Application design of multi-channel low-frequency driver ATA5279 in PKE system

Preface

The main anti-theft methods in the automotive market include engine immobilizer (IMMO), remote keyless entry (RKE), keyless entry (PKE), electronic steering column lock, two-way smart key and GPS satellite positioning, among which IMMO and RKE are the most widely used. Keyless entry system (PKE) technology is developed on the basis of the fairly mature RKE, integrating IMMO and RKE functions. As a new generation of anti-theft technology, PKE is gradually growing and has gradually entered the mid-range car market such as Ford Mondeo and some models of Nissan Teana from the high-end car market such as Mercedes-Benz and BMW. The detection accuracy and system security of the in-vehicle area are important indicators for measuring the performance of the PKE system. This paper uses Atmel's six-channel low-frequency driver chip ATA5279 to transmit low-frequency signals and uses a three-dimensional omnidirectional antenna to receive to achieve accurate positioning of the in-vehicle area.

Working principle of PKE system

The PKE system mainly consists of three parts: body base station, low-frequency antenna and electronic key. The identity of the electronic key is verified through two-way interactive authentication. The vehicle body base station adopts an active working mode, and its behavior does not depend on the instructions of the electronic key. It combines the trigger activation system authentication and area detection of the vehicle body micro switch to decide whether to open the car lock or other actions.

Low-frequency signal wake-up: When the user carries the electronic key and is within the coverage of the low-frequency antenna signal and gives a trigger signal such as pulling the door handle, the vehicle body host sends a coded low-frequency message through the low-frequency antenna. The electronic key receives the low-frequency message through the three-dimensional antenna and verifies the data information. If it matches the data stored in the key, the key will be awakened.

Radio frequency signal verification: After the key is recognized and awakened, the authentication password sent by the vehicle body host will be analyzed, the data will be encrypted using the HITAG2 algorithm and sent back to the host via radio frequency signals. The host compares the received data with the internally calculated data. If the verification matches, the door lock will be unlocked. The authentication process can be completed in tens of milliseconds, and the owner will not feel any lag.

After the user enters the car, he only needs to press the start button and the car engine will start. The verification process at startup is roughly the same as the door opening process, but when starting the engine, the system needs to verify whether the person carrying the key is in the main driving area to prevent children from triggering it by mistake.

System structure

System overall design

PKE system structure is shown in Figure 1. The vehicle body base station control unit uses the 8-bit HCS08-MC9S08DZ60 in LQFP64 package. The MCU has an embedded CAN controller, with 2 SCI, 1 SPI peripheral interface, and an external TJA1040 interface as a CAN Node; the low-frequency communication module uses Ateml ATA5279 to drive 6 separate low-frequency coils and send 125KHz low-frequency signals to achieve close-range communication with the key; the RF receiving module uses MC33596 to quickly receive encrypted and authenticated data transmission between the key and the vehicle body, and the base station chip near the ignition lock uses PCF7991; the vehicle body base station unit uses a 12V power supply and a linear regulator TLE4275 to obtain a 5V power supply to power the system. The electronic key end uses PCF7952 to receive the low-frequency signal from the vehicle body through a three-dimensional antenna, and the receiver control logic analyzes whether to wake up the system, and sends the encrypted data to the vehicle body base station through the RF transmitting unit PCF7900.

Vehicle body base station circuit design

The vehicle body hardware circuit is mainly composed of power supply circuit, main control MCU circuit, CAN node, RF circuit, low-frequency circuit and engine anti-theft locking circuit.

1. CAN bus circuit

In the CAN node design, Philips' high-speed CAN conversion chip TJA1040 is used. MC9S08DZ60 has a built-in CAN bus control unit. The TXCAN and RXCAN of the CAN module are connected to the TXD and RXD of TJA1040 respectively, and STB is connected to the I/O port as the transceiver control signal.

2. RF receiving circuit

The RF circuit receiving part uses Freescale's low-power RF receiving chip MC33596. In the system, MC33596 uses FSK and Manchester encoding to receive the RF signal of the electronic key. MC33596 uses SPI to communicate with MC9S08DZ60. MC9S08DZ60 has a built-in serial peripheral interface module, which is easy to connect. It is used to receive the RF signal sent by the electronic key.

3. Low-frequency transmission

circuit The low-frequency transmission circuit uses Atmel's ATA5279 to drive the low-frequency antenna of the vehicle body. It has excellent EMC performance, thermal and electrical protection for the load, and very low shutdown mode current. ATA5279 can drive six antenna coils at the same time, connected to the controller through the SPI interface, and the main control chip MC9S08DZ60 communicates with it through the analog SPI interface. When realizing regional judgment, ATA5279 can provide 20-stage power management for signal strength detection. The circuit is shown in Figure 2, and the six low-frequency antennas are placed at different positions on the vehicle body. [page]

Figure 1 PKE system structure diagram

Figure 2 Low-frequency signal transmission circuit diagram

Electronic

key circuit The electronic key circuit consists of a central control module, a radio frequency transmission module and a three-dimensional low-frequency receiving module. The radio frequency transmission module transmits remote unlocking and locking commands and signal strength information. The car keyless entry and keyless engine start functions are realized by low-frequency reception and radio frequency transmission.

