Smart Wireless Communications Targets Automotive Safety Applications

Security and confidentiality are now an important part of differentiated application solutions across multiple market segments. The automotive industry is no exception. The rapid acceptance of consumers has further promoted the development of many emerging hotspot applications in the automotive industry. Security and security solutions provide a collaborative bridge between the consumer and automotive markets. Many analysts predict that the global automotive semiconductor market will exceed $17 billion in 2008. With the rapid development of security and confidential applications, this hot application will likely account for nearly one-third of the Total Effective Market (TAM) over the next four years.

A typical remote door opening system used in automotive safety applications includes a controller mounted on the vehicle and a transceiver (or transmitter, ie wireless remote door key) carried by the user. A transceiver (or transmitter) typically includes a microcontroller (MCU), RF components, and human interface devices such as buttons and LEDs. The transceiver (or transmitter) is normally turned off and only works when a button is pressed or when data needs to be sent. Conventional transmitters are used to send data to the controller and are therefore one-way communications. However, this situation is changing. The new smart transceiver can send data as well as receive data, so it is two-way communication. In a two-way communication system, the controller (mounted on the car) and the transceiver (ie the car key) enable automatic communication without the need for a human-machine interface.

A low-cost two-way communication transceiver can be implemented with two frequencies, 125 kHz for receiving data and UHF (315, 433, 868 or 915 MHz) for transmitting data. Since the propagation capability of the 125 kHz signal is not strong, the range of two-way communication is usually less than three meters. Since the transceiver itself can still provide buttons for optional other operations, its range of button information can be sent in one direction (from the transceiver to the controller).

In such applications, the controller uses a 125 kHz frequency to transmit commands while constantly searching for transceiver responses for UHF frequencies in the effective range. Smart transceivers are typically in receive mode, waiting for a valid 125kHz controller command. If a valid controller command is received, the transceiver will respond with a UHF frequency. This is known as the passive remote door opening (PKE) system. The biggest difference between the transmitter in a conventional remote control door opening system and the transceiver in a new passive remote door opening system is that the latter has a 125 kHz circuit for two-way communication. Low-cost PKE transceivers can be implemented using an integrated system-on-chip (SoC) intelligent MCU that includes digital and low-frequency front-end circuitry.

Since the operation of the PKE transceiver relies on automatic communication with the controller, no human-machine interface is required, so the reliability of the system operation is directly dependent on the signal condition between the controller and the transceiver. So the question is, can the transceiver work as reliably as a traditional human-machine interface, and can the price of the transceiver (ie the car key) be similar to the traditional solution?

European luxury car manufacturers have already adopted enhanced security and privacy features, as large Asian OEMs (Toyota, Nissan, Hyundai, Mazda and Daewoo) are also adopting such PKE systems in their mass market automotive platforms. The situation is changing rapidly. The economies of scale that this trend brings will make the performance and price of the PKE system more attractive to the end customer, thus ensuring that the price is acceptable to consumers.

Smart wireless system

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Table 1, PKE Smart Transceiver Requirements and Solutions

In order for the PKE transceiver to work reliably and successfully replace the conventional RKE transmitter, certain conditions need to be met. Table 1 summarizes some of the main requirements and corresponding solutions. Although it seems that PKE transceivers need complex and costly circuits to implement, in reality, with the development of semiconductor technology, intelligent MCUs integrate all the functions needed to implement secure two-way communication, so in fact use relatively simple low The cost circuit can be implemented.

Figure 1. A conventional remote open door (RKE) system in which data is transmitted from the RKE transmitter to the controller and is therefore one-way communication.

Figure 1 shows a traditional RKE system. Once the button is pressed, the RKE transmitter transmits the data. After the controller receives the data, it controls the actuator to open the door if the data is correct.

Figure 2. Intelligent passive remote automatic door opening (PKE) system with two-way communication. The transceiver (key) receives controller commands (125 kHz) using three orthogonally placed LC resonant antennas and transmits the response through the UHF transmitter.

Figure 2 shows an intelligent PKE system. The buttons on the transceiver are used for optional operation, but the action of the door is automatically completed without manual intervention. The two-way communication sequence of the PKE application is as follows:

(a) The controller transmits the command using a frequency of 125 KHz;
(b) The transceiver receives 125 kHz controller commands using three orthogonally arranged 125 kHz resonant antennas;
(c) If the command is correct, the transceiver sends a response (encrypted data) through a UHF transmitter;
(d) The controller receives the response data and activates the switch to open the door if the data is correct.

The challenge for system design engineers is to determine how to increase the transmit range of the 125 kHz controller command to ensure reliable operation while ensuring that the transceiver battery life is long enough.

