Intelligent wireless communication promotes the application solutions of automobile safety system

　　Intelligent wireless communication
　　  requires automatic operation, that is, the user does not need to press any buttons, the system can detect or send signals by itself, 100% independent, can self-learn and adapt in different environments, and can eliminate noise and work normally in noisy environments.

　　The above intelligent wireless communication system has many requirements. The first requirement is small size and low cost. The solution is implemented by an intelligent single-chip microcomputer, which is a combination of digital and analog front ends into one chip. The second requirement is economical two-way communication. The base station command is sent at a low frequency of 125KHz, and the high frequency response is used. The cost of sending at a low frequency is gradually reduced. The third requirement is that the communication distance is more than 2 meters, and its transponder has a high input sensitivity of about 3 millivolts. It works in a noisy environment. Because there is a lot of noise interference in a general environment, it is very important to have a high sensitivity when designing the system. In addition, the direction of the antenna is eliminated, because the control signal cannot always be sent from one direction, especially for portable units. The direction of transmission cannot be controlled, so three-directional antennas XYZ are used on the transponder board, and the signal can be received no matter which direction it comes from. The third requirement is the battery life, because some batteries are used for tire pressure detection systems in cars, and it is impossible to open and replace the battery every 6 months, so a wake-up filter is used to reduce current usage. Finally, there is a data security requirement. The sent signal is encrypted and decrypted when the signal is received. There are many encryption and decryption algorithms. Microchip uses the Keylock algorithm.

　　Figure 1 shows an intelligent passive keyless entry system. The system shown in the figure has similarities and differences with the commonly used system. The base station on the left is composed of a single-chip microcomputer and a high-frequency transmitter and a low-frequency transmitter and receiver. The base station sends a low-frequency command of 125KHz. When the intelligent receiver on the right receives the signal, it will process the signal. If the signal meets certain requirements, it will use high frequency or low frequency as a response. The intelligent receiver has three receiving directions XYZ. No matter which direction the signal comes from, it can receive the signal, and the user does not need to press any buttons. Such an intelligent receiver can automatically receive, send and process signals.
 

　　Figure 2 shows the schematic diagram of the PKE transponder. The PIC16F639 in the figure is composed of PIC16F636 and MCP2030, where MCP2032 is the analog front end and PIC16F636 is another single-chip microcomputer. The main reason for using PIC16F636 and the analog front end together is that PIC16F636 has the function of Keylock encryption and decryption. If the user does not need the encryption and decryption function, the 2030 analog front end and other single-chip microcomputers can be used.

　　Application Examples

　　There are many uses of smart transponders in automotive system applications, such as smart vehicle access systems, engine immobilizer systems (as shown in Figure 3), and tire pressure monitoring systems (TPMS).

　　Smart PKE transponders are not only suitable for use in cars, but can also be used in other places, such as garage door opening and closing, public parking lots. If many cars have smart transponders, the doors will automatically open when the car approaches the parking lot. [page]

　　The display group of the tire pressure detection system (as shown in Figure 4) is mainly composed of three units: one is inside the tire, which is composed of a smart single-chip microcomputer, a tire pressure sensor and a high-frequency transmitter in the lower left corner of the figure; the base station is above the right corner, which is mainly composed of a single-chip microcomputer and a high-frequency receiver; the low-frequency trigger is in the lower right corner, which is generally placed on the body part very close to the tire. When in use, the low-frequency trigger will send a start command to the tire unit every 3 or 4 seconds. When the signal received by the smart single-chip microcomputer in the tire meets the requirements, it will tell the tire pressure sensor to measure the temperature and tire pressure of the tire, and then the high-frequency transmitter will send the tire pressure data to the base station.

　　Programmable digital wake-up filter

　　The purpose of using the wake-up filter is mainly to reduce the operating current, thereby extending the battery life. In general, the digital part is kept in sleep mode to achieve the lowest current usage. The analog front end keeps looking for input signals, and only when the predetermined waveform is reached, that is, the input signal meets the requirements, the analog front end will wake up the filter.

　　Intelligent passive keyless entry (PKE) system design

　　Figure 5 shows a transponder circuit with no battery and backup battery. In some cases, if the battery contact is not good, the system will have no power. The magnetic field can be used to power it briefly, so that the transponder can still work without a battery.

　　The system working requirements are that the transponder needs low-frequency wires, high-frequency transmitters, and some system optional backup electronic circuits. In addition, there must be an intelligent single-chip microcomputer and single-chip microcomputer components; the base station system requires a low-frequency transmitter, a high-frequency receiver, an antenna, a single-chip microcomputer, and the firmware part of the single-chip microcomputer.

　　There are some parameters for the two-way communication distance. The transponder requires antenna tuning and Q, antenna positioning uses a three-dimensional antenna, receiving sensitivity, and modulation depth of the output signal; the base station requires output power and receiving sensitivity.

　　The low frequency of the antenna design generally uses 125KHz, and now uses an LC resonant circuit; the antenna type uses an air-core coil or a ferrite core. The resonant frequency of LC is the same as the carrier frequency of the base station. The range of passive tags is about 1 meter, and the active tag is about 5 meters. The high frequency ranges from 315MHz to 960MHz, the most common are 315MHz and 433MHz, and is engraved on the PCB using dipole wires. The range is relatively much higher, with passive tags being about 5 meters and active tags being about 100 meters.

　　Figure 6 shows a formula for the relationship between magnetic flux and antenna induced voltage. This mainly explains that many factors are considered when determining the induced voltage: for example, the number of turns of the coil, the surface area of ​​the contactor coil, the frequency, and the angle of the receiving wire and the transmitting antenna will all affect the voltage induced by the antenna.

　　Figure 7 shows the relationship between an antenna's induced voltage and distance. The large figure shows that when the base station and receiver are very close, the signal voltage is 200V. The small figure shows that when the distance is 3 meters, the voltage signal only reaches a peak of 5 millivolts. It can be seen that the sensitivity of the signal input is very critical here.

　　We can summarize that an intelligent wireless communication system requires reliable automatic operation, including intelligent two-way communication, low system cost, high sensitivity of low-frequency input (this is more critical), low power consumption and secure data encryption and decryption. The conclusion is that building a system with an intelligent microcontroller can meet all requirements and can therefore be used as a reliable solution.