Design of embedded Chinese display terminal based on vehicle communication 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 vehicle-mounted GPS terminals more conveniently, which is convenient for the center to carry out dispatch 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 the embedded Chinese display is mainly to communicate with the GPS vehicle terminal and the meter through the serial port, receive and process the GPS positioning signal sent by the GPS vehicle terminal and the relevant operation data sent by the meter, and allow users to set and operate the Chinese display through infrared remote control and buttons, and then display the corresponding content on the LCD module, and can provide voice services.

The embedded Chinese display consists of the following parts: Chinese display mainboard, LCD module, infrared receiver, buttons, speakers, light-emitting diodes and connectors.

3 Mainboard hardware

Figure 2 Chinese display structure diagram

The motherboard 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

Road and voice chip and power amplifier circuit.

3.1 MSP430 Series MCU

The MSP430 series of microcontrollers are 16-bit microcontrollers developed by TI. Their outstanding feature is ultra-low power consumption, which is very suitable for various occasions with low power requirements. They are 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 and convenient program update. The emulator 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 on-board 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 on-board environment and large fluctuations in battery power supply, a self-recovery fuse can be connected in series at the power input end to prevent the large current caused by external interference from burning internal components. 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 single-chip computer 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 on-board battery power supply voltage is 12V or 24V, it can be selected according to actual conditions.

Figure 3 Power supply circuit [page]

3.3 Memory

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

3.3.1 The font library requires about 800K bytes.

3.3.2 GPS positioning information needs to be saved as driving records. At present, most GPS modules send GPS positioning data once a second. If every piece is saved as a driving record, it will not have much practical significance. Therefore, positioning data can be saved every one to ten minutes. In this way, 10,000 pieces of driving records can be saved for at least one week, meeting the needs of practical applications. Positioning information includes: time, longitude, longitude mark, latitude, latitude mark, speed, direction, status and additional information, etc.

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

3.3.3 For special needs such as speed limit reminders, road section reminders, and area reminders, the positioning data information of these key points or polygons can be pre-stored. When the vehicle passes through, arrives at, or leaves these areas during driving, the received positioning information can be compared with them and combined with voice broadcast to identify and give reminders. 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 landmarks, etc.

Calculation of key point information volume: 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 be either 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 ready state; when it is at a high level, it indicates that the chip is in the text synthesis state. The voice is output through a 10-bit DAC analog audio output, and the peripheral power amplifier circuit uses the most commonly used LM386 power amplifier circuit to meet the requirements.

Figure 4: Voice chip block diagram

3.5 Other peripheral circuits

Since the current vehicle-mounted GPS terminals and taximeters in China are basically connected to the Chinese character display terminal by serial communication, the corresponding serial communication method must be adopted according to different vehicle-mounted terminals and taximeters. You can consider using MAX202 and other conversion chip circuits, current loop circuits or other solutions. The principle of the current loop circuit is as follows:

Figure 5 Current loop circuit schematic

It is best to use low-power chips powered by 3.3V for MCU and Flash. Therefore, they need to be connected to the LCD module through a driving circuit with a 3.3V~5V level conversion chip as the core. You can also use a suitable solution based on 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.

References:

[1]胡大可编著. MSP430系列16位单片机原理和应用. 北京：北京航空航天大学出版社，2000

[2]胡大可编著. MSP430系列FLASH型16位单片机. 北京：北京航空航天大学出版社，2001

[3] Beijing Juli Technology Co., Ltd. Communication protocol between taximeter and vehicle terminal GPS. Beijing: Beijing Juli Technology Co., Ltd., 2005