Overview of the hottest LIN bus and design example

2022-07-23
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LIN bus overview and automobile door control system design example

power and speed are no longer the only pursuit of consumers for automobile performance. People are more and more concerned about the comfort, safety assurance, functional ease of use, environmental protection, etc. when driving. Therefore, in addition to traditional automobile control units such as car body system and power train, safety system and telematics/infotainment system are also gradually maturing with the progress of electronic technology

in modern automotive electronic systems, electronic control components (ECU) play an important role because they give the vehicle more efficient and intelligent control ability in the above systems. They also realize automatic detection functions such as power supply, lights, doors and windows, and provide greater convenience for driving

there are more than 80 electronic systems and components in the automobile on average. The increasingly complex connection and communication functions between them put forward the demand for bus technology. The traditional connection mode of lamps, engines, solenoid valves, air conditioners and other equipment is cable connection. If the electronic components are also connected by cable, it will inevitably lead to the improvement of connection complexity, the decline of reliability and the increase of overall weight; In addition, the accompanying cable wear and aging will also reduce the safety performance of the car

in order to avoid various troubles caused by cables, the application of standardized bus technology in the in vehicle network becomes an ideal solution. According to different technical characteristics and application fields, vehicle bus technology can be divided into five categories. As shown in Table 1, class I 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), is suitable for communication applications that do not require high real-time performance; The third category includes high-speed can, ttp/c and other technologies, which are suitable for high-speed, real-time and dead cycle control multi-channel transmission networks; The fourth category, such as idb-c, idb-m (D2B, most, idb1394)), IDB wireless (Bluetooth), etc., is generally applied to the on-board Zitong entertainment network; The fifth category has the highest transmission speed and is used for the most critical and real-time personal safety systems, including FlexRay and byteflight

this paper will mainly discuss LIN bus technical specifications and application examples in door control system. Table 1 vehicle network bus standard

lin technology overview

lin bus is called local interconnect network. It is a new Low-speed serial bus with simple structure, flexible configuration and low cost, and a sub bus system of vehicle bus based on serial communication protocol

lin bus is a master-slave node architecture, that is, a master node can support up to 16 slave nodes; The slave node can achieve self synchronization without crystal oscillator or ceramic resonator clock. Lin is based on UART/SCI interface protocol, which can realize extremely low software and hardware costs; The signal propagation time can be calculated in advance to meet the certainty of transmission. The length of bus cable can be extended to about 40 meters at most, and the data transmission rate can reach 20 Kbps

in 1999, after the introduction of Lin version 1.0, new versions (Lin 1.3, Lin 2.0) have appeared continuously, continuously improving the performance and applicability of LIN bus. The vehicle architecture task force under the society of Automotive Engineers (SAE) also proposed the j2602 specification based on Lin 2.0, which shortened the length of software code required by Lin slave nodes, further reduced the complexity of software units in Lin 2.0, and realized more efficient system configuration. In addition, mainstream manufacturers will also introduce improved versions or technologies for Lin performance, such as linsci of Ruyi semiconductor

Figure 1 LIN bus application field

lin is mainly used as an auxiliary network or sub network of can and other high-speed buses, and can provide relatively complete network functions for devices that do not need can, including climate control, mirrors, door modules, seats, smart switches, low-cost sensors, etc. In the occasions with low bandwidth requirements, simple functions and low real-time requirements, such as the control of body electrical appliances, the use of LIN bus can effectively simplify the network harness, reduce costs, and improve the efficiency and reliability of network communication

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 periodic detection signals to the slave node, and the detection results are fed back to the master controller by the slave node. 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 master task and a response processed by the slave task. The header contains a 13 bit synch break field, a synch field generated by the main task, and an identifier field. Each byte field is sent as a serial bit group, with the first bit of the start bit being "0" and the end bit being "1". The signal header executed by the main task will determine the transmission time of each signal according to the schedule of the whole Lin cluster, so as to ensure the certainty of data transmission and avoid the danger of network overload. In the Lin network, only the master node uses the crystal oscillator to provide the system with an accurate basic clock. At this time, the clock will be embedded in the above synchronization field, so that the slave task can be synchronized with the master node. The response part of the Lin signal contains a data file with a length of 2/4/8 bytes and a verification field with a length of one byte (the checksum field is exposed on the light tube due to air saturation)

