Design of Parking Sensor Based on TI MSP430

With the improvement of the intelligent requirements of the vehicle assisted driving system and the network development of the automotive electronic system, the new parking sensor should be able to continuously measure distance and display the obstacle distance, and have a communication function to send data to the bus. .

In the past, the reversing radar design used more components and functions. The reversing radar based on the new high-performance ultra-low-power single-chip MSP430F2274 introduced in this paper can make up for the shortcomings of the previous products.

Overall system design

The system uses the principle of ultrasonic ranging. Ultrasonic ranging instruments generally consist of a transmitter, a receiver and a signal processor. During operation, the ultrasonic transmitter emits an ultrasonic pulse, and the ultrasonic receiver receives the reflected wave reflected by the obstacle, and accurately measures the time from the emission of the ultrasonic wave to the reflection of the obstacle, and the obstacle can be calculated according to the propagation speed of the ultrasonic wave. distance. As a non-contact detection method, ultrasonic waves have the characteristics of small airborne attenuation, strong reflection capability and high penetration. Ultrasonic ranging has the advantages of being free from light, rain, snow and fog in a short range, simple structure, convenient production and low cost. High-performance single-chip microcomputer combined with ultrasonic ranging can realize powerful and easy-to-use reversing radar. TI's 16-bit microcontroller MSP430F2274 has extremely low power consumption and abundant on-chip resources. At the same time, JTAG interface technology can be used to facilitate on-chip flash memory programming and easy software upgrade. It is very suitable as a microcontroller for reversing radar system. The block diagram of the parking sensor system is shown in Figure 1.

Figure 1 Reversing radar system block diagram

Hardware system design

The system is based on the MSP430F2274 microcontroller. The peripheral circuit consists of five parts: ultrasonic transmitting circuit, ultrasonic receiving circuit, sound and light alarm circuit, communication interface circuit and keyboard liquid crystal display circuit.

Figure 2 Main control circuit diagram of reversing radar system

The main control circuit diagram of the system is shown in Figure 2. The MSP430F2274 selected in this system has 32Kb flash memory and 1Kb RAM, so there is no need to expand the memory. The external 32.768kHz crystal oscillator is used as the clock source for the CPU-off state Basic-Timer and is also used as the system's vehicle clock.

The ultrasonic transmission module circuit is shown in Figure 3. It consists of two parts: ultrasonic generation and emission. There are two ways to generate ultrasonic waves: hardware generation and software generation. The commonly used hardware generation method often adopts the following scheme: the ultrasonic wave is generated by the oscillation of the oscillator composed of CD4011, and the ultrasonic transducer is driven to be emitted by the boosting conversion, and the start-up and the stop vibration of the oscillator are controlled by the single-chip microcomputer. This design adopts the software generation method, because the software generation method can reduce the complexity of the hardware, reduce the cost of the system, and has the advantages of flexibility, easy implementation, and good stability. The system uses the timer function of the MSP430F2274 microcontroller to generate a stable PWM (40Hz) pulse wave, and outputs it to the ultrasonic transmitting part through the I/O port P2.3. In the ultrasonic transmitting circuit, the CD4049 includes a total of six non-gates. In Figure 3, only three lines are used. In order to prevent interference or electrostatic breakdown, the entire CD4049 is damaged, and the three non-gate strings on the unused side are used. Get up and do the grounding. When the control terminal outputs a series of fixed frequency pulses, the positive frequency and the reverse voltage of the fixed frequency are applied to the piezoelectric ceramic ultrasonic transducer UCM-40-T, and high-power ultrasonic waves are emitted, and the obtained waveform is better than the other. The effect is more ideal.

Figure 3 Parking sensor ultrasonic transmission module

The ultrasonic transmission module is electrically connected to the two parts of the ultrasonic generation and emission as shown in FIG. There are two ways in which ultrasound can occur: hardware generation and software generation. The commonly used hardware generation method often adopts the following scheme: the oscillation of the ultrasonic wave composed of CD4011 occurs, and the ultrasonic transducer is spurted by the boosting conversion, and the oscillation and the stop vibration of the oscillator are controlled by the single chip microcomputer. This design adopts the software generation method, because the software generation method can reduce the complexity level of the hardware, reduce the cost of the system, and has the advantages of being flexible, easy to implement, and invariant. The system manipulates the timer function of the MSP430F2274 microcontroller to generate a constant PWM (40Hz) pulse wave, and outputs it to the ultrasonic transmitting department via the I/O port P2.3. In the ultrasonic transmission, CD4049 contains a total of 6 non-gates. In Figure 3, only 3 lines are used. In order to prevent interference or electrostatic breakdown, the entire CD4049 is damaged, and the 3 non-doors on the side that is not utilized are strung together. Do grounding. When the control terminal outputs a series of fixed frequency pulses, the piezoelectric ceramic ultrasonic transducer UCM-40-T is fixed at a fixed frequency and a counter voltage, and a high-power ultrasonic wave is generated. The waveform is more ambitious than the results of other systems.

