Hardware System Design of Car Navigation Service Terminal Based on LPC2214

introduction

GPS has been developed in China for more than ten years, but it has not made any major breakthroughs in the civilian field. In recent years, the prospects for civilian use of GPS have become increasingly clear, and user demand is increasing. Currently, there are two applications that are favored: one is civilian navigation terminals, especially car systems; the other is mobile positioning services based on mobile phones.

Navigation is the main way to apply LBS (Location Based Services) to cars. At present, there are practical applications of car navigation in my country. One is the mode of "GPS receiver + simplified GIS engine + map data". The positioning information is obtained through the GPS receiver, and then the map stored in the machine is called to map the positioning information to the electronic map to realize the navigation function. Although this mode is relatively convenient and fast, it also has its own limitations, such as high cost, map resources cannot be shared, and dynamic map updates cannot be achieved. Another mode is the "GPS receiver + GSM module + SMS service" mode, which obtains positioning information through the GPS receiver, sends the positioning information to the control center in the form of SMS, and then dials the center phone through the GSM module to obtain navigation information in voice. This mode has low cost and accurate information, but SMS generally has delays; experiments show that when the network delay is not large, the cumulative probability of a 140-byte SMS with a transmission delay of 6s is 92.86%, and the speed of calling to obtain navigation information is too slow and inconvenient. And this

These problems can be better solved by combining with the operator's existing general packet radio service GPRS. GPRS allows users to send and receive data in end-to-end packet transfer mode without using network resources in circuit switching mode, thus providing an efficient and low-cost wireless packet data service, which is particularly suitable for intermittent, sudden and frequent, small data transmission, and also for individual large data transmission. Therefore, the vehicle terminal uses a high-performance processor and communicates with the information center through GPRS, which can fully realize remote map download and autonomous navigation.

This paper introduces a low-cost vehicle navigation terminal based on ARM processor and supporting GPRS, and gives a detailed description of the hardware design involved.

1 LBS system model and In-vehicle navigation service terminal Introduction

1.1 LBS system model

The LBS system is mainly composed of a global positioning system, mobile terminals, 无线通信\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> wireless communication networks and information service centers, as shown in Figure 1. In the figure, GPS satellites provide positioning information, which is the key to the realization of the entire LBS system; mobile terminals are mainly used to obtain location information, issue location service applications and interpret navigation information; the G3SM communication network is used to realize the communication between mobile terminals and information service centers; the information service center is the core of the positioning service system, responsible for information interaction with mobile terminals and network interconnection of various sub-centers, completing the classification, recording and forwarding of various information and the flow of business information between sub-centers, and monitoring the entire network.

1.2 Terminal Function

This terminal is powered by a car power supply. After the power is turned on, each module automatically starts. The terminal has the following functions: GPS positioning function; real-time download of remote telephone maps; real-time display of positioning information and map data; the main control chip can enter the power-down mode to reduce power consumption, and the main control chip can be immediately awakened by pressing the corresponding function key.

1.3 Main modules of the terminal

The GPS module uses the TIM-LH from Swiss u-blox. This GPS receiver can provide accurate navigation under dynamic conditions in areas with limited sky vision, with a positioning accuracy radius of up to 2.5m. It has two full-duplex serial ports and supports NMEA, RBX and RTCM serial port protocols. It is highly integrated, with a size of 25.4 mm × 25.4 mm and a height of only 3 mm.

The GPRS module uses the Q2406A launched by Wavecom of France. The module supports two frequency bands: 900/1800MHz, with power of 2W (900MHz) and 1W (1800MHz) respectively, and supports WAP (Wireless Application Protocol), IrDA 1.2A protocol and GPRS; it has an AT data set interface, supports data, voice, SMS, fax services, etc., and its data download rate can reach 53.6 khps, and upload rate can reach 26.8 kbps.

