1 Introduction
Automobile bus technology is one of the applications of field bus. Initially, field bus was only used for industrial control. The so-called field bus is a bus topology network applied at the bottom of production. The current problem to be solved by the automobile body control network is to establish a unified, low-cost low-end communication network standard. LIN exists as an auxiliary bus of CAN to realize the layering of the body control network and the body control network at a lower cost.
The goal of the LIN bus is to locate the low-end communication between the nodes of the body network module. Compared with CAN, LIN uses low-cost hardware slave nodes, thereby reducing the cost of the hardware platform. In addition, LIN can fully meet the transmission rate requirements of most low-end application objects. Therefore, UN achieves network communication between switching devices at a lower cost, effectively supporting the control of distributed mechanical and electronic nodes in automotive applications.
2 Hardware circuit design
2.1 System design ideas
The LIN slave node processes the control signal sent by the master node and measures the state of the lamp driving circuit. After receiving the message information from the node, it sends corresponding control signals to the lights and analyzes the status of each light. If a failure occurs, a data message is generated and sent to the general node. After detecting the signal sent by the master node, the LIN slave node first recognizes it through the message frame to see if it belongs to its own message information. If it belongs, first judge whether the message is query information, if yes, return a response message, if it is control information, then control the corresponding lamp, and measure the potential of the measuring point on the lamp driving circuit and Be processed. See if it malfunctions. If a failure occurs, the information is sent to the general node through the LIN bus. Figure 1 shows the hardware circuit design of the LIN node.
2. 2 device introduction
The basic LIN node circuit mainly includes MCU, LIN transceiver, power module and car lamp driving circuit.
2. 2. 1 MCU control unit
In the design, MCU selects MC68HC908QL4. It integrates a slave LIN interface control module SLIC (Slave LIN Interface Controller). In general, SUC can be used as an SCI port. The main features of this device are:
With independent LIN message identifier, 8 B message buffer area;
Automatically adjust the baud rate, frame synchronization;
Automatically process and correct UN synchronization interval (SYNCH BREAK) and synchronization field (SYNCH BYTE);
LIN information without errors can generate up to two interrupts;
Complete LIN error detection and reporting;
High-speed LIN reaches 83.33 Kb / s ~ 120 Kb / s;
Enhanced detection and its generation including ID.
As long as the corresponding register is set according to the needs of the module, it can automatically carry out and send data according to the LIN bus protocol. Compared with the single-chip microcomputer of the SLIC module, this reduces the difficulty of software development. The connection of the MCU module is shown in Figure 2.
2.2.2 LIN transceiver
TJAl020 is selected as the LIN transceiver. TJAl020 is the interface between the LIN master / slave protocol controller and the LIN physical bus, which is mainly used as a vehicle auxiliary network. The baud rate is 2.4 to 20 Kb / s. The transmission data stream input by the controller on the TXD pin is converted into a LIN bus signal by the LIN transceiver, and the conversion rate and waveform are controlled by the transceiver to reduce the extremely low electromagnetic emission (EME). The output pin of the LIN bus is pulled high by an internal termination resistor. The transceiver detects the data stream at the input pin of the LIN bus and sends it to the microcontroller via pin RXD. The main features of TJAl020 are:
With a baud rate of up to 20Kb / s and extremely low electromagnetic emission (EME);
With high electromagnetic interference resistance (EMI) and low slope mode can further reduce EME;
With wake-up source to identify local or remote;
With extremely low current consumption in sleep mode, it can realize local or remote wake-up;
With sending data timeout function;
LIN bus short circuit protection for battery and ground;
With bus termination and battery pins, it can prevent transients in the automotive environment.
Figure 3 shows the circuit design of the LIN module.
2.2.3 Power module
In this design, the voltage regulators of the LIN module all use micro-power, low-dropout voltage regulator LTll2l-5. LTl121-5 is selected to enter the stop mode by inputting a low level to SHDN. At this time, the quiescent current is only 16μA, so when there is no activity on the bus, the purpose of reducing power consumption can be achieved; in addition, the device also has The function of reverse input and output power can prevent reverse current flow even if no diode is added at the output end. Figure 4 shows the power module circuit.
2.2.4 Driving circuit of car lamp
The power driver BTS724G is used to drive 2l W and 5 W headlights. The driver is an N-channel MOSFET power tube designed by Infineon, which integrates a charge pump, current drive, and has a fault feedback function for detecting load current (including overload, overtemperature and short circuit). BTS724G adopts 12 V or 24 V load control, suitable for various resistive, inductive or capacitive loads, especially suitable for loads with high inrush current such as car lights, and can be used as an alternative control method for relays and fuses. BTS724G also has multiple protection functions such as short-circuit protection, overload protection, overvoltage protection, over-temperature shutdown, ground and power-off protection, electrostatic discharge protection and power reverse connection protection. Figure 5 shows the drive circuit.
3 Software design
The lamp control system mainly completes two functions: one is to realize the control of the lamp by the LIN subnode; the other is to realize the diagnosis of the lamp failure. In control, determine whether the system is faulty by analyzing the bus potential and the potential of the input, output, and fault diagnosis pins in the drive circuit.
To enable LIN bus nodes to complete communication tasks effectively and in real time, software design is the key. The use of structured program design scheme has good modularity, portability and modifyability.
The reception of LIN information is interrupted. When the MC68HC90-8Q14 controller detects an information frame that meets the requirements of the node, it first determines what information frame the local node received. If it is control information, it receives 2 bytes of data Information; if it is query information, the status of the local node's lights is sent back to the master node in the form of an information frame to reflect the status of the node. Then it is judged that if it is a received data frame, the corresponding information is read on the data register (SLCDx) in the SLIC module. Finally, the vehicle lights are controlled according to the relevant bits in the data information. After the control signal is sent, the potentials of the lamp drive chip inputs, outputs, and fault diagnosis pins are collected accordingly, and the failure is determined by the analysis of the potential. Then send a fault message. Figure 6 shows the program flow chart.
4 Conclusion
Introduce the online diagnosis system of car lights based on LIN bus and make a simple analysis of hardware modules and software architecture. The LIN bus system has the characteristics of simple structure, reliable performance and low price, which is an inevitable trend of the development of automotive electronic technology. At present, how to use bus technology to improve vehicle performance and reduce manufacturing and maintenance costs in China has become a hot spot for car manufacturers.
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