Consumer demand for security and security is driving the rapid expansion of the automotive electronics market. At the same time, automakers face the challenge of having to implement cost-effective, performance-oriented electronic control modules. Between the car manufacturer's product segmentation goals and the consumer's needs, the car security and security systems set up a coordinated bridge.
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From sophisticated remote keyless gating (RKE) applications to emerging passive keyless gating (PKE), tire pressure monitoring systems (TPMS), electronic toll collection and Bluetooth hands-free systems, in-vehicle wireless systems are constantly evolving result. These wireless connections help improve the performance of your security and security modules. Due to technical cost performance and availability, other special short-range communication systems have been limited in security and security applications.
In addition to the common pressures of accelerating time-to-market and adding functionality, designers face many other challenges, such as cost-effective performance enhancements, power consumption, system size, and encryption security.
For example, we can look at a wireless system that represents one of the many challenges faced by today's system architects: a smart transponder that can receive and transmit data. In this two-way communication system, the base station and the transponder can communicate automatically without manual intervention. This low cost two-way communication transponder can be designed to operate with two frequencies: 125 kHz for receiving data and UHF (315, 433, 868 or 915 MHz) for transmitting data. Due to the nonpropagating nature of the 125 kHz signal, the two-way communication distance is typically no more than 3 meters. Since the transponder still has a button that can perform optional operations, a longer one-way transmission distance (from the transponder to the base station) can be supported when the transmit button is pressed.
In these applications, the base station transmits commands at a frequency of 125 kHz while simultaneously searching for any response sent by the active transponder in the region at UHF frequency. The smart transponder is typically in receive mode and searches for any valid 125 kHz base station command. The transponder transmits a response at UHF frequency if any valid base station command is received. This is what we call a "passive keyless gating (PKE) system." The PKE system uses a 125 kHz circuit for two-way communication. A low-cost, small-volume, low-power PKE transponder can be produced using an integrated system-on-chip (SoC) intelligent microcontroller that includes digital and low-frequency analog front ends.
But as designers accumulate more and more system experience, they face the challenge of making the PKE transponders reliable enough to be a cost-effective alternative to conventional RKE transponders while ensuring it Can you achieve a specific system goal? Although PKE transponders seem to require complex and expensive circuitry to implement, the challenges faced by designers can be solved by using relatively simple, low-cost circuits. These low-cost circuits are centered around a smart PIC microcontroller (PIC16F639) and contain all the features necessary to support secure two-way communication.
The smart PKE system shown in the figure still has buttons that support optional operation, but the main work can be done without any human intervention. The two-way communication sequence of the PKE system is as follows:
The base station transmits a command at a frequency of 125 KHz;
The transponder receives base station commands via three orthogonal 125KHz LC resonant antennas;
If the command is valid, the transponder transmits a response (encrypted data) through a UHF transmitter; if the data is correct, the base station receives the response and activates the switch.
Another challenge for designers is how to achieve system performance enhancements in a cost-effective manner. To achieve enhanced performance, including: communication distance, antenna directivity, small package size, encryption security, and low power consumption under door lock "on/off" conditions. Critical system performance enhancements can be met by increasing the reliable range of 125 kHz base station commands and maintaining long battery operating times.
In battery-powered transponder applications, the maximum communication distance for UHF is approximately 100 meters, but only a few meters with low frequency (125kHz). Therefore, the communication distance of the dual-frequency PKE transponder is mainly limited by the commanded distance of the 125 kHz base station. Due to the non-propagating nature of the low frequency signal, the 125 kHz signal will decay rapidly as the distance increases. For example, if the base station outputs an antenna voltage of about 300 Vpp, the voltage induced by the coil antenna of the transponder about 3 m away is only about 3 mVpp, which is equivalent to the noise level of the application environment. Therefore, how to effectively detect weak signals becomes a difficult performance problem faced by system designers.
To extend the range of the 125 kHz base station command, consider two possible solutions: increase the transmit power of the base station transmitter; or increase the input sensitivity of the transponder. The maximum transmit power of a base station transmitter is generally specified by government regulations. Therefore, if the maximum power transmitted by the base station is within the allowable range, the input signal detection sensitivity is improved, that is, the second method described above is the only effective solution. In order to achieve a two-way communication distance of 3 meters, the transponder input sensitivity must be around 3mVpp.
