LIN bus overview and car door control system design example

Power and speed are no longer the only pursuit of car performance. People are more concerned about the comfort, safety, ease of use, and protection of the environment when driving. Therefore, in addition to traditional car control units such as the Car body and the Power Train, safety systems and Telematics / Infotainment are maturing with advances in electronic technology. .

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In modern automotive electronic systems, electronic control components (ECUs) play an important role in giving cars more efficient and intelligent control capabilities in these systems, as well as automatic detection functions such as power supplies, lights, doors and windows. It provides greater convenience for driving.

The average number of electronic systems and components in automobiles is more than 80. The increasingly complex connection and communication functions between them place demands on bus technology. The traditional connection method of the lamp, the engine, the solenoid valve, the air conditioner and the like is a cable connection, and if the electronic components are also connected by a cable, the connection complexity is increased, the reliability is lowered, and the overall weight is increased; In addition, the accompanying wear and aging of the cable will also reduce the safety performance of the car.

In order to avoid the troubles caused by cables, the application of standardized bus technology in the In-Vehicle Network has become an ideal solution. According to different technical characteristics and application fields, the vehicle bus technology can be divided into five categories. As shown in Table 1, the first type of LIN, TTP/A and other buses have the lowest transmission speed and are suitable for vehicle body control; the second type of medium-speed bus, such as low-speed CAN, SAE J1850, VAN (Vehicle Area Network), etc. The communication application with low real-time requirements; the third category includes high-speed CAN, TTP/C and other technologies, suitable for high-speed, real-time infinite loop control of multi-transport networks; the fourth category is IDB-C, IDB-M (D2B) , MOST, IDB1394)), IDB-Wireless (Bluetooth), etc., generally used in the vehicle-based entertainment network; the fifth type of transmission speed is the highest, for the most critical, real-time personal security system, including FlexRay and Byteflight Wait.

This article will mainly discuss the LIN bus technical specifications and application examples in the gate control system.

Table 1 Vehicle Network Bus Standard


LIN Technology Overview

The LIN bus is called the Local Interconnect Network. It is a new low-speed serial bus with simple structure, flexible configuration and low cost, and a sub-bus system based on serial communication protocol. .

The LIN bus is a master-slave node architecture, that is, a master node can support up to 16 slave nodes (Slave Node); in the slave nodes, a crystal oscillator (Crystal Oscillator) or a ceramic resonator (Ceramic Resonator) clock can also be used. Self-synchronization. Based on the UART / SCI interface protocol, LIN can achieve extremely low hardware and software costs; its signal propagation time can be pre-calculated to meet the determinism of transmission. The length of the bus cable can be extended up to 40 meters and the data transmission rate can reach 20 kbps.

In 1999, after the release of LIN 1.0, new versions (LIN 1.3, LIN 2.0) continued to improve, and the performance and applicability of the LIN bus continued to improve. The Association of Automotive Engineers (SAE)'s Task Force Task Force (Task Force) also proposed the J2602 specification based on LIN 2.0, which shortens the software code length required by LIN slave nodes, further reducing the complexity of software units in LIN 2.0. For more efficient system configuration. In addition, mainstream vendors will introduce improved versions or technologies for LIN's performance, such as STMicroelectronics' LINSCI.


Figure 1 LIN bus application field

LIN is mainly used as an auxiliary network or sub-network of high-speed bus such as CAN. It can provide better network functions for devices that do not need CAN, including Climate Control, Mirrors, and Door Modules. Modules), seat control (Seats), smart switches (Smart Switches), low-cost sensors (Low-cost Sensors), etc. In areas where bandwidth requirements are low, functions are simple, and real-time requirements are low, such as control of body appliances, the use of LIN bus can effectively simplify network wiring harnesses, reduce costs, and improve network communication efficiency and reliability.

LIN network architecture

As mentioned above, the LIN network forms a network topology based on the master-slave node architecture. The master node needs to send a periodic detection signal to the slave node, and the detection result is fed back from the slave node to the master controller. The period is set according to the real-time requirements of event detection.

As shown in Figure 2, the LIN signal consists of a header provided by the main task and a response part (response) processed from the task. Header contains

A 13-bit Synch Break Field, a sync field generated by the main task, and an Identifier Field. Each of the byte fields is sent as a serial byte, the first bit of the start bit is "0" and the stop bit is "1". The signal headers executed by the main task determine the transmission time of each signal according to the schedule of the entire LIN cluster to ensure the certainty of data transmission and avoid the danger of network overload. In the LIN network, only the master node uses a crystal oscillator to provide the system with an accurate basic clock. This clock is embedded in the above synchronization field, allowing the slave task to synchronize with the master node timing. The response portion of the LIN signal contains a Data Filed with a length of 2 / 4 / 8 bytes and a checksum field of one byte in length.


Figure 2 Schematic diagram of LIN signal structure


LINSCI can be integrated into 8-bit MCUs for Header Detection, Identifier and Irrelevant Byte Filtering, Extended Error Detection and Resynchronization Resynchronisation and other functions. Its role is to make the LIN bus function of the slave device more effective.

LINSCI can also achieve higher precision. The Baud Rate prescaler of the LIN bus is generally an 8-bit integer value with limited resolution, making it difficult to achieve an accuracy of 2% for the standard SCI bit time sampling principle. The LIN bus baud rate is generally 10kbps and 20kbps. If the calculation is 20kbps, the CPU frequency is 8MHz. Since the frequency tolerance of LIN is 15%, the quantization error will reach 2.33%. The LINSCI prescaler replaces the 8-bit integer value with a 12-bit unsigned fixed-point value (LDIV), and the quantization error can be reduced to 0.15%.

