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Controller Area Network
A Serial Bus System - Not Just For Vehicles The need for serial communication in vehicles
Many vehicles already have a large number of electronic control systems. The growth of aimed at overall vehicle optimization, it be- automotive electronics is the result partly of comes necessary to overcome the limitations the customer‘s wish for better safety and of conventional control device linkage. This greater comfort and partly of the govern- can only be done by networking the system ment‘s requirements for improved emission components using a serial data bus system.
control and reduced fuel consumption. Con- lt was for this reason that Bosch developed trol devices that meet these requirements the ”Controller Area Network” (CAN), which have been in use for some time in the area has since been standardized internationally of engine timing, gearbox and carburettor (ISO 11898) and has been ”cast in silicon” by throttle control and in anti-block systems several semiconductor manufacturers.
(ABS) and acceleration skid control (ASC).
Using CAN, peer stations (controllers, sen- The complexity of the functions implemented sors and actuators) are connected via a se- in these systems necessitates an exchange rial bus. The bus itself is a symmetric or asymmetric two wire circuit, which can be either screened or unscreened. The electri- dedicated signal lines, but this is becoming cal parameters of the physical transmission increasingly difficult and expensive as con- are also specified in ISO 11898. Suitable bus trol functions become ever more complex. In driver chips are available from a number of the case of complex control systems (such as Motronic) in particular, the number of con-nections cannot be increased much further.
The CAN protocol, which corresponds to thedata link layer in the ISO/OSI reference mo- automotive applications. Unlike cable trees, vering more than one control device. For in- the network protocol detects and corrects stance, ASC requires the interplay of engine transmission errors caused by electromag- timing and carburettor control in order to netic interference. Additional advantages of such a network are the easy configurability of occurs. Another example of functions span- the overall system and the possibility of cen- ning more than one control unit is electronic gearbox control, where ease of gearchan-ging can be improved by a brief adjustment The purpose of using CAN in vehicles is to enable any station to communicate with anyother without putting too great a load on thecontroller computer.
Use of the CAN network in vehicles
There are four main applications for serial communication in vehicles, each having dif- to the cost of the components and wiringrequirements. Typical data rates are ! Networking controllers for engine timing, transmission, chassis and brakes. Thedata rates are in the range - typical of ! In the near future, serial communication communication in order to link compo-nents such as car radios, car telephones, ject, such as vehicle-to-vehicle and vehi- control, air-conditioning, central locking ! At present, CAN can be used for the first three applications, but for diagnosis thepreferred solution is an interface accor-ding to ISO 9141.
Industrial applications of the CAN network
A comparison of the requirements for vehicle Users Group”, which in turn is a member of bus systems and industrial field bus systems the international users and manufacturers shows amazing similarities: low cost, oper- group ”CAN in Automation”. Similar require- ability in a harsh electrical environment, high ments to those of the textile machinery are to real-time capabilities and ease of use are be found in packaging machinery and machi- nery for paper manfacture and processing.
Benz‘s ”S” Class and the adoption of CAN by using CAN in production lines and machine tools as an internal bus system for networ- fast transmissions (up to 1 Mbit/s) has made king sensors and actuators within the line or industrial users prick up their ears. Not only manufacturers of mobile and stationary agri- medical engineering sector, decided in fa- cultural and nautical machinery and equip- vour of CAN because they had particularly stringent safety requirements. Similar pro- been the choice of manufacturers of medical blems are faced by other manufacturers of apparatus, textile machines, special-purpose machinery and equipment with particular re- machinery and elevator controls. The serial quirements with respect to safety (e.g. robots bus system is particularly well suited to net- working ”intelligend” I/O devices as well assensors and actuators within a machine or Apart from the high transmission reliability, the low connection costs per station are afurther decisive argument for CAN. In appli- The textile machinery industry is one of the cations where price is critical it is of essen- tial importance that CAN chips be available ped his looms with modular control systems from a variety of manufacturers. The com- pactness of other controller chips is also an works as early as 1990. In the meantime se- important argument, for instance in the held veral textile machinery manufacturers have joined together to form the ”CAN Textile How the CAN network functions
Principles of data exchange.