Figure: The main operation of the PKE system does not require manual
Antenna directionality problem
Any radio signal radiated by the antenna unit will propagate in a certain direction. If a good antenna is used, the signal will be more directional (or a narrower radiation angle). The low frequency (125 kHz) signal radiated by the LC resonant circuit does not have a better directivity like the high frequency signal, but it still has a certain directivity. Under a given transponder design condition, the communication distance (or induced voltage) of the low frequency signal depends on the degree of inductive coupling of the base station antenna to the transponder antenna. When the two antennas face each other, the coupling degree is optimal.
For hands-free PKE applications, the direction in which the transponder is placed in the user's pocket can be arbitrary, so the probability of the transponder antenna facing the fixed base station antenna is only about 30% (x, y, z direction). But if the transponder has three orthogonal antennas, the probability can be increased to nearly 100%, at which point the transponder can capture the base station signal in any given direction.
The PIC16F639's operation can be effectively controlled to save battery power. In addition, the microcontroller must also operate with minimal circuitry during inactive mode. The PIC16F639 chip in the transponder contains both a low-frequency front end and a digital circuit. The low-frequency front end is always searching for input signals, while the digital part is in sleep mode to conserve battery power. It is only woken up when a valid base station command is received. This can be done by using a special wake-up filter in the low-frequency front end portion. Moreover, the low frequency detection circuit is also programmable so that the output is only available when the input signal has a predefined data packet header.
Power management and package size issues
In addition to using special filters to conserve battery power, the PIC16F639 also features proprietary nanoWatt technology that allows system designers to better control on-chip peripherals, such as several software-selectable speed options. The 8MHz internal oscillator with a frequency reduced to 32KHz. Very low sleep current consumption plus a fast-start internal oscillator to support low power system designs. Its periodic wake-up mechanism includes low-power real-time clock operation, ultra-low power wake-up and extended low-power watchdog timer. With these broad power management features, designers can achieve energy savings in their applications and get tighter control over overall system power at a lower cost.
To facilitate implementation and increase flexibility while maintaining a small footprint, a careful assessment of the integration between the MCU and the analog front end is required. Here, a "double-die single package" solution is adopted, which can support future conversion to different MCUs based on application requirements. Two functional die are internally bonded through a serial peripheral interface. The PIC microcontroller family is available in a variety of package styles. As few as six devices with up to 80 pins are available. Packages up to 20 pins are ideal for space-constrained applications in wireless access systems. The combination of smaller form factors, advanced on-chip peripheral integration, and cost-effectiveness enables system designers to create more powerful systems while meeting the challenges of wireless systems.
The patented KEELOQ global standard encryption technology provides a cost-effective solution for authentication, keyless gating and other remote access control systems. KEELOQ encryption uses an industry-proven code hopping encoding method that changes code and is sent securely when the encoding device is activated. In an implementation based on a pair of encoders and decoders, the encoder is located at the far end and sends a rolling code ID number and counter value. The decoder is located in the receiver and decodes the message sent by the remote encoder. It stores the identification number and counter value of the remote device it is listening to. The decoder only allows access when listening to remote devices.
KEELOQ encryption is a highly secure algorithm implemented by complex formulas and 32-bit random number generators. For the parking lot entrance application, the driver can directly enter the parking lot without stopping the parking, because the system will automatically recognize the PKE transponder within the effective range of about 3 meters.
Designers of future automotive wireless security access systems may encounter various challenges. Cost-effective microcontrollers provide a proven and reliable building block for in-vehicle wireless systems. A low-cost two-way communication transponder implemented with an integrated system-on-a-chip solution is an example of a wireless system that provides drivers with enhanced security features. Without any manual intervention, the PKE transponder can receive low frequency base station commands and respond with encrypted data via the UHF transmitter. This small PKE transponder can be placed in the driver's pocket and can automatically open and close the door without any manual intervention.
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