Design of automotive "black box" based on ARM

Figure 5 Hardware connection between HOLUXGR285 and TMS470R1A256

3.6 Communication between TMS470R1A256 and Benq M22

Benq M22 is an embedded TCP/IP protocol stack that supports GSM/GPRS CLASS4, can work at EGSM900MHz, DCS 1800MHz, PCS 1900MHz, and can support transparent and non-transparent transmission of data services. In terms of packaging, M22 uses a connector interface, and the module comes with an MMCX antenna connector. Because it supports GPRS and even has embedded TCP/IP, it can be used in car driving recorders with high real-time requirements, relatively large data volumes, and relatively fast transmission speeds.

The communication data interface between Benq M22 and the host computer is UART, the level is TTL/CMOS, and the baud rate is a nominal adaptive baud rate of 300~115200bps. As long as it is the nominal baud rate in this range, the module automatically recognizes it without user intervention, and it can be connected to a full serial port or a half serial port for communication. The so-called full serial port means that all nine lines of DB9 need to be connected, and the so-called half serial port only connects RXD, TXD and GND for communication. This system adopts half-duplex mode, so the module's RXD can be directly connected to the TXD of TMS470R1A256, and the module's TXD can be directly connected to the RXD of TMS470 R1A256, as shown in Figure 6.

Figure 6 Hardware connection between M22 and TMS470R1A256

4 System Software Design

After the system is powered on, it will be initialized first, and then self-checked, including data storage, clock and I/O ports. After the self-check, the relevant information will be displayed on the monitor according to the self-check results.

If the self-test fails, "Self-test abnormal" and an error code are displayed.

After the self-check is passed, the driver identity authentication is entered. After passing, the LCD displays the relevant data and waits for the data to be refreshed. The main program flow chart of the system is shown in Figure 7.

Figure 7 System main program flow chart

4.1 Data Collection

After the recorder reads the raw data, it can calculate the total accumulated mileage, the accumulated mileage in the last two calendar days, the accumulated mileage in the last 360 hours, the speed every 200 milliseconds and the speed per minute based on the pulse volume and vehicle characteristic coefficient.

The pulses collected by the recorder come from the speed sensor, which generates 8 pulses per revolution. The number of revolutions that the vehicle makes the speed sensor rotate per kilometer is determined by the vehicle characteristic coefficient. The recorder can obtain the mileage of the vehicle based on the number of pulses recorded.

The driving speed is calculated from the number of pulses. Just calculate the mileage from the number of pulses and then divide it by the time to get the speed.

4.2 GPRS Design

M22 supports GPRS CLASS4, which means the downlink speed is 42.8 Kbps and the uplink speed is 21.4 Kbps. At the same time, M22 also supports the embedded TCP/IP protocol. This embedded TCP/IP protocol solves a relatively complicated process from command mode to data transmission mode.

Because of the embedded TCP/IP, a large amount of car record data can be transmitted in real time, and the operation of the vehicle can be monitored in real time. When using the TCP protocol for data communication, it must go through three stages. The first stage is the connection establishment stage, the second stage is the data sending and receiving stage, and the third stage is the connection release stage.

The PDU mode is used to send short messages in this system, because it not only supports Chinese short messages, but also English short messages. There are three encodings for sending and receiving short messages in PDU mode: 72bit, 82bit and UCS2 encoding.

5 Conclusion

This design adds GPS and GPRS modules to the traditional recorder, and the result of testing the system is that it can realize functions such as precise positioning (civilian standard), remote monitoring, electronic dog and data transmission, realizing the integration of basic recorder and GPS navigation, and meeting the design requirements of the system.