ready”). This is all the CPU has to do to initi-ate data exchange. The message is con- When data are transmitted by CAN, no sta- structed and transmitted by the CAN chip. As tions are addressed, but instead, the content soon as the CAN chip receives the bus allo- of the message (e.g. rpm or engine tempe- cation (”Send Message”) all other stations on rature) is designated by an identifier that is unique throughout the network. The identifier message (”Receive Message”). Each station defines not only the content but also the pri- ority of the message. This is important for message correctly, performs an acceptance test to determine whether the data received are relevant for that station (”Select”). If thedata are of significance for the station con- If the CPU of a given station wishes to send cerned they are processed (”Accept”), other- a message to one or more stations, it passes the data to be transmitted and their identi- A high degree of system and configuration flexibility is achieved as a result of the con- tent-oriented addressing scheme. It is very supports the concept of modular electronics easy to add stations to the existing CAN net- and also permits multiple reception (broad- work without making any hardware or soft- cast, multicast) and the synchronization of ware modifications to the existing stations, provided that the new stations are purely re- needed as information by several controllers ceivers. Because the data transmission pro- can be transmitted via the network, in such a tocol does not require physical destination way that it is unnecessary for each controller addresses for the individual components, it Broadcast transmission and acceptance filtering by CAN nodes Principle of non-destructive bitwise arbitration Non-destructive bitwise arbitration.
In real-time processing the urgency of mes-sages to be exchanged over the network can For the data to be processed in real time differ greatly: a rapidly changing dimension they must be transmitted rapidly. This not (e.g. engine load) has to be transmitted more only requires a physical data transfer path frequently and therefore with less delays with up to 1 Mbit/s but also calls for rapid bus than other dimensions (e.g. engine tempera- allocation when several stations wish to send ture) which change relatively slowly. The priority at which a message is transmitted compared with another less urgent message is specified by the identifier of the message concerned. The priorities are laid down du- ring system design in the form of correspon- dynamically. The identifier with the lowest bi- nary number has the highest priority.
Bus access conflicts are resolved by bitwise arbitration on the identifiers involved by each station observing the bus level bit for bit. In accordance with the ”wired and” mechanism, is no successful bus allocation. More than by which the dominant state (logical 0) over- writes the recessive state (logical 1), the order to allocate the bus at all, the num- competition for bus allocation is lost by all those stations with recessive transmission successful being a purely statistical quan- and dominant observation. All ”losers” auto- matically become receivers of the messagewith the highest priority and do not reattempt In order to process all transmission requests transmission until the bus is available again.
of a CAN network while complying with la-tency constraints at as low a data transferrate as possible, the CAN protocol must im- Efficiency of bus allocation.
plement a bus allocation method that gua-rantees that there is always unambiguous The efficiency of the bus allocation system is bus allocation even when there are simul- determined mainly by the possible applica- tion for a serial bus system. In order to judge as simply as possibly which bus systems aresuitable for which applications the literature The method of bitwise arbitration using the includes a method of classifying bus alloca- identifier of the messages to be transmitted tion procedures. Generally we distinguish uniquely resolves any collision between a number of stations wanting to transmit, andit does this at the latest within 13 (standard ! Allocation on a fixed time schedule. format) or 33 (extended format) bit periods for any bus access period. Unlike the mes- gardless of whether this participant needs thod of conflict resolution ensures that no bus capacity is used without transmittinguseful information.
! Bus allocation on the basis of need. The bus is allocated to one participant on Even in situations where the bus is over- loaded the linkage of the bus access priority standing, i.e. the allocation system only to the content of the message proves to be a considers participants wishing to transmit beneficial system attribute compared with master, round robin or bitwise arbitration).
spite of the insufficient bus transport capa- city, all outstanding transmission requests are processed in order of their importance to dure specified by CAN is classified as al- The available transmission capacity is uti- Another means of assessing the efficiency of lized efficiently for the transmission of useful bus arbitration systems is the bus access data since ”gaps” in bus allocation are kept very small. The collapse of the whole trans-mission system due to overload, as can With methods of this type the bus is allo- plementation of fast, traffic-dependent bus immediately or within a specified time fol- access which is non-destructive because of tralized bus control. All major communicationmechanisms, including bus access control, Non-destructive bus access can be further are implemented several times in the sys- tem, because this is the only way to fulfil thehigh requirements for the availability of the In summary it can be said that CAN imple- ments a traffic-dependent bus allocation sys- nisms are present in the system only once (centralized) or more than once (decentral- structive bus access with decentralized bus access control, a high useful data rate at thelowest possible bus data rate in terms of the bus busy rate for all stations. The efficiency station (inter alia for centralized bus access of the bus arbitration procedure is increased by the fact that the bus is utilized only by effect in the event of a failure of the master station. This concept has the disadvantage that the strategy for failure management isdifficult and costly to implement and also that These requests are handled in the order of the importance of the messages for the sys- tem as a whole. This proves especially ad- vantageous in overload situations.
