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精密實驗室系列:CAN、LIN 收發器及 SBC

TI 精密實驗室 (TIPL) 線上教室內容完善,是類比訊號鏈設計人員的不二之選。此課程系列為工業、汽車和許多其他應用中的常用協定提供了技術訓練。啓動 CAN 和 LIN 系列以了解 CAN 和 LIN 標準,包括其實體層電氣訊號特性,以及框架結構和資料通訊協定。

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      Hello. And welcome to the TI Precision Lab on controller area network communication, usually referred to as CAN communication. This video will introduce the method that CAN transceivers use to communicate across a CAN bus, particularly in automotive applications. This introductory video will include an overview of CAN, an introduction to the standard, a look at the physical layer, and a discussion of different CAN protocols.

      This is an example of a typical automotive body network. The central body control or any similar master device in an automobile needs to communicate with other devices in the car, devices that are frequently located far from the master itself, in order to complete tasks. The CAN bus shown here in blue allows components to talk to each other seamlessly in the automobile. The LIN bus shown in yellow allows for further expansion to peripheral devices.

      Using this bus network, the climate control system can, for example, interface with the power seat systems to activate the seat warming heaters, or the control unit can activate the brake lights when the brake pedal is depressed. This bus hierarchy was designed to save costs in wiring. Wiring is one of the most expensive components in a car. It is heavy and often assembled by hand. So wiring has a direct impact on an automobile's overall cost of production. Automobile designers use CAN to minimize wiring in the car, reducing manufacturing cost and improving fuel efficiency.

      CAN is the main bus used in today's automobiles. It allows different modules, like power seats, door modules, and climate control units to talk to each other. CAN with flexible data rate or CAN FD is the most popular method of communication over this bus. CAN and CAN FD implement a differential two-wire interface. Classical CAN operates up to 1 megabit per second while CAN FD operates up to 5 megabits per second. LIN is a sub bus that operates at lower data rates.

      The CAN standard defines both a protocol and a physical layer for asynchronous serial communication in multipoint bus applications. Each node consists of a CAN transceiver and a CAN controller in an MCU. Communication occurs over a differential bus at up to 1 megabit per second for classical CAN or 5 megabits per second for CAN with flexible data rate. CAN is designed to be used with twisted pair cabling. The network should be wired in a bus topology, limiting stubs as much as possible.

      The bus for CAN and CAN FD should be properly terminated at both ends with resistors that match the impedance of the network. Proper termination helps reduce signal integrity issues like reflections. If nodes may be removed from the bus, designers must use care when choosing where to place the termination resistors. Termination can be accomplished in different ways. Standard termination uses a single termination resistor at each end of the bus.

      An alternative method is split termination, shown on the right, which improves signal integrity and electromagnetic emission characteristics. It can also eliminate fluctuations in the bus common mode voltage levels while allowing differential fluctuations to remain. CAN's bus topology with twisted pair cabling usually has a 120 ohm characteristic impedance. So proper termination usually involves using a 120 ohm termination resistor.

      Since CAN networks connect CANH and CANL to power sources like VCC and ground, the power ratings of the termination resistors should take into account the short circuit overcurrent protection of the CAN transceivers in the network. CAN is a two-wire differential signal. The differential voltage, VD, is the difference between the high side signal, CANH, and the low side signal CANL. A value of logic 1 is represented by a low VD, referred to as a recessive state. And a value of logic 0 is represented by a high VD, referred to as a dominant state.

      Drivers can actively pull the bus to the dominant state, but the bus can only return to a recessive state passively via dissipation across the termination resistors when no driver is driving a dominant signal. We will discuss the characteristics of dominant and recessive states and their impact on message arbitration in our TI Precision Lab titled CAN Physical Layer and Hardware and our TI Precision Lab titled CAN Protocol and CAN FD.

      Automotive CAN interfaces are dictated by the ISO11898 standard, which includes six parts. Each part addresses some specific aspect of CAN shown here. Part two, for example, is referred to as ISO11898-2 and covers the CAN physical layer for high speed CAN. Part five adds low power mode requirements to the ones outlined in part two. Partial networking requirements are outlined in part six.

      Naming conventions used in the ISO11898 standard are similar to the terminologies frequently used by car makers. Many times, car makers will use terms like low speed, medium speed, and high speed to describe CAN data rates. These names can be relatively arbitrary and can even vary between different car makers. These names can understandably generate confusion, since the ISO Standard uses similar nomenclature.

      Low speed fault tolerant CAN, called LSFT CAN, is defined by part three of ISO11898 and operates up to 125 kilobits per second, a speed similar to the automotive definition of low speed CAN shown above. For all three of these carmaker data rate definitions, however, most carmakers are actually implementing high speed CAN, or HS CAN, which usually operates at speeds up to 1 megabit per second, and is defined by parts 2, 5, and 6 of ISO11898.

      Note that high speed CAN fully overlaps the operating range of low speed fault tolerant CAN. CAN transceivers are bounded on the low end to around 10 to 40 kilobits per second by TXD and RXD time out times. Currently, many automobiles use classical high speed or HS CAN. CAN with flexible data rate or CAN FD is an enhancement to classical CAN, which increases the usable bandwidth to as high as 5 megabits per second.

      There are many additional higher layer CAN based standards and protocols, which govern applications, including aeronautical, agricultural, embedded control, industrial automation, military, marine, and safety critical applications. These standards build upon the ISO11898 standards outlined in this presentation, and many have additional test standards that govern electromagnetic compatibility and electrostatic discharge requirements.

      To find more CAN and CAN FD technical resources and to search for CAN and CAN FD products, visit ti.com/CAN. Also, be sure to check out our other TI Precision Labs videos for CAN, LIN, and SBC. Thank you for watching.

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      Learn how to design with CAN/LIN transceivers and multi-function system basis chips (SBCs). (4)
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      精密實驗室系列:CAN、LIN 收發器及 SBC

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      Learn how to design with CAN/LIN transceivers and multi-function system basis chips (SBCs). (4)