Figure 4 Parking sensor ultrasonic receiving module

Amplifier CX20106, which consists of preamplifier, limiting amplifier, bandpass filter, detector, integrator, and integral type. The preamplifier has an active gain control function, which can have a higher gain when the ultrasonic sensor receives a weak voltage from a far-reflecting signal, and the amplifier does not overload when the proximity input signal is strong. Adjust the external resistor R3 of the chip pin 5 and set the intermediate frequency of its filter to 40 kHz, which is a good result. When the collar receives a signal that matches the intermediate frequency of the filter, its output pin 7 outputs a low level, and the output pin 7 is directly connected to P2.2 of the MSP430F2274 to trigger a discontinuity.

Figure 5 reversing radar sound and light alarm circuit diagram

The alarm module uses a simple sound and light alarm circuit, as shown in Figure 5. First set a threshold value. When the distance between the tail and the obstacle is less than the set minimum distance, the red indicator light flashes and the green indicator light goes out. The MCU sends a PWM pulse to its port. As the distance decreases, the frequency of the flash and the buzzer is increased by controlling the duty cycle of the PWM pulse to alert the driver.

Figure 6 reversing radar communication interface circuit diagram

The communication interface circuit is shown in Figure 6. With Maxim's MAX3232 chip, the peripheral circuitry is very simple, requiring only five 0.1μF capacitors. The circuit isolates the serial port output signal of the single chip into an RS-232 signal and sends it to the bus, and can also realize the communication between the system and the computer.

Figure 7 Reversing radar keyboard display circuit diagram

The keyboard and display circuit are shown in Figure 7, which consists of a keyboard and a liquid crystal display. The keyboard adopts a free-standing button, and has three buttons, a setting button, a up button, and a down button. It is possible to set various parameters such as alarm value, working mode, and clock. The liquid crystal display circuit adopts ZJM12864BSBD, a low-power dot-matrix LCD. The display format is 128 dots (columns) × 64 dots (rows). It has multi-function instructions and is easy to use. It can display clock, distance and alarm prompts in real time. Information is convenient and intuitive.

System software design

The software adopts modular design. The French method consists of main modules, ranging sub-methods, keyboard display sub-methods, clock display sub-methods and other modules. During the debugging process, each function module and sub-method are used to debug the entrepreneurial policy of college students. After each sub-method has completed the designated functions, the four entrepreneurial teams of the University of China Venture Capital University of Ningbo have been invested by the venture capital fund of the business students, and then comprehensive debugging. The software flow of the system is shown in Figure 8.

Figure 8 system software flow chart

When the ultrasonic wave emits ultrasonic waves, the counter starts counting. When it encounters the obstacle feedback signal to the ultrasonic receiver, after the CX20106 receives the power, a low pulse occurs, and the signal is transmitted to the MCU for interruption and misappropriation. From the sub-method, the measurement distance is calculated, the display sub-method is used, and the distance and the alarm are displayed on the liquid crystal according to the measured values ​​of the divergence. When the distance is greater than 2m, the display shows “safe” and the measured distance, and continues to measure; when the distance is less than 1m, “” and the measured distance are displayed, and the buzzer is driven by the P1.2 bit; when the distance is less than 2m and greater than 1m, the display is displayed. “Focus on” and measure distance. In order to strengthen the anti-interference ability, the system continuously transmits ultrasonic waves three times, from which the maximum and minimum values ​​are removed, and the center value is compared with the exact distance. If you can measure about three data per second, you can still meet the timeliness requirements.

Conclusion

The reversing radar adopts the high-performance MSP430F2274 single-chip microcomputer, and fully manipulates its on-chip capital to make the system functionful. The peripheral chip used is reduced, and the system reliability is improved. The reversing radar is used in a car. When the driver reverses the vehicle, the distance from the obstacle is known from the liquid crystal display, and the error of the rear view mirror is small and the field of view is narrow, which eliminates the hidden trouble caused by the reverse.