Since bus access is prioritized on the basis For these reasons and to circumvent the pro- of the messages, it is possible to guarantee blem of the reliability of the master station low individual latency times in real-time sys- Message frame for standard format (CAN Specification 2.0A) Message frame formats.
extension) bit, which indicates either stan-dard format or extended format, a bit re- served for future extensions and - in the last frame formats, the only essential difference 4 bits - a count of the data bytes in the data being in the length of the identifier (ID). In the standard format the length of the ID is11 bits and in the extended format the length The ”data field” ranges from 0 to 8 bytes in is 29 bits. The message frame for transmit- length and is followed by the ”CRC field”, which is used as a frame security check for The ”ACK field”, comprises the ACK slot with the start bit ”start of frame”, this is (1 bit) and the ACK delimiter (1 recessive followed by the ”arbitration field”, which con- bit). The bit in the ACK slot is sent as a re- tains the identifier and the ”RTR” (remote cessive bit and is overwritten as a dominant transmission request) bit, which indicates bit by those receivers which have at this time whether it is a data frame or a request frame received the data correctly (positive acknow- without any data bytes (remote frame).
ledgement). Correct messages are acknow-ledged by the receivers regardless of the The ”control field” contains the IDE (identifier result of the acceptance test. The end of the message is indicated by ”end of frame”.
ciency in bit coding. The synchronisation ”Intermission” is the minimum number of bit edges are generated by means of bit stuf- periods separating consecutive messages. If fing, i.e. after five consecutive equal bits there is no following bus access by any sta- tion, the bus remains idle (”bus idle”).
stuff bit with the complementary value,which is removed by the receivers. Thecode check is limited to checking adher- Detecting and signalling errors.
Unlike other bus systems, the CAN protocol If one or more errors are discovered by at least one station (any station) using the but instead signals any errors that occur. For above mechanisms, the current transmission error detection the CAN protocol implements is aborted by sending an ”error flag”. This prevents other stations accepting the mes-sage and thus ensures the consistency of The CRC safeguards the information inthe frame by adding redundant check bits After transmission of an erroneous message has been aborted, the sender automatically re-attempts transmission (automatic repeat tested against the received bits. If they do request). There may again be competition for not agree there has been a CRC error.
bus allocation. As a rule, retransmission willbe begun within 23 bit periods after error de- tection; in special cases the system recovery the transmitted frame by checking the bitfields against the fixed format and the However effective and efficient the method described may be, in the event of a defective checks are designated ”format errors”.
station it might lead to all messages (inclu-ding correct ones) being aborted, thus blocking the bus system if no measures for self-monitoring were taken. The CAN proto- col therefore provides a mechanism for dis- tinguishing sporadic errors from permanent errors and localizing station failures (fault confinement). This is done by statistical as- sessment of station error situations with the aim of recognizing a station‘s own defects only by the recipients, that the ACK field negatively affected. This may go as far asthe station switching itself off to prevent mechanisms for error detection at the bit Data reliability of the CAN protocol.
The ability of the transmitter to detecterrors is based on the monitoring of bus The introduction of safety-related systems in automobiles brought with it high require- ments for the reliability of data transmission.
The objective is frequently formulated as not bit received. This permits reliable detec- permitting any dangerous situations for the tion of all global errors and errors local to driver to occur as a result of data exchange throughout the whole life of a vehicle.
This goal is achieved if the reliability of the The coding of the individual bits is tested data is sufficiently high or the residual error at bit level. The bit representation used probability is sufficiently low. In the context of bus systems data, reliability is understood as the capability to identify data corrupted by transmission faults. The residual error pro- corruption will remain undetected. The resi- bability is a statistical measure of the impair- dual error probability should be so small that ment of data reliability: it specifies the proba- on average no corrupted data will go unde- bility that data will be corrupted and that this tected throughout the whole life of a system.
Residual error probability as a function of bit error probability Calculation of the residual error probability For example, if a CAN network operates at a requires that the errors which occur be clas- data rate of 1 Mbit/s, at an average bus ca- sified and that the whole transmission path pacity utilization of 50 percent, for a total be described by a model. If we determine the residual error probability of CAN as a func- average message length of 80 bits, then the tion of the bit error probability for message lengths of 80 to 90 bits, for system configura- 9 x 1010. The statistical number of unde- tions of, for instance, five or ten nodes and tected transmission errors during the opera- with an error rate of 1/1000 (an error in one ting life is thus in the order of less than 10-2.
Or to put it another way, with an operating bit error probability is approximately 0.02 - in time of eight hours per day on 365 days per the order of 10-13. Based on this it is possible year and an error rate of 0.7 s, one unde- tectable errors for a diven CAN network.