Figure 2 Schematic diagram of Lin signal structure

linsci

linsci can be integrated into 8-bit MCU to realize header detection, identifier, irrelevant byte filtering, extended error detection, resynchronization and other functions. Its function is to make the LIN Bus function of the slave device play more effectively

linsci can also achieve higher accuracy. The baud rate prescaler of LIN bus is generally an 8-bit integer value with limited resolution, which makes it difficult to reach the standard SCI bit time. The error rate required by the principle of developing assembled perforated plastic tiles with high flexural and tensile strength using Korean raw materials and technology is 2%. The LIN bus baud rate is generally 10kbps and 20kbps. If it is calculated as 20kbps, assuming that the CPU frequency is 8MHz, since the Lin frequency tolerance is 15%, the quantization error will reach 2.33%. The prescaler of linsci replaces the 8-bit integer value with a 12 bit unsigned fixed-point value (i.e. ldiv), and the quantization error can be reduced to 0.15%

Figure 3 linsci data structure figure

there are many factors involved in realizing the optimization of the Lin system. Although the Lin network based on standard SCI has excellent performance, the bandwidth and CPU load required for Lin data transmission, the frequency accuracy required for application, and the stability and effectiveness of the Lin interface should be considered. In addition, it is necessary to strengthen the hardware technology

st linsci can achieve higher efficiency and lower cost through these means. First, the enhanced hardware SCI port reduces the CPU load and improves the system performance accordingly. The low cost is mainly obtained by high integration. The 1MHz oscillator, fast 10 bit ADC with operational amplifier and configurable restart circuit with low-voltage detector are internally integrated, which simplifies the external circuit and system design and reduces the manufacturing cost. At the same time, 8KB extended memory can operate under a single supply voltage, which not only provides faster programming ability, but also reduces the complexity of the circuit board

example of automobile door control system architecture

take automobile door control system as an example. As shown in Figure 4, the door control system of medium and high-end models at present mainly includes door lock, dead lock latch, power window, footstep light, switch panel illumination, etc. Its main node is a central body ECU connected to the body can network. Each door has a door module, that is, the body with four doors provides DM driver, DM passenger, DM rearright and DM rearleft with door lock, power window and other functions; The other two front doors also have MMR and MML left and right rear-view mirror control modules. The central switch panel at the driver's end is an independent slave node, which controls all power windows, manual door locks, rear-view mirrors and other functions

Figure 4 Schematic diagram of Lin network door control system

the application scenario of automobile door control system puts forward the following requirements for Lin network: when the master controller receives the effective signal sent from the remote key, it must start the door control system, and the slave node usually receives it through the CAN bus; When the correct key is used to open the front door, the door control system is also started; The slave node will react directly without communication with the master controller; The polling function of the switching panel ensures the response to the active switching of various driving device controls, such as power windows, rearview mirror adjustment, door locks, etc; Query all slave nodes to get the position status of window lifting and closing, as well as the door opening and closing conditions; And the sleep mode control (i.e. battery supply operation mode) of the system to all slave nodes. Therefore, the MCU of the door control system also needs to be consistent with the above functions. For example, anti pinch function, PWM control of motor and window position monitoring must be provided for window lifting; The door lock motor can be controlled by SPI interface; The action of pulling out the car key and opening the door will cause interference and even damage to the electronic universal testing machine. It can provide contact monitoring of the power supply mode and control functions of the rearview mirror and the switch panel

Figure 5 functional architecture of gating module

there are also some factors to be considered in the parameter setting of the above functions, such as the accuracy of timing and real-time action. Taking the action of manually opening the car door lock as an example, a quick response is required from the time the key is inserted into the door lock to the time it is opened, and the acceptable delay must be less than 200ms. During this period, the transmission motor needs about 100ms to open the door lock, so the time left for MCU to start from the low power mode, detect the key, and trigger the transmission device is only 100ms. The LIN bus baud rate is generally 10kbps or 20kbps. If the fastest 20kbps is used, the CPU response time must be less than 1ms to ensure the success of data transmission. In addition, there are real-time requirements for the security (such as anti trap) and convenience (such as door lock detection) of the system

the accuracy of timing is to achieve correct operation and process. The door module needs a time reference with tolerance less than 3%, which is required by the complex algorithm of the anti pinch function of the window

power consumption and energy saving are key factors for most ECUs. As for the door control system, the system still needs to conduct interval monitoring and inquiry after the vehicle is shut down, which will cause

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