1. Low-frequency receiving circuit

The main control chip used by the key is PCF7952 produced by Philips. It includes a security authentication chip, a highly sensitive three-dimensional low-frequency interface, and supports received signal strength indication (RSSI) to determine the location of the ID device. The on-chip remote unlocking code generation is implemented in the hardware authentication calculation unit. Support FLASH programming. The three-dimensional low-frequency omnidirectional antenna ensures that as long as the key is in the same position, no matter how it is placed, the field strength value of the position is unchanged.

2. Radio frequency transmission circuit

When the low-frequency verification is passed, the radio frequency module on the key side should respond accordingly. In order to ensure that the system can have high transmission power and receiving sensitivity under low current consumption, the system uses NXP's highly integrated, single-chip transmitter PCF7900. The data rate can reach 40kbps. PCF7900 is connected to PCF7952 through a three-wire serial port (SPI), and SDIO, SCK, EN, and CLOCKOUT are connected to the P20, P23, P24, and P15 pins of PCF7952 respectively. The electronic key circuit is shown in Figure 3.


Figure 3 Electronic key circuit diagram

Positioning of electronic keys

The PKE system brings comfort and convenience to users by positioning electronic keys. The owner only needs to carry the electronic key with him. When there is a trigger action, the PKE system will automatically detect the key and identify the identity. For the PKE system, position detection is the most critical step of the PKE system. Position detection directly affects the security of the access control system and the anti-theft locking system. Only with accurate positioning can the main controller make correct judgments on the events that have occurred and notify the relevant actuators to take timely and correct actions.

As shown in Figure 4, the PKE system needs to detect and judge three areas: the door, the area outside the car near the trunk, the area inside the car, and the main driving position. When the owner pulls the door or opens the trunk outside the car, the micro switch will be triggered, thus triggering the two-way authentication and field strength detection process; the identification of the area inside the car is the difficulty of the entire PKE system design, and it is necessary to accurately determine the position of the key, thereby determining the state of the door; if the key is detected in the main driving position, the engine can be started normally. [page]

There are two ways to detect the area. One is to adjust the sensitivity of the low-frequency signal and then make a fuzzy judgment based on whether the communication is stable. The accuracy is limited but it is easy to implement. The second is to judge based on the strength detection of the received low-frequency signal, and calculate the relative distance between the key and the low-frequency antenna in the car according to the size of the low-frequency signal. As shown in Figure 4, a total of six low-frequency antennas are placed for area detection. The specific location of the key is accurately located through the cross-coverage range of multiple low-frequency antennas. If the electronic key is awakened by the low-frequency signal and authenticated in the identification area, it will measure the field strength of its location and transmit the corresponding information to the vehicle body base station through radio frequency. The base station compares the received field strength with the originally set threshold value to determine the location of the electronic key at this time.


Figure 4 Area detection and antenna position

System low-frequency circuit Software design and test analysis System software design The main work of the system software design is entry control. Engine start control is similar to entry control. When there is a trigger signal such as a micro switch, the body base station will actively send a low-frequency wake-up signal containing an ID. If the ID is consistent with the ID stored in the key, the electronic key is successfully awakened and sends a radio frequency confirmation signal to the body base station; after receiving the confirmation signal, the body base station sends an encrypted low-frequency signal containing a random number, and the electronic key responds to the corresponding radio frequency encryption signal to complete the identity authentication. After the identity authentication is completed, the low-frequency antenna will send a field strength query command, and the electronic key will send the corresponding field strength information to the body base station through the radio frequency signal. The body base station will analyze and determine the position of the electronic key and will only perform the corresponding action within the effective area. When opening the door, the effective area refers to the safe distance between the electronic key and the car, which does not require too high precision. When the engine is started, the effective area refers to whether it is in the main driving position, which requires higher precision. Its workflow is shown in Figure 5. Figure 5 PKE entry function program flow chart Test analysis By placing six low-frequency antennas inside the body and using ATA5279 to drive and manage them, the area detection accuracy can be effectively improved. The experimental results show that the effective accuracy can reach 5cm when detecting the main driving position in the car. By strengthening the encryption, decryption, random number generation, digital signature, key negotiation and application protocol formulation in the wireless communication process, the confidentiality, identity authentication and information integrity of both parties can be achieved. The low-frequency wake-up mechanism effectively minimizes the RF transmission operation of the electronic key, thereby effectively saving battery power and extending the service life of the electronic key. Conclusion This paper proposes a car keyless entry system based on ATA5279. The system adopts two-way interactive authentication, and any error will lead to the end of communication, which effectively prevents the possibility of being intercepted by other receivers and greatly improves the anti-theft and anti-robbery performance. The research in this paper shows that the system can correctly identify the owner under any circumstances and can automatically open or close the car lock. At the same time, the system uses the ATA5279 low-frequency driver chip, combined with a three-dimensional low-frequency receiving device, which can better solve the problems of high-precision detection of the in-car area and the battery life of the electronic key. Therefore, it has a higher application value and market prospects.