two different voltages for 0 and 1 bits

Voltage constant during bit interval

No transition i.e. no return to zero voltage

Absence of voltage for zero, constant positive voltagefor one.

More often, negative voltage for one value andpositive for the other

This is NRZ-L

Fewer transitions (on average): less EMI, butrequire less oscillator drift.


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Bit Coding : NRZ (Non-Return-To-Zero code) does not

ensure enough edges for synchronization Stuff Bits are inserted after 5 consecutive bits of the

same level

Stuff Bits have the inverse level of the previous bit.

No deterministic encoding, frame length depends ontransmitted data

Number of consecutive bits

with the same polarity

CAN Bit Coding & Bit Stuffing


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Types of Frames

Communication is performed using the following

five types of frames.

Data frame Remote frame

Error frame

Overload frame Interframe space


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Of these frames, the data and the remote

frames need to be set by the user.

The other frames are set by the hardware part

of CAN.

The data and the remote frames come in two

frame formats: standard and extended.

The standard format- 11-bit ID,

The extended format - 29-bit ID.

Types of Frames....


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Frame Types and Roles of Each FrameFrames Roles of Frames User Settings

Data frame This frame is used by transmit unit to send

message to the receive unit

Necessary

Remote Frame This frame is used by the receive unit to

request transmission of message that has

the same ID from the transmit unit

Necessary

Error frame When error is detected, this frame is used tonotify other units of the detected error

Unnecessary

Overflow frame This frame is used by the receive unit to

notify that it has not been prepared to

receive frame yet

Unnecessary

Interframe space This frame is used to separate a data or

remote frame from preceding frame

Unnecessary


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Structure of Data Frame


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Data Frame

The data frame is used by the transmit unit to send a

message to the receive unit, and is the most fundamental

frame handled by the user.

The data frame consists of seven fields.

(1) Start of frame (SOF) This field indicates the beginning of a data frame.

(2) Arbitration field

This field indicates the priority of a frame.(3) Control field

This field indicates reserved bits and the number ofdata bytes.


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Data Frame

(4) Data field This is the content of data. Data in the range 0 to

8 bytes can be transmitted.

(5) CRC field

This field is used to check the frame for atransmission error.

(6) ACK field

This field indicates a signal for confirmation that

the frame has been received normally.(7) End of frame

This field indicates the end of a data frame.


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Data Frame

Start of Frame(SOF)

This field indicates the beginning of a frame.

It consists of one dominant bit.


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Data Frame

Arbitration field:-This field indicates the priority ofdata.


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Data frame : Arbitration field

The standard format ID consists of 11 base ID

bits (ID28ID18). These ID bits are transmitted

sequentially beginning with ID28.

The 7 high-order bits cannot all be recessive.

IDs set to 1111111XXXX are prohibited.

Up to 2,032 discrete IDs can be set.

In no case can multiple units on the bus transmit

data frames with the same ID at the same time.


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Data FrameControl fieldThis 6-bit field indicates the number of data bytes in a

message to be transmitted.


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Continued.


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Data Frame

Data field (common to both standard and extended formats)This field indicates the content of data.

Zero to 8 bytes of data set in the control field can be

transmitted.

The data is output beginning with the MSB side.


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Data Frame

CRC field (common to both standard and extended formats)This field is used to check the frame for a transmission error.

It consists of a 15-bit CRC sequence and a 1-bit CRC delimiter

(separating bit).


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Data Frame: CRC field

CRC sequence

The CRC sequence consists of a CRC valuegenerated by the polynomial

P(X) = X15 + X14 + X10 + X8 + X7 + X4 + X3 + 1

The CRC calculation range includes The start of frame, arbitration field, control field, and

data field.

On the receive unit side too, a CRC value is calculatedin the same range of fields.

The calculated CRC and the received CRC sequenceare compared, and if they do not match, an error isassumed.


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Data FrameACK field

This field indicates a signal for confirmation that the framehas been received normally.

It consists of 2bits, one for ACK slot and one for ACK

delimiter.


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Data Frame: ACK Field

ACK field of the transmit unit

The transmit unit sends the ACK slot and ACK

delimiter in recessive bits.

