Understanding Automotive Electronics 8th - Chapter 9

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Communication with vehicles (while moving) began with the introduction of AM radio receivers in the 1930s. In this same general era, two-way radio communication via radio was used by law enforcement agencies. The evolution of civilian two-way radio communications advanced relatively slowly until the
N463 Flat panel display Audio output Cellular Driver KB system pnone nterfac Entertainment/ Comfort Instrumentation communications Digital data link manage Powertrain Tri control computer contro Engine/transmission ABS/ Electronic traction steering FIG 9. 1 Representative in-vehicle communication topology measurement is potentially available for use in optimizing performance. Furthermore, redundant sen sors for measuring primary variables can be eliminated by an integrated electronics system for the ve hicle. For example, wheel speed measurements are primary variables for ABS systems and are also useful in electronic transmission control, vehicle speed display, and others and are useful for such ap- plications as trip computer or fuel range. Any networking for IVC involves digital communication systems having both a specific hardware requirement and the software for controlling this communication The physical link connecting vehicular electronic systems can have many forms. This link is termed a bus or, often, a medium In addition to the physical bus, there is also a specific set of requirements for the format of any message that is sent along the bus. This format and all specifications for it are termed the protocol for the system There are several Ivn configurations and protocols for application in land vehicles. Each has a very recise set of specifications for the hardware and message format In this chapter a number of IvN systems are discussed and compared with respect to performance and cost There are several components that are common to all types of IvNs. The bus for each is either a wire or a pair of wires or optical cable that passes through the vehicle close enough to all relevant electronic systems/subsystems to connect to them In addition there must be an interface between the bus and each device capable of receiving or transmitting messages to the device. In certain cases, there is a separate digital bus controller that determines how each device is enabled for sending a message 464 CHAPTER 9 VEHICLE COMMUNICATIONS There are two major strategies for determining individual device access to the bus for sending messages:(1) time-division multiplexing in which each device has a specific time interval in a com plete cycle or(2)event-driven access in which a device transmits a message when a specific event occurs In this latter case, it will frequently occur that two or more devices transmit simultaneously such that their messages overlap. When this occurs, the bus controller (if one exists) determines priority and sends commands for the devices to repeat sending the event-driven messages. In the absence of a bus controller some form of network arbitration is required for determining the priority of the use of the ivn whenever there is conflict between subsystems for its use. This arbitration feature can be handled by the system manager subsystem(see Fig 9. 1) if one exists in the system or is automatic in other cases One of the characteristics of an ivn is that messages are sent in a serial mode each message is digital and consists of multiple fields. One field is an identifier, another field contains data, and the remaining fields are IVN-specific and are discussed below for each ivn presented in this chapter CAN We begin with an ivn that is called controller area network(CAN). This IvN was developed for vehicle use in the 1980s and has broad application in automotive systems including power train, suspension, and braking systems among other vehicle model-specific systems Essentially, CAN IVN provides a sophisticated communication system between various subsys- tems. Among the issues of importance for such a communication system are the protocol and message format. It is highly advantageous to have a standard protocol for all automobiles. The Society of Automotive Engineers (SAE) has developed a standard specification for Can. The CAN IVN operates asynchronously at a data rate of up to 1 mbps for a distance of 40 m The basic message structure is derived assuming that the majority of data on the link are regularly sent. This means that the content of each message is known(only the actual data vary). The standards and specifications for the Can network are given in a document published by Sae, which is given the designation(in the latest version at the time of this writing )J-2284-3 In the can concept, each electronic subsystem that is connected to the can(called ECU in J-2284-3)incorporates communication hardware and software, permitting it to function as a commu nication module referred to as a gateway can is based on the so-called broadcast communication mechanism in which communication is achieved by the sending gateway(which we call a transceiver) transmitting messages over the network(e. g, wire interconnect). Each message has a specific format (protocol) that includes a message identifier. The identifier defines the content of the message, its priority, and is unique within the network. In addition to the data and identifier, each message includes