The car black box is the car driving recorder, which is used to record the state of the car while driving. With the successful use of black box on the plane, more and more countries are using this technology in cars. As early as 1990, the European Community passed legislation to install black boxes on cars, requiring the 15 member states of the European Community to install the device in 9 million commercial vehicles in use within 10 years. Cars and black scorpions have also been widely used in countries and regions such as the United States, ***, Hong Kong and Malaysia [1]. From the perspective of the use effect, the car black box can not only accurately reflect the state of the car at the time after the accident is released, but also take a significant effect in preventing car accidents. On October 1, 2004, China introduced a new "Safety Technical Conditions for Motor Vehicle Operation" (GB7428-2004), which clearly requires passenger cars, heavy-duty trucks and semi-trailers used for road operations to be installed and used. National standard car tachograph. This article refers to the address: http:// At present, there are more than 60 enterprises that have passed the national standard test for the production of automobile black box. Two-thirds of them are OEM or sell other products. Only 18 companies with real technology, funds and production lines are compared. Famous is Sichuan Da Kexing, Beijing Weihang, Huizhou Ruishen, etc. However, most of its car black box uses 8-bit or 16-bit single-chip microcomputer as the processor, using assembly language, and the application program is relatively simple. Although some companies have developed the second-generation car black box, the function is also more, but it still does not use the operating system to the above, so there is still a defect in scalability and stability. In foreign countries, Russia's newly developed car black box can be used after the accident, the physical changes in vehicle space movement will be clearly recorded by the "black box", thus helping people to analyze how the accident occurs, how the car can be improved in the future. In terms of embedded systems, ICE in Europe has successfully applied embedded systems to car black boxes. 1 The overall structure of the system 1.1 Demand Analysis According to the latest standards of the country's automobile driving recorder: the vehicle driving recorder can collect, record, store, travel speed, mileage measurement, recording, storage, etc. of the real-time clock, date and driving time, and data for accident suspects. The recorder shall continuously record and store the vehicle travel speed value corresponding to the real time of 20 seconds before parking and the vehicle brake status signal at intervals of no more than 0.2 seconds [2]. 1.2 System structure The main hardware used in this system is: 32-bit Samsung S3C44B0X processor, signal processing circuit, RS232 standard interface and USB1.1 standard interface, 4M Flash, 8M SDRAM, 4K FRAM, operating keyboard and expansion interface. The operating system uses the uClinux operating system. The system structure is shown in Figure 1. 1.3 Structure Description 1, 32-bit Samsung's S3C44B0X processor, built-in RTC (real-time time chip) can realize the date and clock of Beijing time, the date and clock are used to mark all functions (recording, storage, output, etc.) for the recorder The date and clock, along with its powerful other features, provide excellent support for system applications and future upgrades or extensions. 2. The signal processing circuit processes the signals collected by the car black box to meet the requirements of our system. 3, the system is equipped with RS232 standard interface and USB1.1 standard interface, using USB interface to complete the communication of data from the car black box to the computer, making the communication between data more convenient and rapid. 4, the main storage unit is 4M Flash used to store uClinux operating system and recorded data, 8M SDRAM and a 4K FRAM. When the car is running, the recorder continuously records and stores the vehicle traveling speed corresponding to the 20s real-time time before the parking and the 8 semaphores of the over-braking for recording at a time interval of 0.2 s, and stores it in the FRAM memory. These data will be used as accident suspect data. At the same time, the recorder will sample from the FRAM at an interval of 0.5 min, and store the result of the sampling into the flash memory. 5, the software uses uClinux operating system, can better meet the system stability, scalability and the diversity of application procedures. 2 key technologies 2.1 Signal data acquisition and processing In order to reduce the cost and achieve practicability, the signals used in this system are taken directly from the various sensors on the car. The main signals collected are the vehicle speed signal, the brake signal and the signal of each light. 2.1.1 Collection and processing of vehicle speed signals The acquisition of the vehicle speed signal of this system is the signal on the ABS wheel speed sensor on the car. At present, the ABS sensor wheel speed sensor widely uses a variable reluctance electromagnetic sensor. It is found through research that the signal emitted by the ABS sensor is a sine wave signal, and the frequency of the signal is proportional to the speed of the car. Therefore, we need to measure the frequency of the signal. Converted to the corresponding voltage value and then sent to the A/D converter. At the same time, because the car has a very large interference to the signal sent by the ABS sensor while driving, it is necessary to process the signal before the frequency conversion. The processing of the signal is (1) through the limiting circuit. The faster the wheel speed, the larger the amplitude of the sine wave signal. Therefore, the amplitude of the signal is limited to within plus or minus 5 by two Zener diodes. (2) Pass the filter circuit. Since the car generates a large high-frequency interference signal when it is running, it uses two An active filter with step feedback to remove high frequency interference signals. (3) Converting the sine wave signal into a square wave signal through a shaping circuit. (4) Finally, the frequency conversion circuit is passed. The conversion process is shown in Figure 2. 2.1.2 Acquisition and processing of brake signals and individual light signals Braking and lighting signals on the car are collected directly from the brake circuit or lighting circuit on the car. The output is a ground-frequency signal with a voltage of 12V, so we achieve resistance through an opto-isolator. Interference and regulation. 