ACK field of the receive unit

The receive unit that has received a correct

message sends a dominant bit in the ACK slot of

the received frame to notify the transmit unitthat it has finished receiving normally. This is

referred to as sending ACK or returning ACK.


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Data Frame

End of frame

This field indicates the end of a frame. Itconsists of 7 recessive bits.


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Remote Frame

Difference Between Data Frame &


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Difference Between Data Frame &

Remote Frame

The remote frame does not have a data field, and that theRTR bit in its arbitration field is of a recessive level.

The data frame without a data field and the remote framecan be discriminated by the RTR bit.

What does the data length code of the remote frame thathas no data fields indicate?

It indicates the data length code of the corresponding dataframe.

For what is the data frame without a data field used?

For example, this data frame may be used by each unit toconfirm or respond for connection periodically, or to placereal information in the arbitration field itself.


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Error Frame

Error flag

There are two types of error flags:

Active-error flag: 6 dominant bits

Passive-error flag: 6 recessive bits

O l d F


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Overload FrameThe overload frame is used by the receive unit to notify

that it has not been prepared to receive frames yet.It consists of an overload flag and an overload delimiter.

(1)Overload flag - 6 dominant bits.

(2)Overload delimiter - 8 recessive bits.

Error flag overlapping

Depending on timing as for the error flag, overload flags may overlap

one on top of another, up to 12 bits in total length.

I t f S


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Interframe Space

Used to separate the data and the remote frames

Are separated from the preceding frame by an inserted

Interframe space

Continued


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Continued.(1) Intermission

Consists of 3 recessive bits.

If a dominant level is detected during an intermission, anoverload frame must be transmitted. However, if the thirdbit of an intermission is of a dominant level, theintermission is recognized as the SOF.

(2) Bus idle

Consists of a recessive level. There are no limitations on itslength (it can be zero bits in length).

While in this state, the bus is thought to be free, and anytransmit unit can start sending a message.

(3) Suspend transmission (transmission pause period) Consists of 8 recessive bits.

This field is included in only an interframe space if theimmediately preceding message transmit unit was in an

error-passive state.


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Priority Resolution by Arbitration


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Priority of data and remote frames


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CAN Message Format

SOF: Start Of Frame

CRC: Cyclic Redundancy Code

ACK: Acknowledge

EOF: End of Frame

IFS: Inter Frame Space

DATA Frame

Message Field Details


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Message Field Details

ARBITRATION FIELD

The ARBITRATION FIELD consists of the IDENTIFIER andthe RTR-BIT

The IDENTIFIERs length is 11 bits (std Message),29 bitsfor extended frame.

Lower value implies higher priority

CONTROL FIELD

The CONTROL FIELD consists of six bits. It includes theDATA LENGTH CODE and two bits reserved for futureexpansion.

DATA FIELD

The DATA FIELD consists of the data to be transferred

within DATA FRAME. It can contain 0 to 8 bytes


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Field Details

CRC FIELD

Contains the CRC SEQUENCE followed by a CRC

DELIMITER(15 bits)

ACK FIELD

END OF FRAME

EACH DATA FRAME and REMOTE FRAME is

delimited by a flag sequence consisting of 7

recessive bits.

Arbitration Mechanism


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Arbitration Mechanism CAN is multi master network

It uses CSMA/CD + AMP (Carrier Sense MultipleAccess /Collision Detection with Arbitration on

Message Priority)

Network nodes transmits synchronously

All nodes start sending their identifier fields at the

same time.

If node hears a dominant bit in the identifier when it

tries to send recessive bit, it stops transmitting It stops transmitting, letting the other node, with a

higher priority message, continue uninterrupted.

A bi i


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Arbitration

Two nodes on the network are not allowed tosend message with the same id

If two nodes try to send a message with same id at

the same time arbitration will not work.

One of the transmitting nodes will detect that his

message is distorted outside arbitration field.

The nodes will then use the error handling of CAN,

which in this case ultimately will lead to one of thetransmitting node being switched off ( bus-off

mode).


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Remote frames

Remote frames can be described as a requestfor information.

A frame with the RTR bit set means thetransmitting node is asking for information of the

type given by the identifier A node which has the information available should

then respond by sending the information onto thenetwork.

Depending on the implementation of the CANcontroller the answer may be sentautomatically.