This terminal uses an ARM microprocessor as the main control chip, combined with GPRS, to achieve real-time map download and fast processing. LPC2214 is a 16/32-bit microprocessor based on the ARM7TIDMI (Thumb) core launched by Philips. The 128-bit memory interface and unique acceleration structure enable 32-bit code to run at the maximum rate clock; its high-speed computing capability provides a guarantee for the fast processing of map data, and its cost performance is much higher than that of ordinary single-chip microcomputers. LPC2214 has an 8/16/32-bit external memory interface, which can configure 4 external memories, each of which can reach 16 MB, providing conditions for large-scale data storage and downloading of embedded operating systems; and the chip has a power control module, which can put the terminal into power-down mode when navigation is temporarily not needed, greatly reducing power consumption. In addition, the Jun chip has two serial ports, an embedded on-chip programmable phase-locked loop PLL, and the CPU's maximum operating frequency can reach 60MHz. [page]

2 Design and analysis of key hardware of vehicle navigation service terminal

The hardware structure of the vehicle navigation service terminal is shown in Figure 2. The serial port O of LPC2214 is connected to the GSM/GPRS module, and the serial port l is connected to the GPS module. The liquid crystal 显示器\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> display (LCD) is connected to the ARM chip in parallel and adopts 8-bit data transmission mode; the keyboard adopts matrix scanning mode and is controlled by 8 GPIO port lines.

After the terminal is powered on, log in to GPRS through software settings; after receiving GPS data, the GPS module automatically sends it to the main control chip through serial port 1; after receiving GPS data, the main control chip selects the required information and displays the positioning information on the LCD. When surrounding map information is needed, press the corresponding function key, the main control chip scans the keyboard signal, passes the positioning information to the GSM/GPRS module through serial port 0, and sends it to the information service center in the form of a text message, downloads surrounding map data through GPRS, and the main control chip reads the map data through serial port 0, and displays it on the LCD after processing, realizing autonomous navigation.

2.1 Design and analysis of power supply circuit

The terminal system needs 电源模块\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> a power module to provide three voltages: 3.3V, 1.8V and 5V. Among them: the core power supply voltage of LPC2214 is 1.8V; the I/O port power supply voltage of LPC2214, the power supply voltage of TIM-LH and Q2406A are 3.3V; the power supply voltage of the LCD display is 5V. This terminal is powered by a car power supply, and the voltage is generally 12V or 14V. The LM2576 series power supply chip of National Company is selected to obtain 5V and 3.3V voltages. The specific plan is: select LM2576-5.0 to obtain 5V voltage, LM2576-3.3 to obtain 3.3V voltage, and then the 3.3V voltage is used as the input voltage to obtain 1.8V voltage through the power supply chip LMlll7-1.8.

The power line and ground line are common parts of all circuits. Since the working current of each part of the circuit is coupled with each other through the power supply and the lead,

, thus forming interference, so the ground and power lines of the printed circuit board PCB (Printed Circuit Board) should be carefully designed. When designing the PCB, the wiring width of the power line can be increased as much as possible according to the size of the current; at the same time, the loop resistance can be reduced so that the direction of the power line and the ground line is consistent with the direction of data transmission. Since the anti-interference ability of the multi-layer board is significantly higher than that of the double-layer board, it is possible to consider setting up a power layer and a ground layer. In the case where multiple power supply voltages are required on a circuit board, the power layer can be divided according to different voltages; when laying out the PCB, the devices using the same power supply voltage should be distributed in the corresponding area as much as possible. The ground layer is tightly coupled with the power layer, and appropriately extended beyond the power layer, which can also effectively suppress common-mode interference; of course, multiple power layers can also be set, but the cost will increase significantly. The circuit board of this terminal adopts the method of setting a single power layer and dividing the power layer. The divided power layer is shown in Figure 3.

In addition, the power input end of the circuit board should be connected to a 10-100μF decoupling capacitor; a 0.01-0.1μF decoupling capacitor should be connected between the VCC port of each chip and the ground, which can also effectively suppress noise interference. Practice has proved that the above methods are more effective in suppressing noise and interference on circuit boards with signal frequencies below 10 MHz.

2.2 GPS module peripheral circuit

After the GPS module TIM-LH is powered on, it sends GPS data through the serial port. The data formats include RMC (Recommended Minirnum Specific GNSS Data), GGA (Global Positioning System Fix Data), etc. You can select the corresponding format of data according to your needs. The default baud rate of the serial port 1 of the TIM-LH chip is 9 600 bps, and the default baud rate of the serial port 2 is 57 600 bps, which can be changed by software; if a serial port is not used, the corresponding serial port input terminal should be connected to a pull-up resistor.