error-checking bits(e. g, cyclic redundancy check CrC) and beginning and end-of-file bits. In the most recent version of J-2284-3, the message identifier is 29 bits The Can communication system has great flexibility, permitting new subsystems to be added to an existing system without modification, provided the new additions are all receivers. Each system connected to Can may be upgraded with new hardware and software at any time with equipment that was not available at the time the car left the manufacturing plant or even when it left the dealer. Essentially, the Can concept with its open architecture frees the development of new telematics ap plications from the somewhat lengthy development cycle of a typical automobile model with the help of AUTOSAR. Furthermore, it offers the potential for the aftermarket addition of new subsystems CAN 465 The SaeJ-2284-3 standard is a recommended practice document(one of many published by sae that defines the can protocol in terms of its physical layer and portions of the data link layer, message format, etc. It primarily focuses on a minimum standard level of performance from the CAN IVN implementation by any automotive manufacturer. All of the ECUs associated media are to be designed to meet component level requirements. By meeting component level requirements, the system level performance requirements are assured Physically, the can consists of a twisted pair of wires can h and can l whose voltages are specified by a pair of states: (1)dominant state and(2)recessive state. The Can_h bus wire is fixed to a mean voltage level during the recessive state and is driven positive during the dominant bit state The Can_l bus wire is fixed to a mean voltage level during the recessive state and driven in the negative voltage direction during dominant bit state The recessive state is represented by an inactive state differential voltage(v dift) between CaN_H and can L that is approximately o. the recessive state represents a logic 1-bit value. The dominant state is represented by a differential voltage between CAN_H and can_l greater than a minimum threshold value. The dominant state overwrites the recessive state and represents a logic 0-bit value These voltages are depicted in Fig. 9.2 Fig. 9.2A Shows the individual voltages on the two lines. The differential voltage is depicted in Fig. 9.2C. The rejection of EMI is shown in Fig 9.2B in which the differential voltage V is unaffected by EMi, which changes both Can_H and can_l by the same amount. The Sae J-2284-3 standard gives a number of definitions of terms by which the can ivn can be understood. The term "media refers to the physical structure/ configuration that conveys the electrical transmission between ECUs on the network and as stated above is an unshielded twisted pair of wires The term physical layer"refers to the transmission of a bit stream over the physical media and deals with electrical, mechanical, functional, and procedural characteristics to access the physical media. The term"protocol refers to a set of conventions for the exchange of information between ECUs on the CAN. It includes the specification of frame administration, frame transfer, and the physical layer In this context, the frame is the formal arrangement of the sequence of bits over a specified time interval that constitutes the message The message format includes a message identifier(formerly 11 bits but later 29 bits). The actual encoding of the identifier is manufacturer-specific. The identifier defines the content of the message and its priority. The message also includes a field for the information being sent in the form of 8 data bytes. A set of error-checking bits is also included that might be of the form of"check sum of the bits The can is capable of supporting data transfer between ecus from a minimum of two to a maximum of 24. The topology of the can is depicted in Fig. 9.3, which illustrates a Can with N ECUS The configuration of the Can shown in Fig. 9.3 includes a connection to an off-board diagnostics tool (ECUN-2)via a data link connector DLC). Each ECU is connected to the can via a stub whose length(L1) must satisfy 0<L1<1m The stub length to the dlC L2 has the same requirement as L1. The off-board stub length(L3)must satisfy 0<L3 m 466 CHAPTER 9 VEHICLE COMMUNICATIONS min. 1 us Voltage 5V CAN H 3.5V 2.5∨ CAN L 1.5V 0V Dominant Time Recessive Dominant Recessive (A) CAN H (B) CAN L SV (C) FIG9.2 CAN voltage levels. (A)CAN voltages, b)individual can voltages with external interference, and ( c)cAN differential voltage off-board ECU (N-2) ECU2 ECU3 ECU stub d DLC (N-1) ECU1 ECU 尺 can bUs N FIG 9. 3 can bus architecture CAN 467 The distance between and two ECUs including cable stubs(d)must satisfy 0.1<d<33m The Can must be terminated at either end with a resistance r. that has tolerance range 118<RL<132 The nominal value for R is 120 Q2. This resistance is connected between can H and can L wires In addition each ecu must present no more than 100 pf capacitance to ground and no more than 50 pF differential The physical media parameters for an unshielded twisted pair are also given in SAe J-2284-3 The characteristic impedance of the twisted pair zo must satisfy 108≤20<1329 The resistance/unit length R,must be less than 0.070 Q2 /m. The propagation delay for the media must be less than 5. 