Presentation CAN

8/4/2019 Presentation CAN

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CONTROLLER AREA NETWORK (CAN)

Umesh Mangalekar 1




Umesh Mangalekar 2

Objective:

To understand

Development of CAN

Basic Concept of CAN Frame Formats - Overview Error detection - Overview

CAN Protocol Typical CAN Implementations

CAN – User Benefits

CAN – Application Examples




Umesh Mangalekar 3

Implementation of more and more electronic devices in modern motor vehicles

for More safety and comfort for the driver, Reduction of Fuel Consumption,Exhaust Emission

Development of CAN




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These different control systems & their controls were connected point to point by wiring.

The requirement for information exchange grown to such an extent thata cable network with a length of up to several miles and many connectors were required

Problems --- Increased material cost, Production time and reliability

Development of CAN




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Solution: Connection of control systems with serial bus system. This bus system has tofulfill some special requirement due to its use in vehicle

With the use of CAN, point to point wiring is replaced by one serial bus system connectingall control systems. This is accomplished by adding some CAN- specific hardware to eachcontrol unit that provides the “rules” or the protocols for transmitting and receivinginformation via the bus.

Development of CAN




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Advanced serial bus system to efficiently supports distributed control systems. Initially developed by Robert Bosch GmBH, Germany in late 1980. Holds license for CAN CAN is internationally standardized by ISO and SAE

CAN protocol uses Data Link Layer and Physical Layer in ISO_OSI model There are also higher level of protocols are availableWidely used in Automotive and Industrial market segment There are about 140 million nodes are in use by 2000

Overview of CAN




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Multi-master bus with Open, linear structure with one logic bus lineNumber of nodes not limited by protocol

Bus Nodes do not have specific address, instead address information is contained inidentifiers of transmitted messages, indicating the message content and priority of message No. of nodes may be changed dynamically without disturbing communication of other nodes Multicasting and broadcasting capability

Basic Concept of CAN




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Provides sophisticated error detection and error handling mechanism viz. CRC Checkand high immunity against electromagnetic interference. Erroneous messages areautomatically retransmitted. Temporary errors are recovered. Permanent errors are followedby automatic switch-off of defective nodes

CAN protocol uses Non – Return to Zero or NRZ bit coding CAN protocol uses bit stuffing for synchronization High data transfer rate of 1000kbps for max bus length of 40m using twisted wire pairs Message length is short with 8 data bytes per message. Bus access is handled via advanced serial communications protocol Carrier SenseMultiple Access/Collision Detection (CSMA/CD) with non destructive arbitration. Collision of message is avoided by bitwise arbitration without loss of time

Basic Concept of CAN




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Two Bus states – Dominant and Recessive Bus logic uses WIRED AND mechanism Dominant bits (equivalent to logic level zero) overwrites the recessive bits (equivalent tothe logic level one)

CAN Bus Characteristics – Wired AND




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Only if all nodes transmits recessive bits(ones), the bus is in recessive state

CAN Bus Characteristics – Wired AND

As soon as one node transmits dominantbit (zero), the bus is in dominant state




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Carrier Sense Multiple Access with Arbitration on message priority Arbitration avoids the collision of messages whose transmission was started by more than

one node simultaneously and makes sure that the most important message is sent firstwithout time loss. It is not permitted for different nodes to send messages with same identifier as arbitrationcould fail leading to collisions and errors.

Bus Access and Arbitration – CSMA/CD w/AMP




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Frame Formats - Overview




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Frame Formats – Data Frame

Generated by CAN when node wishes to transmit the data Start of Frame used for Hard synchronization of all nodes 11 bit identifier field reflects the contents and priority of the messages Remote Transmission Request (RTR) is used to distinguish the Data Frame

(RTR=Dominant) from Remote fram (RTR=Recessive) Control Field 6 bits 1st bit IDE (Identifier Extension) at dominant state to specify the standard frame Next bit is reserved and is in dominant state Next 4 bits are Data Length Code (DLC) used to specify the number of bytesdata contained in message (0-8 bytes)




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Frame Formats – Data Frame

Data Field contains the data to be sent Cyclic Redundancy Field (CRC) consists of 15 bits CRC sequence and 1 bit recessiveCRC Delimiter. It is used to detect possible transmission errors

Acknowledge Field contains ACK slot – transmitting node sends out a recessive bit. Anynode that has received the error free frame acknowledges the correct reception of the frameby sending back the dominant bit. Recessive acknowledge delimiter completes the acknowledge slot and may not beoverwritten by dominant bit. 7 recessive bits (End of Frame) ends the Data Frame




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Frame Formats – Remote Frame

In general data transmission, data source node (e.g. Sensor) send out the data frame. It is also possible that the destination node can request data from the source by sendingthe remote frame. Differences ---

1. RTR frame is transmitted as dominant in Data frame 2. There is no Data field in remote frame. Data frame and Remote frame with the same identifier being transmitted at the sametime, the data frame wins the arbitration due to the dominant RTR field following theidentifier. So node that transmits remote frame received the desire data immediately.




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Frame Formats – Error Frame

Generated by any node which detects bus error Two fields = Error flag field followed by Error Delimiter field Error delimiter consists of 8 recessive bits, allows bus node to restart the bus

communication cleanly after an error. Error Flag field has 2 forms, depends upon the error status of node that detects the error. Error Active node detects a bus error, interrupts transmission of the current message bygenerating the active error flag. Error Passive node detects a bus error, transmits a passive error flag followed by errordelimiter field.