The schematic diagram of the peripheral circuit of TIM-LH is shown in Figure 4. The external reset port REXET_N is effective at low level and is very sensitive. If it is not used, leave it empty. It cannot be connected to a high level. The TIM-LH module automatically resets after power-on. Usually, the reset signal is used when the module is not working properly. In the design, it can be reset by controlling the power supply. This will make the control more flexible and reliable. In addition, the module rarely freezes. If you want to restart, you can also control it through the program using commands. When designing the PCB, the area around the GPS chip should be copper-clad to shield external interference.

2.3 GSM/GPRS module and processor interface design and SIM card interface circuit

The GSM module and the ARM chip communicate via a standard serial port, with a maximum baud rate of 115 200 bps. The output level of the serial port of the processor LPC2214 is 3.3V, and a level conversion is required in the middle, which is achieved by using MAXIM's MAX3232. The serial port communication between LPC2124 and the GSM module uses the simplest three-wire connection method, that is, the three ports of ground, send data and receive data are connected, and the receive data and send data ports must cross each other.

The GSM module and SIM card mainly communicate data through the SIMCLK and SIM-DATA signal lines. The schematic diagram of the SIM card interface circuit is shown in Figure 5. When designing the PCB, it should be noted that in order to improve stability, the distance between the GSM module connector and the SIM card holder pin should not exceed 20cm. In addition, to achieve the best effect, copper should be applied under the SIM bracket, and the copper should be connected to the GND pin of the SIM card; the capacitors C101 and C102 between SIMVCC and SIMGND should be as close to the pin as possible to meet the specification requirements. When choosing a SIM card, choose one that supports GPRS services and activate GPRS services.

2.4 Key points of LPC2214 peripheral circuit design

When designing the LPC2214 peripheral circuit, pay special attention to the use of the P0.14 pin. The Flash boot loader code is executed when the device is powered on or reset. The loader can execute the ISP command processor or the user application code; the low level of P0.14 after reset is considered as an external hardware request to start the ISP command processor. If P0.14 is sampled to a low level and the watchdog overflow flag is set, the external hardware request to start the ISP command processor will be ignored. If there is no external request (P0.14 is sampled to a high level after reset), a valid user program will be searched. If a valid user program is found, the execution control is transferred to the user program; if not found, the automatic baud rate program is called.

When pin P0.14 is used as the ISP hardware request, since P0.14 is in high-impedance mode after reset, the user needs to provide external hardware (pull-up resistor or other device) to put the pin in a certain state; otherwise, it may cause unexpected entry into ISP mode.

In addition, since an external crystal oscillator is used, the crystal oscillator should be placed as close as possible to the oscillator input and output pins of the chip during PCB design to reduce the impact of external interference.

3 Hardware circuit debugging

When debugging the circuit board, the focus should be on overall and global wiring error troubleshooting, such as short circuit and wrong connection of the power line. When debugging the serial port, the serial port line on the circuit board can be led out, and the various serial ports of the terminal can be debugged through the serial port communication with the computer . It should be noted that the serial port of the computer is a standard RS-232 serial port. Among them, the debugging of the GSM/GPRS module is completed through AT commands. After the serial port debugging is completed, the software program can be designed to test the performance of the circuit. Figure 6 describes the positioning signal received by the GPS module.

4 Conclusion

This article introduces the hardware design of the in-vehicle navigation service terminal based on ARM. The terminal software is recommended to be written in C language, which can be developed from the bottom up by yourself or

The μClinux operating system is tailored for more advanced development, so that the terminal has more powerful functions and a more friendly interface. The experiment proves that the hardware design of the terminal is feasible. It only takes 5 seconds to log on to the network, the data download speed can reach 38.4kbps, and the upload speed can reach 19.2kbps. With the further improvement of GPRS services, the vehicle navigation service terminal will inevitably develop in the direction of supporting remote electronic map updates and real-time image communication, so the terminal will have broad application prospects.