Design of embedded Chinese display terminal based on vehicle-mounted GPS

1 Introduction 

In recent years, GPS navigation and positioning technology has been developing continuously, and more and more vehicles have been equipped with vehicle-mounted GPS terminals. Today, vehicle-mounted GPS terminals have become an important part of the urban traffic management network. However, after users installed vehicle-mounted GPS terminals, the lack of friendly human-computer interaction equipment has affected user use and the popularization of vehicle-mounted GPS terminals, limiting the functional expansion of vehicle-mounted GPS terminal products. By installing a Chinese character display terminal, users can use the vehicle-mounted GPS terminal more conveniently, which is convenient for the center to conduct scheduling management and make up for the shortcomings of previous products. 

2 Composition of embedded Chinese display



Figure 1 Structure diagram of vehicle-mounted GPS system 

The function of embedded Chinese display is mainly to communicate with GPS vehicle-mounted terminal and meter through serial port, receive and process GPS positioning signals sent by GPS vehicle-mounted terminal and related operation data sent by meter, and let users set and operate Chinese display through infrared remote control and buttons, and then display the corresponding content on the LCD module, and can provide voice services. 

Embedded Chinese display consists of the following parts: Chinese display motherboard, LCD module, infrared receiver, button, speaker, light-emitting diode and connector. 

3 Mainboard hardware



Figure 2 Chinese display structure diagram 

The mainboard of the embedded Chinese display consists of the following parts: parallel port Flash, power supply circuit, LCD module interface and drive circuit, serial port communication circuit, watchdog circuit and voice chip and power amplifier circuit. 

3.1 MSP430 series microcontroller 

The MSP430 series microcontroller is a 16-bit microcontroller developed by TI. Its outstanding feature is ultra-low power consumption, which is very suitable for various occasions with low power requirements. It is the lowest power consumption among all internal integrated flash memory (Flash ROM) products in the industry, and the power consumed is only 1/5 of other flash microprocessors (Flash MCUs). At a working voltage of 3V, its current consumption is less than 350uA/MHz, and the standby mode is only 15 uA/MHz. It has five energy-saving modes. Its 64K program space ensures that it can accommodate sufficient code length, and the internal FLASH that can be written during program execution can be used to store important data such as center number, telephone number and GPS vehicle terminal settings. The microcontroller has anti-ESD protection and strong anti-interference ability. Online programming, easy program update. The simulator has a simple structure and can be made by yourself. The operating temperature range of this series of industrial-grade microcontrollers is -40" + 85 degrees Celsius, which can meet the application requirements of vehicle-mounted equipment. 

3.2 Power supply circuit 

In order to be compatible with the 12V and 24V voltages provided by different vehicle-mounted batteries, the voltage converter used in the power supply circuit should be able to meet the input voltage between 9V and 36V. Due to the harsh vehicle environment and large fluctuations in battery power supply, in order to prevent the large current caused by external interference from burning internal components, a self-recovery fuse can be connected in series at the power input end. The basic power consumption of the motherboard of this design idea is about 50mA. When the LCD backlight is on, the power consumption is about 180mA. Therefore, a self-recovery fuse with a fuse current of 350mA can be selected to meet the requirements. In order to prevent the microprocessor or microcontroller from malfunctioning or being damaged due to instantaneous pulses, such as electrostatic interference, a TVS tube can be connected in parallel at the positive input end of the power supply. Since the vehicle-mounted battery power supply voltage is 12V or 24V, it can be selected according to the actual situation.




Figure 3 Power supply circuit [page]

3.3 The memory 

Flash needs to save GPS positioning information, key points, fonts, call records, short messages, operational data and other data. The Flash capacity requirement analysis is as follows: 

3.3.1 The font library requires approximately 800K bytes. 

3.3.2 GPS positioning information needs to be saved as a driving record. Currently, most GPS modules send GPS positioning data once a second. If each record is kept as a driving record, it does not have much practical significance. Therefore, the positioning data can be retained every one to ten minutes. In this way, 10,000 records can at least save about a week's driving records to meet actual application needs. Positioning information includes: time, longitude, longitude mark, latitude, latitude mark, speed, direction, status, and additional information, etc. 

Calculation of positioning information: 10,000 × 48 bytes = 480,000 bytes 

3.3.3 For special needs such as speed limit prompts, road section prompts, and area prompts, you can pre-store the positioning data information of these key points or polygons. When the vehicle passes through, arrives at, or leaves these areas during driving, the received positioning information will be compared with them and combined with voice broadcast to identify and give prompts. The information of key points includes: time of entering the key point, time of leaving the key point, latitude of the lower left corner, longitude of the lower left corner, latitude of the upper left corner, longitude of the upper left corner, latitude of the lower right corner, longitude of the lower right corner, latitude of the upper right corner, longitude of the upper right corner, and other flags, etc. 

Calculation of key point information: 52 bytes × 200 = 10400 bytes (save 100 key points, reserve 100 key point capacity) 

Polygon information includes: vertex number, vertex longitude, vertex latitude and other information. 

Polygon: 10 × 9 vertices × (2 + 10 + 11) = 2070 bytes (up to 10 9-gons can be saved) 

Therefore, the capacity should be at least 1MB, and it is best to use parallel communication Flash to meet the high-speed data transmission with the MSP430 series microcontroller. 

3.4 Voice chip and power amplifier circuit 

In order to shorten the development cycle, it is possible to consider using a complete speech synthesis system integrated inside the processor. Taking iFlytek's voice chip as an example, the chip supports the synthesis of any Chinese text. Chinese encoding can use GB2312 or Unicode. The level logic is TTL level. The commands and data sent by the host computer are received through the serial port (UART). The commands and data are encapsulated and sent in the form of frames. The maximum length of the data allowed to be sent is 200 bytes. After receiving the command or completing the text synthesis, the voice chip sends feedback information to the MCU through the serial port TXD2. The MCU can learn whether the chip has correctly received the command and whether the chip has completed the text synthesis by receiving the feedback from the chip. When the chip READY signal pin is at a low level, it indicates that the chip is in the standby state; when it is at a high level, it indicates that the chip is in the text synthesis state. The voice is output through the 10-bit DAC analog audio, and the peripheral power amplifier circuit uses the most commonly used LM386 power amplifier circuit to meet the requirements.



Figure 4 Schematic diagram of the voice chip 

3.5 Other peripheral circuits 

Since the current domestic vehicle-mounted GPS terminals and taximeters basically use serial communication to connect to the Chinese character display terminal, the corresponding serial communication method must be used according to different vehicle-mounted terminals and taximeters. You can consider using conversion chip circuits such as MAX202, current loop circuits or other solutions. The principle of the current loop circuit is shown in the figure below:



Figure 5 Schematic diagram of the current loop circuit 

MCU and Flash are preferably powered by 3.3V low-power chips, so they need to be connected to the LCD module through a 3.3V"5V level conversion chip as the core drive circuit. You can also use a suitable solution according to the LCD module. 

4 Conclusion 

Today, when vehicle-mounted GPS technology is already very mature, the development trend of vehicle-mounted GPS systems is to provide more friendly human-computer interaction functions and more complete services. The embedded Chinese character display terminal can realize the expansion of the vehicle-mounted terminal through the serial port, providing a friendly human-computer interaction interface and personalized settings for the vehicle-mounted GPS terminal, which greatly facilitates the user's use, and expands the functions and resources of the vehicle-mounted GPS terminal, and can provide users with richer service content. Therefore, the embedded Chinese character display terminal will greatly promote the popularization of vehicle-mounted GPS systems and promote the development of vehicle-mounted GPS systems.