Design of an embedded visual reversing device

At present, the reversing device commonly used in my country is ultrasonic reversing radar. Although this device can accurately measure the distance between the rear of the car and the obstacle behind the car, due to the visual blind spot, the driver cannot determine the exact location of the obstacle, let alone perceive pits or low obstacles. The research trend at home and abroad is to use digital image processing technology on the basis of reversing radar, and use powerful embedded processors to develop a vehicle-mounted visual reversing device that combines the advantages of detecting the distance to the object behind the car and monitoring the image behind the car. This type of new device is relatively expensive and is currently only used on mid-to-high-end cars. To this end, a vehicle-mounted visual reversing device based on the Intel PXA270 hardware platform and embedded Windows CE operating system is proposed.

1 System Structure

The hardware circuit diagram of the visual reversing device proposed in this paper is shown in Figure 1, which is mainly composed of Intel embedded processor PXA270, video acquisition, ultrasonic ranging and other circuits. Two pairs of ultrasonic transducers, signal conditioning circuits and microcontrollers complete the distance measurement of obstacles and send them to the main system in the car through the LIN bus; the video composite signal collected by the camera is sent to the video decoding chip through a coaxial cable for A/D conversion, and the video signal in YUV422 format is generated and input into the fast capture camera interface of PXA270; the main system processor uses character overlay technology to play the image signal containing parameters such as obstacle distance on the TFTLCD screen.

1.1 Embedded Processor PXA270

The embedded processor PXA270 used in this system has a maximum main frequency of 624MHz and has added WirelessMMXTM technology to improve multimedia processing capabilities. It also adds IntelSpeedStep dynamic power management technology to minimize the power consumption of mobile devices while ensuring CPU performance; and has a variety of external interfaces: such as AC'97 controller, LCD controller, CIF interface, SD card interface, etc.

1.2 Video Capture Circuit

The CIF interface of the main processor PXA270 in the car can only process digital signals, so the analog signal collected by the camera must be converted first. The video input decoding module of this system is constructed using TI's video decoding chip TVP5150 and peripheral circuits. Its main function is to send the standard PAL TV analog signal collected by each CCD camera to the video decoder, complete the pre-processing of video image clamping and anti-aliasing filtering, conversion of analog video signals to digital YUV4:2:2, and separation of brightness/chroma, horizontal/vertical synchronization and other signals. The decoding chip and PXA270 are connected through the CIF interface. PXA270 accesses the internal registers of TVP5150 through the I2C bus to coordinate the working communication between the processor and the decoder.

1.3 Ultrasonic ranging circuit

The distance measurement circuit is mainly composed of ultrasonic transmitting circuit and receiving circuit, and the principle block diagram is shown in Figure 2. The module single chip uses Freescale's MC68HC908QL4, which has high reliability and strong anti-interference ability. It contains 4kB flash memory, four-channel 10-bit A/D converter, and integrated LIN controller. Ultrasonic waves detect the distance between the vehicle and the object, and transmit the data to the main processor in the car via the LIN bus. Since ultrasonic ranging only provides the driver with information behind the car when the car is reversing, and the speed of the car is slow when reversing, it can be considered as stationary compared to the speed of sound. Therefore, the pulse ranging method is used to measure the round-trip time between the ultrasonic wave and the target, which is simpler to calculate the distance.

(1) Ultrasonic transmitting circuit The ultrasonic transmitting circuit is shown in Figure 3. When the car driver turns the handle to the reverse gear, the ranging circuit starts to work, and the microcontroller sends out a 40kHz square wave signal, which is amplified by the driving circuit and transmitted through the probe. At this time, the counter starts counting.

(2) Ultrasonic receiving circuit The ultrasonic receiving circuit is shown in Figure 4. A special preamplifier CX20106 is used, which consists of a preamplifier, a limiting amplifier, a bandpass filter, a detector, an integrator, and a shaping circuit. The preamplifier has an automatic gain control function. When the ultrasonic signal propagates in the air and encounters an obstacle, it is reflected. The echo is received by the receiving probe and converted into an electrical signal. It is input into the external interrupt port of the microcontroller through a cable, so that the counter stops counting, thereby calculating the distance of the obstacle.

1.4 LIN bus transceiver interface circuit design

LIN is a low-cost serial communication network used to realize distributed electronic system control in automobiles. It is an auxiliary bus network that can greatly save costs in situations where the bandwidth and speed of the CAN bus are not required. LIN communication is based on the data format of SCI asynchronous serial communication, adopts a single master controller/multiple slave device mode, and only uses a 12V signal bus and a node synchronization clock line without a fixed time reference. TJA1020 is a commonly used LIN master/slave protocol controller and LIN bus physical interface chip. The interface circuit is shown in Figure 5.

2 Software Design

2.1 Customization of system platform

Windows CE is an efficient and scalable operating system designed by Microsoft specifically for various embedded systems and is widely used in various embedded products. To customize the Windows CE operating system of the device, first import the BSP board support package provided by Intel in the PlatformBuilder development environment according to the hardware configuration, and develop your own OEM hardware adaptation layer and components. After the operating system image is successfully established, the platform is transferred to the target device for testing. Finally, the software development kit is output to develop applications in the EVC++ environment.

The WindowsCE customization process is shown in Figure 6.

2.2 Camera driver design

The driver described in this article is implemented under Windows CE. The main task of the camera driver is to control the flow of video data in the hardware and provide a standard interface for the camera application. In order to simplify the programming difficulty, considering that the working mode of the CIF interface is relatively independent, a single-chip driver model of the stream interface driver type is adopted: that is, a dynamic link library containing the entry point of the driver is created for each stream interface driver, and the file I/O and power management functions are handed over to the kernel for use.

The flowchart of the camera driver is shown in Figure 7, and its working contents are as follows:

(1) Responsible for querying the information of the camera decoder through the I2C bus and adjusting the settings of the camera decoder;

(2) Establish and control the DMA transmission channel, and transfer the data information in the three FIFOs in the CIF interface to the memory through DMA, thereby achieving fast and high-quality data transmission;

(3) Provide an interface that can be used by applications.

2.3 Ultrasonic ranging software design

The ultrasonic ranging software mainly includes ranging and data sending, and its flow chart is shown in Figure 8.

3 Results

After the successful joint debugging of software and hardware, a reversing test was carried out on an actual vehicle. The video signal actually captured by the camera is displayed on the LCD as shown in Figure 9. The numbers in Figure 9 are ultrasonic ranging data. The test shows that the system operates reliably. When reversing, it can not only clearly display the panoramic view behind the car in real time, but also accurately measure the distance between the car and the obstacles behind the car, which basically meets the design requirements.