Introduction to the Controller Area Network (CAN)

1 Application ReportSLOA101 - August 20021 Introduction to the Controller area Network (CAN)Steve CorriganHPLABSTRACTA Controller area Network (CAN) is ideally suited to the many high-level industrial protocolsembracing CAN and ISO 11898 as their physical layer. Its cost, performance, andupgradeability provide for tremendous flexibility in system design. This paper presents a briefintroduction to the CAN operating principles, the implementation of a basic CAN bus usingTexas Instrument s CAN transceivers and DSPs, and a discussion of the robust errordetection and fault confinement mechanisms. Some of the properties of CAN, especiallyrelating to the electrical layer and features of transceiver products, are then discussed at atutorial.

2 2 The CAN Standard3.. 3 Standard CAN or Extended CAN4.. The Bit Fields of Standard CAN and Extended CAN4.. Standard CAN4.. Extended CAN5.. 4A CAN Message6.. Arbitration6.. Message Types7.. The Data Frame7.. The Remote Frame8.. The Error Frame8.. The Overload Frame8.. Error Checking and Fault Confinement8.. 5 The CAN Bus9.. CAN Transceiver Features12.. Supply Voltage12.. High Short-Circuit Protection12.. High ESD Protection12.. Wide Common-Mode Range13.. Common-Mode Rejection13.. High Input Impedance14.. Controlled Driver Output Transition Times14.

3 Low Current Standby and Sleep Modes14.. Thermal Shutdown Protection15.. Glitch Free Power Up and Power Down15.. Trademarks are the property of their respective to the Controller area Network (CAN) Unpowered Node Does Not Disturb the Bus15.. The Relationship Between Bus Length and Signaling Rate15.. 6 Conclusion16.. 7 Additional Reading16.. List of Figures1 The Layered ISO 11898 Standard Architecture3.. 2 Standard CAN: 11-Bit Identifier4.. 3 Extended CAN: 29-Bit Identifier5.. 4 The Inverted Logic of a CAN Bus6.. 5 Arbitration on a CAN Bus7.. 6 Details of an Electronic Control Unit9.. 7 CAN Dominant and Recessive Bus States of the SN65 HVD23010.

4 8 CAN Bus Traffic11.. 9 CAN Test Bus11.. 10 CAN Transceiver Power Savings12.. 11 Common-Mode Noise Coupled onto 4 Twisted-Pair Bus Lines14.. List of Tables1 CAN Versions4.. 2 Maximum Signaling Rates for Various Cable Lengths15.. SLOA1013 Introduction to the Controller area Network (CAN)1 IntroductionThe CAN bus was developed by BOSCH1 as a multi-master, message broadcast system thatspecifies a maximum signaling rate of 1M bit per second (bps). Unlike a traditional Network suchas USB or Ethernet, CAN does not send large blocks of data point-to-point from node A to nodeB under the supervision of a central bus master.

5 In a CAN Network many short messages liketemperature or RPM are broadcast to the entire Network , which allows for data consistency inevery node of the application report explains the CAN message format, message identifiers, and bit-wisearbitration a major benefit of the CAN signaling scheme. CAN bus implementation is examinedand typical waveforms are CAN StandardCAN is an International Standardization Organization (ISO) defined serial communications busoriginally developed for the automotive industry to replace the complex wiring harness with atwo-wire bus. The specification calls for signaling rates up to 1 Mbps, high immunity to electricalinterference, and an ability to self-diagnose and repair data errors.

6 These features have led toCAN s popularity in a variety of industries including automotive, marine, medical, manufacturing,and CAN communications protocol, ISO 11898, describes how information is passed betweendevices on a Network , and conforms to the Open Systems Interconnection (OSI) model that isdefined in terms of layers. Actual communication between devices connected by the physicalmedium is defined by the physical layer of the model. The ISO 11898 architecture defines thelowest two layers of the seven layer OSI/ISO model as the data-link layer and physical layer inFigure LayerData-LinkLayerPhysicalLayerLogic Link ControlMedium Access ControlPhysical SignalingPhysical Medium AttchmentMedium Dependant InterfaceDSPor ControllerCANT ransceiverCAN Bus-LineCAN Controller ,Embedded or SeparateEmbeddedCANC ontrollerElectrical Specifications:Transceivers, Connectors,CableISO 11898 SpecificationImplementationFigure 1.

7 The Layered ISO 11898 Standard Architecture1 Robert Bosch GmbH, to the Controller area Network (CAN)In Figure 1, the application layer establishes the communication link to an upper-levelapplication specific protocol such as the vendor independent CANopen protocol. This protocol issupported by the international users and manufacturers group, CAN in Automation (CiA).Additional CAN information is located at the CiA website, There are many similaremerging protocols dedicated to particular applications like industrial automation or of industry-standard CAN-based protocols are KVASER s CAN Kingdom,Allen-Bradley s DeviceNet and Honeywell s Smart Distributed System (SDS ).

8 3 Standard CAN or Extended CANThe CAN communication protocol is a carrier-sense multiple-access protocol with collisiondetection and arbitration on message priority (CSMA/CD+AMP). CSMA means that each nodeon a bus must wait for a prescribed period of inactivity before attempting to send a +AMP means that collisions are resolved through a bit-wise arbitration, based upon apreprogrammed priority of each message in the identifier field of a message. The higher priorityidentifier always wins bus first version of the CAN standards listed in Table 1, ISO 11519 (Low-Speed CAN) is forapplications up to 125 kbps with a standard 11-bit identifier.

9 The second version, ISO 11898(1993), also with 11-bit identifiers provides for signaling rates from 125 kbps to 1 Mbps while themore recent ISO 11898 amendment (1995) introduces the extended 29-bit identifier. The ISO11898 11-bit version is often referred to as Standard CAN Version , while the ISO 11898amendment is referred to as Extended CAN Version The Standard CAN 11-bit identifierfield in Figure 2 provides for 211, or 2048 different message identifiers, while the Extended CAN29-bit identifier in Figure 3 provides for 229, or 537 million 1. CAN VersionsNOMENCLATURESTANDARDMAX. SIGNALING RATEIDENTIFIERLow Speed CANISO 11519125 kbps11-bitCAN 11898:19931 Mbps11-bitCAN 11898:19951 Bit Fields of Standard CAN and Extended CANSOF11-Bit IdentifierRTRIDEr0 DLC0.

10 8 Bytes DataCRCACKEOFIFSF igure 2. Standard CAN: 11-Bit IdentifierThe meaning of the bit fields of Figure 2 are: SOF The single dominant start of frame (SOF) bit marks the start of a message, and isused to synchronize the nodes on a bus after being idle. Identifier The Standard CAN 11-bit identifier establishes the priority of the message. Thelower the binary value, the higher its Introduction to the Controller area Network (CAN) RTR The single remote transmission request (RTR) bit is dominant when information isrequired from another node. All nodes receive the request, but the identifier determines thespecified node.