Interface circuit design of reversing radar unit

In the design, the reversing radar uses the PIC18F258 microcontroller as the control core to control the operation of the ultrasonic transmitting and receiving circuit and the temperature correction circuit. The PIC18F258 microcontroller itself has a CAN bus, which can reduce the complexity of the circuit. The ultrasonic transmitting circuit emits 40 kHz ultrasonic waves. After being reflected by the obstacle, the probe receives the reflected wave. The microcontroller calculates the distance between the obstacle and the car based on the difference in transmitting and receiving time, and sends it to the instrument unit in the form of a message through the CAN bus and displays it. It can also be sent to the audio unit for voice alarm. When the distance is less than a certain value, the corresponding braking unit will automatically brake. The temperature compensation circuit uses the digital temperature sensor DS18B20, which uses the relationship between the speed of sound and temperature to correct the speed of sound, thereby eliminating the impact of temperature changes on the sound.

Interface circuit design of reversing radar unit

There are generally two forms of hardware components of CAN units. One form is a microcontroller with integrated CAN controller plus a transceiver; the other form is a general-purpose microcontroller plus an independent CAN controller plus a transceiver. This design adopts the former form, which does not occupy the port resources of the microcontroller and can simplify the design of the interface circuit. Considering that cars work in a very special environment, we chose Mierochip's PIC18F258 microcontroller, which meets the temperature range of the car and has high cost performance. The chip integrates a CAN controller, which can simplify the hardware design of the system and improve the system's efficiency. reliability. The hardware circuit of the reversing radar unit and CAN bus interface is shown in Figure 3, which is mainly composed of PIC18F258 microcontroller, 6N137 high-speed photocoupler, and PCA82 C250 bus transceiver.

In order to improve the system's anti-interference ability and signal transmission ability, the 6N137 high-speed photoelectric coupling circuit can be used to achieve electrical isolation between units on the bus. The two power supplies Vcc and V'cc of the high-speed optocoupler must be completely isolated using a power isolation circuit.