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Remote Transfer Frame

SOF Arbitration

Field

Control

FieldCRC

Field

ACK

Field

EOF

Remote FrameInter frame Space Inter frame Space

Or Overload Frame

Or Error Frame


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Error Control

CAN bus is very reliable. The error control, signaling and fault confinement

defined in the CAN standards

Data Link layer functionality

The information is correct and consistent

The built in error detection of the controllers

together with the error signaling .

Faulty nodes will go to modes where they do

not disturb the traffic on the bus.


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The CAN Error Process

The error is detected by the CAN controller ( a transmitter or a receiver).

An error frame is immediately transmitted.

The message is cancelled at all nodes.

The state of CAN controllers are updated

The message is re-transmitted.

If several controllers have messages to send ,

normal arbitration is used.


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Error Detection

Error detection is handled automatically by

the CAN controller. The detected errors are

Bit Errors

Bit stuffing errors

Receiving node detects Bit stuffing violation

Bit Errors

Transmitting node detects the bit on the bus is notwhat it has sent and which is not a part arbitration field

or ack field


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Message Errors

Checksum Error

Receiving node checks CAN message for checksum

error

Frame Error

Error will be flagged if receiver not finds

predefined bit value at certain point within CAN

frame Acknowledgement Error

Transmitter determines that message has not

been acknowledged and ACK error is flagged


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CAN Controller Modes Error active

Normal operating mode for a controller

Messages can be received and transmitted

On detecting an error an active error flag is sent

Error passive

A mode entered when the controller has frequent problems intransmitting or receiving messages

Messages can be received and transmitted

On detecting an error while receiving, a passive error flag issent

Bus off

Entered if the controller has serious problems withtransmitting messages.

No messages can be transmitted or received until the CAN

controller is reset by host microcontroller or processor.


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Error States and Counter Values

E St t f U it


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Error States of a Unit

E Si li


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Error Signaling When error is detected by a node it sends an error flag

on the bus This prevents any other node from accepting the message

and ensures consistency of data throughout the network

The active error flag consists of six low bits and is usedif the node transmitting the error frame is in active

error state.

As low is dominant all other nodes will detect bit stuffing

violation and send their own error flag

After this, nodes that want to transmit(including the

one sending the interrupted message) will start to do

so.

Error Signaling


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Error Signaling

If the CAN controller is in error passive mode the

error frame will consists of six passive(high) bits. Since the error flag consists of passive bits, the bus is not

affected.

If no other node detected an error, the message will be

sent un interrupted.

This ensures that a node having problems with receivingcan not block the bus

All of this advanced error handling is doneautomatically by CAN controller, without any needfor the host microcontroller to do anything. This isone of the biggest advantages.


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Error frame

Error Frame

Error Flag

Superposition of Error Flag

Data Frame

Interframe

Space or

Overload

Frame

Error Delimiter

Basic CAN Controller


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Basic CAN Controller

Interesting messages are filtered out using two registers, thatoperate on the message identifier. Each bit in the identifierare checked against the filter. If the message matches the

filter it is stored in one of the receive buffer.

Bus

Interface

CAN

Protocol

Controller

Acceptance

Filtering

Data ControlRegister

Transmit

Buffer

Receive

Buffering

Host

Inte

rface

Main

Controller

CAN

Bus

Higher Layer Protocol


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Higher Layer Protocol

The CAN protocol defines only the physical andlow data link layers.

The HLP defines:

Start-up behavior

Definition of message identifiers for the different nodes

Flow control

Transportation of messages greater than 8-bytes

Definition of contents of data frames Status reporting in the system

Some HLP:CANopen, DeviceNet, SAE J1939 (Heavy

vehicles)

CAN Applications


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CAN Applications Building Automations

Heating control, Air conditioning(AC), Security(fire,

burglar), Access control, Light control Domestic & Food distribution appliances

Washing machines, Dish cleaner, self service bottledistributors connected to internet.

Automotive & Transportation Dash board electronics, Comfort electronics

Production Automation Control and link of production machines, production control,

Tool machines. Medical

Agriculture Harvester Machines Seeding/Sowing machines, Tractor control, Control of live

stock breeding equipment.


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Thank You

controller area network_dgk.pptx

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