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Frame Formats – Overload Frame

Generated only during the interframe space. Two fields = Overload flag field followed by Overload Delimiter field Overload frame can be generated by a node if the due to internal conditions, the node is

not yet able to start reception of the next message. A node may generate maximum of two sequential Overload frames to delay the start ofthe next message.




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Frame Formats – Interframe space

Interframe space separates a preceding frame from following data or remote frame. It consists of 3 recessive bits called as intermission field. This time is provided to allow the nodes, time for internal processing before the start of the

next message.




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Error detection - Overview

CAN protocol provides sophisticated error detection mechanism.




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Error detection – Cyclic Redundancy Check (CRC)

If node detects the mismatch betweenthe calculated and the received CRCsequence, then CRC error has occurred.

Node B discards the message andtransmits an error frame to requestretransmission of garbled frame.




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Error detection – Acknowledge Check

With Acknowledge check, the transmitter checks in the Acknowledge Field of a messageto determine if the acknowledge slot, which is sent out as recessive bit, contain dominant

bit. If this is the case, other node had received the frame successfully. If not, the acknowledge error has occurred and message has to be repeated. No error frame has generated.




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Error detection – Frame Check

If transmitter detects a dominant bit in one of the four

segments, CRC delimiter, Acknowledge delimiter, End of Frame,Interframe space Then a form error has occurred and a error frame isgenerated. Message will then be repeated




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Error detection – Bit Monitoring

All nodes perform bit monitoring. If transmitter sends a dominant bit but detectsrecessive bit on a bus or vice versa, an error frame isgenerated and message is repeated.




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Error detection – Bit Stuffing

If six consecutive bits with the same polarity are detected between start of frame andCRC delimiter, the bit stuffing rule has been violated. Stuff error occurs and error frame is generated and message is repeated.

Undetected errors – Example

To understand the error detection capabilities of CAN, imagine a vehicle equipped withCAN running 2000 hrs per year at a CAN bus speed of 500kbps with 25% bus load This will result in 1 undetected error every 1000 years




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CAN Protocol versions

The original CAN specifications (V1.0, 1.2 and 2.0A) specify 11 bit identifier. Also known as Standard CAN With this 2048 different messages (identifier = 0 – 2047) can be identified.

16 messages with lowest priority (2032-2047) are reserved. CAN Version 2.0B adopted to remove the message number limitation and to meet the SAE J1939std for the use of CAN in trucks CAN 2.0B is also called as Extended CAN Contains 29 bit identifier which allows over 536 millions message identifier. 29 bit identifier consists of 11 bit identifier (BASE ID) and 18 bit extended identifier (ID Extension)




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CAN Protocol Message Coding

CAN Protocol uses Non-Return-To-Zero or NRZ bit coding

Signal is constant for one whole bit time and only one time segment is need to represent one bit

Zero corresponds to dominant bit, placing the bus in dominant state.

One corresponds to recessive bit, placing the bus in recessive state.




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Relation Between Baud Rate and Bus Length

Max CAN Bus speed is 1MBaud, can be achieved using bus Length of up to 40m

For Bus length longer than 40m, Bus speed must be reduced.

For bus length more than 1000m, special drivers should be used.




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ISO Physical Layer

For CAN bus line, medium must be chosen that is able to transmit two possible bit statesi.e. dominant and recessive

Most Common and cheapest – Twisted wire pair

Bus lines are then called as CAN_H and CAN_L

Alternate – Optical Fiber Recessive state will be represented by the signal Light off and Dominant state will be represented by the signal Light on




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Typical CAN Implementations

Application itself is controlled by a microcontroller e.g. Siemens SAB 80C166

For communication, microcontroller has to be connected with CAN controllere.g. Siemens Full Controller 81C90/91

To meet the requirements of ISO 11898 CAN standard, a CAN transceiver chiphas to be used to connect the node to the CAN bus line




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CAN – User Benefits

Low Cost - Serial Bus with two wire – good price/performance ratio

Reliable - Sophisticated Error handling & detection mechanism - high reliability transmission

Real Time - Short Message length ( 0 – 8 data bytes /message)- Low latency between transmission request and actual start of the transmission- Inherent Arbitration on Message Priority (AMP)

Flexible - CAN nodes can be easily connected or disconnected (i.e. plug and play)- No limitation of number of nodes

Fast - Max data rate is 1Mbit/s @ 40m bus length

Broadcast capability

- Message can be sent to single / multiple nodes- All nodes simultaneously receive the common data

Standardization- ISO - DIS 11898 (High Speed Applications)- ISO - DIS 11519-2 (low Speed Applications)




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CAN – Application Examples

Motor vehicles (Cars, Buses, Trucks)

- Enables communication between ECUs like engine management system,anti-skid braking, gear control, active suspension

Utility vehicles- Construction vehicles, forklifts, tractors- CAN used for power trains and hydraulic control

Trains - High need of data exchange between electronic subsystem control units- i.e. acceleration, braking, door control, error messages etc,

Industrial automation- For connecting all kinds of automation equipments

(control units, sensors, actuators)- Machine controls

Medical Equipment- Computer tomographs, X-ray machines, dentist and wheel chairs

Building automation- heating, air-conditioning, lighting, surveillance etc.- Elevator and escalator control




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References

http://www.mjschofield.com, 2009, Homepage for Controller Network Area

http://www.can-cia.org, 2009, Homepage for organization CAN in Automationhttp://www.s3.kth.se, 2009, Paper on Vehicle Applications of CAN

http://www.mjschofield.com/

http://www.can-cia.org/

http://www.s3.kth.se/






























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Presentation CAN