2.2 USB interface design USB is the Universal serial bus, which is a fast and flexible bus interface. Compared with other communication interfaces, the most prominent feature of the USB interface is its ease of use and fast transfer rate. So the USB interface is being adopted by more and more systems. In order to realize the system, the project transmits the data stored in the car black box record to the computer or PDA through the USB interface. We consider the whole system as a U disk, so that the data can be transmitted conveniently and quickly. The USB organization defines a series of device classes and device subclasses according to different attributes of the device. The advantage of combining these devices of the same attribute is that the PC host-based driver can be developed at the same time. Currently, the following classes are defined in USB: Audio, Communication, Display, Human Interface, Mass Storage, Image, Printer, Power, PC Legacy, Physical interface. Because what we need to do in this project is to treat the entire system as a USB flash drive for data transfer, we need to follow the Mass storage protocol. At the same time, the Mass storage protocol is supported in Microsoft Windows, so USB mobile devices only need to follow the Mass storage protocol to organize data and process commands, which can realize data exchange with PC. At the same time, the FAT16 file system is used in the storage organization of Flash, so that data can be exchanged directly through the removable disk in the Windows browser. The USB organization now defines two specifications for the Mass Storage protocol transfer: 1. USB Mass Storage Class Control/Bulk/Interrupt (CBI) Transport 2. USB Mass Storage Class Bulk-Only Transport. However, CBI cannot be used for high-speed equipment, so the second transmission method, Mass Storage Class Bulk-Only, is used in our system. In this transfer mode, you need to pay attention to the setting of some values ​​in the interface descriptor. Set the value of bInterfaceClass to 08, which means that the device is a Mass storage Class. Set the value of bInterfaceSubClass to 06, which indicates that the USB device follows the SCSI protocol during transmission. This is because the General Mass Storage Device is a random access, block/sector storage based device. It can only store and retrieve data from the CPU. The interface of this device follows the SCSI-2 standard Direct Access Storage Device protocol. The value of bInterfaceProtocol is set to 50, that is, the USB device is in Bulk-Only Transport mode. When the device is plugged into a USB, USB searches for the device and asks the device to provide the appropriate descriptor. After the USB Host obtains the above descriptor, the configuration of the device is completed, the Mass Storage device that is Bulk-Only is identified, and then the Bulk-Only transmission mode is entered. In this mode of transmission, there are three types of data that are transferred between the USB and the device, CBW, CSW, and normal data. The Command Block Wrapper (CBW) is a command sent from the USB Host to the device. The command format follows the command block specified by bInterfaceSubClass in the interface. This is the SCSI transfer command set. The USB device needs to extract the SCSI command from the CBW and execute the corresponding command. After the completion, the host sends a response to the host. The Command Status Wrapper of the command execution status, Host determines whether to continue sending the next CBW or data according to the CSW. The host asks the USB device to execute the command to send data. In this case, the specific data needs to be transmitted. After the completion, the CSW is issued to enable the Host to perform the next step. The operations performed by the USB device can be as shown in Figure 3. What needs to be noted here is that the read and write feature of Flash is that it can be read randomly, but it cannot be written randomly. Flash must be erased before writing, and a wipe must erase a block or a sector, so you must save the data in the same block or sector before erasing a block or sector. If the data is erased, saved, and written every time the data is received, the system task will be very heavy and cannot respond to the request of the USB Host in time. Therefore, this needs to be solved by setting a buffer of a certain size. 3 uClinux operating system In the design of embedded systems, the choice of operating system is crucial, and different operating systems are required for different system requirements. The system is mainly considered stability and security, so the uClinux operating system is used. The uClinux operating system is basically the same as the Linux operating system. (1) It is a license-free fee with high stability, safety and tailorability. (2) Support powerful network functions, which makes our system more scalable, such as implementing GPRS and other network functions in the future. (3) It has rich open tools, providing C, C++, JAVA and many other developments. Tools, this provides great convenience for the development of our application. (4) There are a large number of documents for our reference. The biggest difference between them is that uClinux is specially designed to meet the processor without MMU, that is, uClinux can not use the virtual memory management technology of the processor but adopts the real memory management strategy. There are three main aspects to be aware of during the migration process. 1. If the processor core is not supported by uClinux, this requires us to add a new architecture to uclinux/linux-2.4.x/arch/. This workload will be huge. But in our system the ARM7TDMI architecture is already in the uclinux/linux-2.4.x/arch/armnommu/ directory. 2. Find a configuration file for the processor that is close to your processor in the same architecture directory to make it suitable for your processor. If you don't have the right one, you need to write it yourself. The same amount of work is also quite large. 3. If some peripheral circuits of this system are not supported by uClinux, you need to write your own device driver. 4 Conclusion This car black box is built on the 32-bit Samsung S3C44B0X microprocessor core and uClinux operating system embedded system, which can better meet the system stability and the diversity of application procedures, so it can be more complete, accurate and diverse. The record of the driving state of the car provides an accurate basis for the analysis of the accident, and it also plays an important role in suppressing traffic accidents. In addition, in this system, we make the whole system into a U disk, so that the recorded data can be conveniently and quickly transferred to a computer or PDA.
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