The Local Interconnect Network (LIN) consortium developed a standard for a serial, low-cost communication concept, in conjunction with a development environment, that enables the car manufacturers and their suppliers to create, implement and handle complex hierarchical multiplex systems in a very cost-effective manner. The LIN specification covers the transmission protocol, the transmission medium, the interfaces for development tools and application software. LIN supports the interoperability of network nodes from the viewpoint of hardware and software, with predictable EMC behavior.
With lower system cost, lower power consumption, weight reduction and faster time to market for innovative electronic solutions being among the many challenges facing the embedded designers in the automotive industry, LIN is being implemented in vehicles around the world. LIN defines a low-cost, serial communication system for distributed electronic systems, such as window controls, seat movement, mirror positioning, LED lighting and other body oriented applications. LIN advances basic networking capability to subsystems that have historically been considered uneconomical for smart actuators and controls. LIN complements the existing portfolio of automotive multiplex networks led by CAN. By 2010, it is estimated that an average of 20 LIN nodes per vehicle will exist, making LIN nodes the second largest number of networked nodes within the vehicle (after CAN).
Protocol Concept
LIN is a holistic communication concept for local interconnect networks in vehicles. The specification covers the definition of the protocol and the physical layer, as well as the definition of interfaces for development tools and application software. LIN enables a cost-effective communication network for switches, smart sensors and actuator applications within the vehicle where the bandwidth and versatility of CAN is not required. The communication protocol is based on the SCI (UART) data format, consisting of a single-master/multiple-slave concept, a single-wire 12V bus, and clock synchronization for nodes without a stabilized time base. This integration concept of communications and development environment allows the implementation of a seamless chain of development and design tools, while enhancing the speed of development and the reliability of the network.
LIN Applications
Body-control electronics improve the comfort and safety of vehicle occupants. Innovative body-control electronics support the ability of car manufacturers to produce smarter vehicles that are more pleasing to drive, more reliable and safer. Body-control electronics improve the safety of the driver by simplifying the operation of the vehicle and releasing the driver from the distractions of secondary activities.
As noted in Figure 1, typical applications for the LIN bus are assembly units such as doors, steering wheels, seats, motors and sensors in climate control, lighting, rain sensors, smart wipers, intelligent alternators, switch panels or RF receivers. These LIN-enabled applications are easily connected to the larger car network, and become accessible to all types of diagnostics and services. The commonly used analog coding of signals is being replaced by digital signals, leading to an optimized wiring harness.
Generally, actuators and sensors are hardwired, with one electronic control unit (ECU) utilizing CAN connectivity in a centralized body-control system. One ECU exchanges signals via a CAN link with other main ECUs. Hardwiring is chosen if the local actuators and sensors require a high computing performance. In systems where the local performance can be low, an alternative distributed system based on smart actuators and sensors can be used. This partitioning is chosen in order to achieve a scalable system architecture with universally applicable components.
This partitioned architecture is cost effective if the additional cost for the local intelligence and networking can be offset by cost savings in production and development, due to a lower variety of electronic components. The key enablers for this type of architecture are the sub-bus LIN standard.
The Key Features of LIN are:
- Low-cost, single-wire implementation
- Enhanced ISO 9141, VBAT-Based
- Speeds up to 20 Kbit/s (limited for EMC reasons)
- Single Master / Multiple Slave Concept
- No arbitration required
- Low-cost silicon implementation based on common UART/SCI interface hardware
- Self synchronization in the slave nodes without crystal or ceramics resonator
- Significant cost reduction of hardware platform
- Guaranteed latency times for signal transmission
With LIN residing in low-end applications, two factors are extremely critical: (a) the communication cost per node must be significantly lower compared with CAN; and (b) the performance, bandwidth and versatility of CAN are not required.
The main cost savings of LIN versus CAN are derived from: (1) the single-wire transmission, (2) the low cost of implementation as hardware or software in silicon, and (3) the avoidance of crystals or ceramic resonators in slave nodes. These advantages are compromised by a lower bandwidth and the restrictive single-master bus access scheme.
Lighting performance and aesthetics of a vehicle are improved with LEDs. Ambient lighting is an application area that is benefitting from the cost effectiveness of the LIN network. Additionally, ambient lighting placed in strategic areas within the vehicle has the ability to create a mood within the car as well as outside of the car. The lower costs of LEDs as well as the embedded ICs that control them lead to usage in a variety of areas, such as the cup holder, door sills or footwells, since the LED technology enables the deployment while still satisfying space and power budgets.
Microchip Technology’s RGB LED Reference Design, part number APG000027, shows how to create an interior ambient-lighting module that controls remote RGB LED devices residing on a LIN bus, while supporting communication with a central Body Control Module. The module features include:
· Multi-color mixing from 7 to 16,383 colors
· 1,023 levels of intensity
· Constant voltage / current drive
· LIN 2.0 / J2602 bus slave capability
LIN Communication Basics
A LIN network comprises one master node and one or more slave nodes (see Figure 2). All nodes include a slave communication task that is split into a transmit and a receive task, while the master node includes an additional master transmit task. The communication in an active LIN network is always initiated by the master task.