5 ns/m. The basic can bit time requirements are a critical specification In Sae j-2284-3 the bit time(tbit must satisfy 1990< tbit <2010 ns. A further constraint is that the nominal bit time must be a programmable multiple of the system clock period For precise timing details, the reader is referred tO sae j-2284-3 The sae J-2294-3 also has specific requirements concerning electromagnetic compatibility The electromagnetic radiation from the can and susceptibility to interference from other Can electronic/electrical systems is specified in the sae J-2284-3 standard It is typical of SAE standard documents (including J-2284-3)that they evolve over time to accommodate technology advances and changes resulting, for example, from government-mandated regulatory changes. Regardless of such evolution, the basic concepts for the can network will remain the same The interface electronic block diagram is depicted in Fig. 9.4. In this figure, the Can transceiver and controller are commercially available chips. The microcontroller refers to the ecu3 depicted in Fig 9.3 and controls the vehicular electronic system connected to the can bus as shown above. also shown in Fig. 9.4 are the 120 Q2 bus termination resistors(denoted rT) CAN BUS TRANSCEIVER The can bus transceiver is a commercially available integrated circuit that handles the exchange of data between modules that are connected to the bus It has a pair of terminals with one connected to the CAN_ Hline and the other to the Can_L line. It is capable of both receiving and transmitting data along the can bus. For an understanding of its operation, it is helpful to refer to Fig. 9.5, which depicts the CAN_H and can_l voltage waveforms associated with a pair of modules si and s2 In this figure, the time axis shows that successive bit times are alternately recessive or dominant. For convenience, each bit time is depicted as I us duration. The graphs labeled Txn or rxn are the logic levels for transceiver in depicting what that transceiver is sending onto the bus (i. e, Txn)and receiving from the bus(i.e, Rxn). These two line voltages are shown together in the graph whose ordinate is abeled vcan. The voltage on Can_H is denoted as vch and is given by the solid line, and on can_l, the voltage is denoted as Vcl and is given by the dashed line. During the recessive time bits, the two voltages are approximately equal recessive v V CC 468 CHAPTER 9 VEHICLE COMMUNICATIONS Microcontroller CAN controller TXO X1 RXO Rx1 7s0 Rso Rs1R 5V CAN transceiver 100nl CAN L CAN H Bus termination Bus terminatio CAN H R T can bus lines T CAN L FIG9.4 CAN interface block diagram during the dominant time bit when a device is transmitting the two voltages are given by dominant Vcc sV 22 The differential voltage(V diff) during the active dominant time bit is given by diff=VCH-VCL SV An international standard calls for the minimum value of sv to be 1.5 V. during recessive bits both Txn and rxn are logic high. For the two modules whose voltages are depicted in Fig. 9.5, the receiver logical states(RxI and rx2) during the first dominant time bit are both low. Each transceiver generates the corresponding electrical signal in response to the VCH and Vcl during dominant time bits Control of the transceiver is done by a microprocessor-based subsystem(or IC). There are commer cially available integrated circuit can bus control devices. The IC manufacturer normally also has available development system that permits the user to program it to fit the IvN requirements However, it is also possible to implement this control as a part of the vehicle system controller CAN 469 Tx X2 δv/2 δv/2 CAN v Recessive Dominant Recessive Dominant tu(s) FIG.9.5 CAN voltage waveforms. CAN ELECTRONIC CIRCUITS Fig 9.6 depicts a representative circuit for a Can transceiver. The upper portion of Fig. 9.6 containing two FETS is representative of the transmit circuitry, and the lower portion of Fig. 9.6 with operational amplifier(op-amp) is representative of the receiver circuitry. In an ideal case during the recessive state, the voltages on both Can wires would be Vcc/2 with V diff =0. In the dominant state Vdiff=dV. In this case, the dominant state would correspond to V diff>0. However, in practice with multiple nodes on a can network there can be times when a small difference vdiff exists in a recessive state due to small fluctuations and small but nonzero interference/noise In practical CAN applications, it is helpful to require V diff to exceed a threshold V th to correctly identify dominant state and to minimize the probability of errors in the two states. In the representative receiver circuit of Fig. 9.6, this goal is achieved with the use of a zener(z) diode in the output of an op-amp comparator circuit connected to Can_H and CAN_L. It can be shown with reference to the discussion of operational amplifier circuits from Chapter 2 that voltage vo is given by +1 R Rs+R R with r chosen to be RI=R/R 470 CHAPTER 9 VEHICLE COMMUNICATIONS CC CAN H CAN L C H FET H Transmit (dominant time) C L R R Op-amp R、 √z1>R R3 FIG. 9.6 Illustrative cAn transceiver circuitry Vo is given by V=GV where G=R/R The voltage VR across the resistance R3 connected between the zener anode and ground is given by R=0 Vo<V GVad-VzVo≥vz where vz= zener voltage The threshold voltage for correctly detecting dominant state and not incorrectly detecting recessive state is determined by the parameters G and vz. For example, if the threshold voltage was chosen to be

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