Figure 3 LINSCI data structure diagram

There are many factors in achieving optimization of a LIN system. Although the LIN network established by the standard SCI has excellent performance, the bandwidth and CPU load required for LIN data transmission, the frequency accuracy required for the application, and the stability and effectiveness of the LIN interface should be considered. the elements of. In addition, hardware technology enhancements are also necessary.

ST's LINSCI can achieve higher efficiency and lower cost through these means. First, the enhanced hardware SCI port reduces CPU load and improves system performance. Low cost is primarily achieved by high integration, with a 1MHz oscillator, a fast 10-bit ADC with op amp, and a configurable restart circuit with a low-voltage detector, simplifying external circuitry and system design and reducing manufacturing cost. At the same time, 8KB of extended memory can operate at a single supply voltage, in addition to providing faster programming capabilities, but also reduces the complexity of the board.

Automotive door control system architecture example

Take the example of a car door control system. As shown in Figure 4, the current door control systems for mid- to high-end models mainly include locks, dead locks, power windows, Footstep lights, and switch panels. Lighting (Switch Panel Illumination), etc. The main node is a central body control unit (Central Body ECU) connected to the vehicle body CAN network. Each door has a door module, that is, the body of the four-door is DM-Driver (driver position), DM- Passenger (co-pilot position), DM-RearRight (right rear door) and DM-RearLeft (left rear door) provide functions such as door locks and power windows; the other two front doors also have MMR and MML left and right mirror control modules. The Central Switch Panel on the driver's end is an independent slave node that controls all power windows, manual door locks and rear view mirrors.

Figure 4 Schematic diagram of LIN network gate control system

The application scenario of the car door control system puts forward the following requirements for the LIN network: when the main controller receives the valid signal from the remote control key, the gating system must be activated, and the slave node usually receives through the CAN bus; when the correct key is turned on At the front door, the door control system is also activated at the same time; the slave node will directly respond without communication with the master controller; the panel's polling function is activated to ensure that the response is controlled for each drive device, such as a power window, Active switching of rearview mirror adjustment, door lock, etc.; inquiry to all slave nodes

Function to get the position status of the window lift, as well as the switch status of the door; and the system's sleep mode control for all slave nodes (ie battery supply operation mode). Therefore, the MCU of the door control system also needs to comply with the above functions. For example, the anti-Pinch function, the PWM control of the motor and the window position monitoring must be provided for the lifting of the window; the door lock motor can be controlled by the SPI interface; For the removal and opening of the car key, it is possible to provide contact monitoring of the power supply mode, as well as control functions of the rear view mirror and the switching panel.


Figure 5: Functional structure of the gate module

For the parameter setting of the above functions, there are also some factors to be considered, such as the accuracy of the timing and the real-time behavior. For example, the action of manually opening the car door lock, from the key to the door lock to open, requires a quick response, the acceptable delay must be less than 200ms. During this time, the drive motor takes about 100ms to open the door lock, so all the time left for the MCU to start from low power mode, detect the key, and trigger the transmission is only 100ms. The baud rate of the LIN bus is generally 10 kbps or 20 kbps. If the calculation is based on the fastest 20 kbps, the response time of the CPU must be less than 1 ms in order to ensure the success of data transmission. In addition, there are real-time requirements for system security (such as anti-pinch) and convenience (such as door lock detection).

The accuracy of the timing is to achieve the correct operation and process. The door module requires a time reference with a tolerance of less than 3%, and the complex algorithm of the anti-Pinch function requires this accuracy.

Power consumption and power savings are key factors for most ECUs. In the case of a gated system, the system still needs to perform intermittent monitoring and interrogation actions after the vehicle is turned off, which will result in continuous consumption of electricity. The monitoring delay interval setting is difficult to choose, because the time interval is too long, which will cause the reaction delay; if it is too short, it will increase the power consumption of the system.

Fail-safe design

Fault and safety are also the focus of the system design, such as the Fail-Safe mechanism of the bus line during short circuit. Because the LIN bus does not have Fault Tolerant compared to the car body CAN bus system, each node must have the ability to resolve the shorted bus line, and the reaction action must follow a specific procedure.

The L9638 is a LIN transceiver from ST that provides additional safety faults to handle short-circuit faults. When the MCU finds a shorted LIN bus line, the ECU can self-shut down; and the transceiver can still restart after eliminating the short circuit condition.

in conclusion

With a flexible configuration, LIN can deliver comprehensive performance in a wide range of applications. For example, the LIN protocol is built in hardware (LINSCI) to increase system reliability and simplify LIN driver code. The design of the MCU is also a key. Take the ST72F361 as an example. It provides an advanced SCI interface on a standard MCU and supports LIN functions. In addition to reducing the CPU load, it also eliminates the need for costly and precise timing resources.

The LIN bus is a low-rate transmission standard and does not have the performance of a CAN bus. It is mainly located in scenarios other than the critical applications of CAN (high speed, high efficiency, high fault tolerance, etc.). When designing the vehicle electronic system, it is necessary to properly select the appropriate technical standards according to specific needs and technical requirements, in order to allow LIN and CAN to exert their own unique advantages and save costs. The LIN bus has won a unique market space with its low cost and high reliability. It is expected that the application of the LIN bus will occupy a considerable proportion in the new European vehicles.

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