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LIN Introduction

LIN (local interconnect network) was specified in the years 2000 (LIN 1.1) to 2003 (LIN 2.0) by a group of renowned car manufacturers. It is used as an addition to CAN as a particularly low-price serial communication system in all cases where a low bit rate (up to 20kbit/s) is sufficient and no safety-critical functions are required. Typical reaction times in LIN networks are approx. 200ms.

LIN does not just describe a communication protocol, but rather all aspects of a communication system. In addition to the actual protocol specification, this also includes specifications for a configuration language (LIN configuration language) and the interface to the application program. In addition, the sepcification V2.0 describes the capabilities of a LIN node via the so-called "node capability" as well as the diagnosis and configuration.

In general, LIN is used within local subsystems. Typically these are applications of car body electronics, such as air conditioning systems, doors, seats, sunroof. Such sub-networks are connected as units of a CAN network via a LIN/CAN gateway with LIN master function.

LIN is a serial single-wire communication protocol based on the standard SCI interface (UART). Access to the LIN bus is controlled by a master, due to which a maximum latency time can be guaranteed. A special feature of LIN is the synchronization of the slave devices via the bus, which makes the implementation of low-cost nodes without quartz clocking possible. Data are transmitted by a 12V single wire line with a maximum transmission rate of 20kbit/s.

A typical LIN system has up to 16 nodes, due to the low number of identifiers (64) and the relatively low bit rate. The clock synchronization, the simple UART communication and the single wire medium are responsible for the low costs of a LIN system.

Communication principle and message format

The communication in a LIN network is carried out according to the master/slave principle. The LIN master unit defines which message is transmitted on the bus. For this, the master works through a message table, the so-called scheduling table. This defines the order in which the messages are transmitted. The transmission of a LIN telegram begins with a 13-bit long dominant level (sync break) transmitted by the master, which is used to identify the start of the frame. Then the master transmits a sync field (alternate 1-0 sequences). This can be used by the slave node for clock synchcronization.

After the sync field, the identifier is transmitted by the master. This consists of a 6-bit long message ID and a 2-bit long parity field. In addition to the ID, the message ID contains an optional message length information (2, 4 or 8 data bytes). The slave, which is addressed by the message ID, now transmits its data bytes (1-8 data bytes) followed by a checksum.

Specific LIN components

Some semiconductor manufactures already offer specific microcontroller derivatives and LIN drivers which support the low-cost connection to the LIN bus. Furthermore, so-called "system basic chips" are being developed which, in addition to the LIN transceiver, also contain the voltage controller for the microcontroller, voltage supply and several digital inputs.

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