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Significance of LIN bus in vehicle control apps

Posted: 28 Nov 2014     Print Version  Bookmark and Share

Keywords:Controller Area Network  CAN  LIN  communications protocol  microcontroller 

When Galvin Manufacturing Corp. (now Motorola) launched the fitted car radio in the 1930s, few could envision what the future would look like. For years the only electronic component in a car was the radio. The infotainment system in today's automobile is just one of the many systems that are electrically controlled: the number of electronic control units (ECU) in a modern car today can be as high as 80 (figure 1). No wonder the market for automotive electronic components has been on the rise and is expected to hit $18.5B by 2018.

In the initial years, the practice was to have stand-alone autonomous electronic systems. It was soon realised that there was a need for a mechanism to enable systems to communicate with each other. This led to the introduction of networks such as serial communication channels (bus) that would co-ordinate the individual electrical systems and lead to improved functionality of the vehicle as a whole.

 Multiple electrical systems in a car

Figure 1: Multiple electrical systems in a car.

In 1983, Bosch began developing the Controller Area Network (CAN) bus and officially released the protocol in 1986. Even today, when different vehicle bus standards are available, CAN continues to be the most popular. In a CAN network, all the nodes (originating from different ECUs) act as master (i.e., there is no master-slave topology) and are not assigned specific addresses. Rather, messages contain 'identifiers'.

At a given time, multiple nodes may transmit data to the CAN bus. The message identifier then helps determine the priority of the messages. The highest priority message will pull the CAN bus to the dominant state and all the other nodes stop transmitting. The nodes are in fact transceivers and, based on specific functions, they may look out for specific messages from the bus, apart from transmitting messages. Thus, information flow happens between different nodes connected to a CAN bus.

CAN is highly reliable because of multiple error checks, such as stuffing errors, bit errors, checksum errors, frame errors, and acknowledgment errors. In terms of data rate, CAN supports up to 1MB/s. Thus CAN is the default choice for connecting critical function ECUs in vehicles (e.g., gear box, temp sensor, etc.).

However, the role of electronics in automobiles is not limited to these critical units only. Over the years, the market for automotive body electronics has been increasing. Example body control applications are seats, windows, smart wipers, and climate control sensors. For body electronics, the key requirement is to ensure improved comfort and safety of the vehicle. These are systems that may not require as high reliability as the critical ECUs do, but they still require an automotive standard for communicating over a network.

The different systems and the type of network they require are categorised below:
 • Conventional body and power train applications use protocols with real-time properties, and mainly require CAN.
 • Multimedia applications require higher bandwidth, speed, and even wireless interconnection. Bluetooth, MOST, or Firewire are some of the networks used for these applications.
 • Safety critical applications require protocols that are reliable and fault tolerant. TT-CAN (Time-Triggered CAN) and FlexRay are typical of such networks.
 • Smart sensors and actuators in seats, windows, wipers, and even some complex ECUs have simpler communication needs. These applications are usually addressed by custom OEM protocols and don't require a CAN or FlexRay interface for communication.

For the last category, where OEMs use their own protocols, difficulties arise such as the complexity and expense for suppliers to the OEMs to design different systems without the benefit of a standard. Thus arose the formation of the LIN consortium by different automakers in the late 1990s. The consortium eventually implemented a standard for networking in these types of systems called LIN in 2002.

Unlike the more sophisticated and widely used CAN bus, LIN (Local Interconnect Network) is a serial network protocol used for communication between components in vehicles and arose out of the need for an inexpensive serial communications protocol between microcontroller-based sensors within an automobile.

LIN vs CAN
CAN is more expensive to implement than LIN. Factors that contribute to the higher cost of CAN include:
 • Each node in a CAN network requires a clock generator or crystal
 • At a silicon level, CAN is more complex to implement
 • 2-wire transmission is used.

Most importantly, the entire expensive architecture is overkill for applications that demand neither high reliability nor high data rates.

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