A particular feature of LIN is its synchronization mechanism, which allows the clock recovery by slave nodes without the need for a crystal or ceramic resonator. The specification of the line driver and receiver follows the ISO 9141 single-wire standard with some enhancements. The maximum transmission speed is 20 Kbit/s, resulting from the requirements of EMC and clock synchronization.
A node in a LIN network does not make use of any information about the system configuration, except for the denomination of the master node. Nodes can be added to the LIN network without requiring hardware or software changes in other slave nodes. The size of a LIN network is typically under 12 nodes (though not restricted to this), resulting from the small number of 64 identifiers and its relatively low transmission speed. The clock synchronization, the simplicity of UART communication and the single-wire medium are the major factors for the cost efficiency of LIN.
The LIN master node sends out a message header that comprises the synchronization break, the synchronization byte and the message identifier. Exactly one slave task is activated upon reception and filtering of the identifier, and starts the transmission of the message response. The response comprises two, four or eight data bytes, and one checksum byte. The header and the response form one message frame.
The identifier of a message denotes the content of a message, but not its destination. This communication concept enables the exchange of data in various ways: from the master node (using its slave task) to one or more slave nodes, and from one slave node to the master node and/or other slave nodes. It is possible to send signals directly from slave to slave, without the need for routing through the master node or broadcasting messages from the master to all nodes in a network. The sequence of message frames is controlled by the master. The number, sequence and frequency of messages in the scheduling frame of the master determine, along with the baud rate, the system response time and time behavior. Careful system design is necessary since if the master missed a slave message, this message would reach the master earliest at the next schedule sequence due to the master-slave concept.
The LIN protocol provides a dedicated synchronization pattern with the start of every message frame that allows slave nodes without a crystal or ceramic resonator to synchronize their local time base to that of the master.
LIN Physical Layer
The LIN bus is a single-wire bus that is supplied via a termination resistor from the positive battery node Vbat. The bus-line transceiver is an enhanced implementation of the ISO 9141 standard. The bus can take two complementary logical levels: the dominant value with an electrical voltage close to ground represents a logical ‘0’, and the recessive value with an electrical voltage close to the battery supply voltage represents a logical ‘1’.
The bus is terminated with a pull-up resistance of 1 kOhm in the master and 30 kOhm in a slave. The termination capacitance is typically 220 pF in the slave nodes. The capacitance in the master node is higher, in order to make the total line capacitance less dependent on the number of slave nodes.
Significant electrical parameters of the LIN physical layer are listed in Table 1.
The LIN physical layer specification puts high performance requirements on the transceiver. The switching of the transceiver must not disturb other electronic components. Special care has to be taken to meet the EMC requirements of the car OEMs. Wave shaping or edge rounding is applied to minimize radiated emissions of the transceiver. For example, the MCP202X LIN/SAE J2602 transceivers from Microchip Technology have proven robustness and world-class ESD performance to enable reliable communication in even harsh environments, with minimal or no external components required. In fact, their industry-leading low emissions eliminate the need for external shielding, which is of particular interest to noise-sensitive systems such as car radios, in-car entertainment systems, GPS navigation, mirrors, steering-wheel control and garage-door openers.
Summary
From the release of the LIN 1.3 specification in December 2002 to the release of LIN 2.1 in November 2006 and SAE J2602 in August 2004, LIN has gained significant global interest beyond it initial roots in the European market.
Factors contributing to the success story of the LIN Standard include:
· LIN being introduced during a period where electronic content in cars is rapidly expanding, since many of today’s innovations in cars are enabled through the utilization of advancing electronic concepts. Consumer demand for more comfort and safety in the vehicle has fueled this market development. Regulatory guidelines enforce this trend. At the same time, car makers are demanding cost-effective implementations from their suppliers.
· LIN being a cost effective bus concept for sub-networks in cars is essential. LIN helps to optimize system cost and increases system efficiency. This open serial-bus standard for cost-efficient communication was developed that ensured the simultaneous availability of a consistent tools concept and appropriate software interfaces.
· The concept of the LIN consortium motivated many semiconductor suppliers to develop a wide range of semiconductor solutions. LIN Bus Standard 2.1 addresses standardized tests to ensure interoperability of solutions developed by semiconductor vendors and system suppliers. The Society of Automotive Engineers (SAE) initiated the task force J2602 to develop LIN standards to support implementation in the North American market.
· A wide availability of tools to support the design, development, simulation, analysis, calibration and testing of distributed vehicle networks with their associated ECUs.
· Additional information about the LIN Standard is available at www.lin-subbus.org.
Captions
Figure 1: Target Applications for LIN
Figure 2: LIN Network Configuration
Figure 3: LIN Message Frame
Click here for the illustrations:
Figure 1, Figure 2, Figure 3, Table 1
