With the increasing use of electronic technology and computer technology in automotive electronics, traditional mechanical meters have gradually been replaced by electronic meters. Electronic meters have the advantages of small size and light weight, which can effectively save limited space in the car. When displaying car information in digital, text or graphic mode, the display is intuitive and the amount of information is large, and different display contents can be modified according to different needs. This article refers to the address: http:// Head up display Figure 1 system design box Figure 2 Serial Protocol Timing Figure 3 basic software Summary This paper mainly discusses the basic implementation framework of the car head-up display and the main components required for implementation. The design and implementation of the CAN bus interface, display controller interface and power interface are discussed. Through the design scheme proposed in the actual engineering paper, it is proved that it has the advantages of high degree of realization, low cost and good effect. MLF is fully capable of providing high-quality AC adapter with the power arrange from 3W to 120W .All of them meet the worldwide safety and EMC standards as UL FCC CE GS BS CCC SAA, KC, PSE etc , and comply with the latest Rohs &Reach also. The AC adapter widly use in the field of telecommunications, audio/veido products ,IT & computer peripherals ,home appliances , LED lights ,toys ,and medical equipments .MLF has different designs to suit the conditions of the AC adapters used in various lines.and each power state there are several models for selection. meanwhile MLF also welcome OEM/ODM projects including customized AC adapters. DC Adapter,Usb Adapters,Wall Mount Adapters,Power Adapters,45W Series AC Adapter Meile Group Limited , http://www.hkmeile.com
HUD is the abbreviation of English Head Up Display, which means “head-up display†or “head-up displayâ€. The initial application is on the fighter plane. It is an integrated electronic display device consisting of electronic components, display components, controllers and high-voltage power supplies. It can pass flight parameters, aiming attacks, self-tests and other information in the form of images and characters. The optical component is projected onto a display device on the combination glass directly in front of the cabin. Since the driver does not have to look down at the dashboard, the HUD can greatly shorten the driver's meter reading time. Compared to traditional dashboards, people can use HUDs to more effectively detect events that occur in front of the road.
1 The overall function head-up display system adopts LED direct projection type, and adopts reflection film reflection mode at the front windshield. The display light is reflected by the translucent reflector located in front of the driver's seat and is reflected in the driver's eyes. The driver recognizes the road condition and the display image in front of the road through the reflector in an overlapping manner. The display image can be imaged at an infinity point through the lens system, so the time required for the movement of the line of sight and the focus adjustment can be shortened. The system has CAN bus communication function, which can receive and indicate the speed, speed, overspeed alarm, cruise status, gear and other information through the bus. 
2 Implementation 1 Device Selection The system has three main components, a CAN transceiver, a microprocessor, and an LED display driver. The main control board needs to obtain information such as the vehicle speed, oil quantity, and door switch through the CAN bus interface. The interface circuit includes a protocol transceiver and a bus isolation amplifier. Since the STM8S series MCU has its own CAN protocol communication interface, the SJA1000 series CAN bus transceiver used in the previous design can be omitted here. Only the CAN physical layer interface device TJA1040 can be added to realize the complete communication link with the whole vehicle. As an automotive grade chip, TJA1040 can operate in the temperature range of -40 to +125 °C, can withstand 6kV ESD voltage shock, and has differential transmission and reception functions, can support communication rates up to 1Mb/s, single chip implementation Data transmission, isolation, and protection functions of the CAN node. The microprocessor selects ST's STM8S206xx series, which integrates a CAN controller on-chip and supports the CAN 2.0B protocol. The I/O pin driving capability of the MCU is limited, and the direct driving of the LED segment code requires a large current. Therefore, it is necessary to add an LED driving circuit ST2225A between the MCU and the LED segment code screen to enhance the driving current of the LED screen. The ST2225A is a dedicated circuit for LED drive control in CMOS technology with continuous backlight control and serial data input.
2 system design system uses vehicle 12V power supply, with ACC key control and external switch control. When the external switch is in the off state, the HUD system will not enter the working state (in the off state) regardless of whether the car is started or not. There are two cases when the external switch is in the open state. When the car is turned off and the key is removed, the system enters the sleep state, which consumes only a very small amount of electricity; when the car is started, the system enters the working state, and the data is read from the CAN bus and displayed on the LED display.
The two ends of the CAN network usually need to connect two terminating resistors, which are used to prevent data from returning like an echo after reaching the line terminal and interfering with the original data. In order to prevent external electromagnetic interference or external radiation disturbance, the two data lines are usually wound in parallel, the potential is opposite, and the voltage and total are equal to the normal value. 
The protocol for displaying the serial port data input of the driver chip ST2225A is as shown in FIG. 3, and the input data is 35-bit data starting with a "1". When the 36th data transfer is completed, the load signal is generated in synchronization with the clock enable (DATA ENABLE), and 35 bits of data are loaded into the latch. The clock disable can generate a reset signal to clear the data in the shift register and wait for the next set of 35-bit data. The serial communication clock frequency can be set to 500Kbps. 
In order to achieve continuous backlight control, the brightness of the LED is set by a variable resistor to set the reference current. Figure 4 shows the interface circuit diagram of the ST2225A. A total of 82 LED lights need to be driven due to the large amount of content to be displayed. The ST2225A has only 35 outputs, so it is necessary to use several of the pins as common terminals to adopt a time-sharing driving method. However, the more the common end, the smaller the output duty cycle, the brightness of the LED light will be affected, so the public end can not be too much. After trial and verification, the three common terminals do not have much influence on the brightness of the LED lamp. In order to maintain the original brightness of the LED lamp, the power supply voltage can be appropriately increased.
3 Software flow After the system is started, the screen displays a prompt message. The user enters the time. After the date information is confirmed, the display starts to display the date information. At this time, the car display speed is zero. After the car is started, the system displays the processed approximate vehicle speed and the mileage information that has been traveled in real time. During the driving process, the system analyzes the mileage of the car by analyzing the CAN information frame sent from the body bus. The MCU uses the timed interrupt to periodically calculate the count value to obtain the speed and other information. The program continuously adjusts the speed value and the threshold value. Compare and give an alarm prompt if it is exceeded.
September 03, 2019