Extended format CAN messages
extension (ID extension). Thus the CAN pro- standardized signals and messages as well tocol allows the use of two message formats: as data transmission protocols for various data rates. lt became apparent that stanard- ization of this kind is easier to implement have to coexist on one bus it is laid down when a longer identification field is available.
which message has higher priority on thebus in the case of bus access collisions with To support these efforts, the CAN protocol dithering formats and the same base identi- was extended by the introduction of a 29-bit identifier. This identifier is made up of the ex- priority over the message in extended for- isting 11-bit identifier (base ID) and an 18-bit CAN controllers which support the messages ing transmitted or whether a specific mes- sage is being requested from a station.
In place of the RTR bit in standard format the CAN controllers which only cover the stan- SRR (substitute remote request) bit is trans- dard format (Version 2.0A) are used on one mitted for frames with extended ID. The SRR network, then only messages in standard for- bit is always transmitted as recessive, to en- mat can be transmitted on the entire net- sure that in the case of arbitration the stan- dard frame always has priority bus allocation controllers which only support standard for- sages have the same base identifier.
Unlike the standard format, in the extended format and ignore them (Version 2.0B pas- format the IDE bit is followed by the 18-bit ID extension, the RTR bit and a reservedbit (r1). The distinction between standard format andextended format is made using the IDE bit All the following fields are identical with stan- (Identifier Extension Bit) which is transmitted dard format. Conformity between the two for- as dominant in the case of a frame in stan- dard format. For frames in extended format controllers which support the extended for- mat can also communicate in standard for- The RTR bit is transmitted dominant or re- Message frame for standard format (CAN Specification 2.0A) Implementations of the CAN protocol
Communication is identical for all implemen- ters allow a limited acceptance filtering tations of the CAN protocol. There are differ- (8 MSB of the identifier). Suitable choice of ences, however, with regard to the extent to these register values allows groups of identi- fiers or in borderline cases all ID‘s to be sage transmission from the microcontrollers selected. If more than the 8 ID-MSB‘s are necessary to differentiate between mes-sages then the microcontroller following theCAN controller in the circuit must comple- CAN controller with intermediate buffer.
ment acceptance filtering by software.
CAN controllers with intermediate buffer may CAN controllers with intermediate buffer (for- place a strain on the microcontroller with the acceptance filtering, but they require only a mented as hardware the logic necessary to small chip area and can therefore be pro- create and verify the bitstream according to duced at lower cost. In principle they can protocol. However, the administration of data accept all objects in a CAN network.
sets to be sent and received, acceptance fil-tering in particular is carried out to only alimited extent by the CAN controller.
CAN controller with object storage.
Typically, CAN controllers with intermediate CAN objects consist mainly of three compo- buffer have two reception and one transmis- nents: identifier, data length code and the sion buffer. The 8-bit code and mask regis- CAN controllers with object storage (formerly tion to this, they can only administer a limited called fullCAN) function like CAN controllers with intermediate buffers, but also administercertain objects. Where there are several si- multaneous requests they determine, for ex- combine both principles of implementation.
ample, which object is to be transmitted first.
They have object storage, at least one of They also carry out acceptance filtering for which is designed as an intermediate buffer.
incoming objects. The interface to the fol- For this reason there is no longer any point RAM. Data to be transmitted are written into the appropriate RAM area, data received areread out correspondingly. The microcontrol-ler has to administer only a few bits (e.g.
CAN slave controllers for I/O functions.
As well as CAN controllers which support all CAN controllers with object storage are de- functions of the CAN protocol there are also signed to take as much strain as possible off CAN chips which do not require a following the local microcontroller. These CAN control- microcontroller. These CAN chips are called lers require a greater chip area, however, SLIO (serial link I/O). CAN chips are CAN and are therefore more expensive. In addi- slaves and have to be administered by aCAN master. Physical CAN connection
The data rates (up to 1 Mbit/s) necessitate a Integrated driver chips in accordance with sufficiently steep pulse slope, which can be ISO 11898 are available from several com- implemented only by using power elements.
panies (Bosch, Philips, Siliconix and Texas Instruments). The international users and manufacturers group (CiA) also specifies se- manufacturers group CAN in Automation re- commends the use of driver circuits in ac- Physical CAN Connection according to ISO 11898 Source Proof
This text was kindly provided for us by theusers and manufacturers organisation CiA(CAN in Automation e.V.).
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Source: http://www.poli.usp.br/d/pmr2410/CAN_intro.pdf