Two-way communication range requirements for input sensitivity

In battery-powered transceiver applications, the UHF signal (315/433/915 MHz) has a maximum communication distance of approximately 100 meters, but for low frequency signals (LF, 125 KHz) it is only a few meters. Therefore, the communication range of the dual-frequency PKE transceiver is mainly limited by the command transmission range of the 125 KHz controller. Due to the transmission characteristics of the low frequency signal, the 125 kHz signal decays rapidly with the transmission distance. For example, assuming the controller outputs an antenna voltage of about 300 Vpp, the transceiver antenna at about three meters receives only about 3 mVpp, which is about the same level as ambient noise. Detecting weak signals is a challenging problem for system designers.

Some may have questions about why UHF is not used in both directions for two-way communication. The answer to this question is: First, the cost of implementing a UHF receiver in a transceiver is much higher than that of a 125 kHz detection circuit. Second, a distance of three meters is sufficient for most PKE applications.

To increase the transmission range of the 125 kHz controller command, there are two possible solutions to consider: increasing the transmit power of the controller or increasing the input sensitivity of the transceiver. However, due to government regulatory requirements, the maximum power of the transmitter is limited. Therefore, assuming that the controller achieves the maximum transmit power within the allowable range, then increasing the input signal detection sensitivity is the only effective solution. In order to achieve a two-way two-way communication distance, the input sensitivity of the transceiver must be about 3mVpp.

The low frequency antenna of the transceiver (eg 125 KHz) uses an LC resonant circuit. The LC resonant circuit induces a voltage when the magnetic field of the electromagnetic wave emitted by the controller antenna passes through the coil antenna of the transceiver. The voltage of the receiving coil is determined by:

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Where fo is the resonant frequency, N is the number of turns of the coil, S is the cross-sectional area of ​​the coil, Q is the quality factor of the coil, Bo is the magnetic field strength, and a is the direction angle between the transmitter and the receiving antenna coil. The tuning frequency fo of the antenna is given by:

In the case where the physical limit of the LC resonant circuit is given, the input receiving voltage of the transceiver is maximized when the LC circuit is tuned to the carrier frequency of the controller command (125 kHz), or the antenna coil (inductor L) is facing Controller antenna.

Antenna direction problem solution

Any RF signal radiated from the antenna has a certain direction angle when it propagates. If a good antenna is used, it will have good directivity (ie, a narrow radiation angle). The low frequency (125 kHz) signal radiated from the LC resonant circuit is directional without the high frequency signal, but still contains the directional field component. For a particular transceiver design, the communication range (inductive voltage) of the low frequency signal depends on the coupling of the controller and the transceiver wires. The best case for coupling is when the two antennas are placed face to face.

For PKE applications that do not require human intervention, the transceiver (key) may be at any angle in the owner's pocket. Therefore, the probability that the transceiver antenna is optimally coupled to the controller antenna fixed to the vehicle is 30% (x, y, z direction). If the transceiver has three orthogonally arranged antennas, this possibility is increased to approximately 100%. The three antennas are placed in the x, y, and z directions, respectively. By employing three orthogonal antennas, the transceiver is able to receive signals transmitted by the controller from any direction.

Figure 3. Illustration of the transceiver antenna orientation problem. When the transceiver antenna is perpendicular to the magnetic field strength B, the transceiver receives the largest induced voltage, at which point the transceiver circuit and the controller antenna are in a face-to-face position.

Extend battery life with a special wake-up filter

Since the MCU integrates most of its functions, it also consumes the most power. Therefore, in order to save power consumption and extend battery life, it is necessary to carefully manage the work of the MCU. In inactive mode, the number of circuits in the MCU that are active must be as small as possible. The intelligent MCU in the transceiver includes both a low frequency (LF) front end and a digital portion. The LF front end section constantly looks for input signals. At the same time, the digital circuitry is in sleep mode to reduce battery drain. The digital circuit portion is only woken up when the correct controller command is received. This can be done by using a special wake-up filter in the front end of the LF. The output is generated by programming the LF detection circuit such that only the input signal has a pre-set header flag.

Figure 4. Input signal and detector output when the input matches the wake-up filter preset timing. The detector output wakes up the digital circuitry.

Figure 4 shows an example. The LF detector can produce an output when the controller command matches the pre-programmed filter timing. The demodulated detector output wakes up the digital circuitry. Figure 5 shows the situation when the input (controller command) does not match the pre-set wake-up filter requirements. Therefore, at this time, the detector output is invalid and the digital circuit portion is not woken up. The wake-up filter is used to prevent the digital circuit portion from being erroneously woken up due to noise or other input signals. This reduces operating current and extends battery life.

Figure 5. Input signal and detector output when the input does not match the wake-up filter preset timing. The output of the input detector is invalid, so the digital circuit portion will not be woken up.

Intelligent PKE transceiver application

Figure 6 shows an example of a PKE transceiver constructed using a smart MCU. Thanks to its versatile intelligence and its low cost, smart transceivers can be used in a variety of applications, especially in the automotive and security industries.

Figure 6. Example of a passive remote door open (PKE) transceiver configuration with a smart MCU. The transceiver uses three orthogonally placed antennas to detect input signals from the x, y, and z directions.

(a) Automotive industry:

Intelligent passive remote control door opening (PKE) system remote control garage door lock and door opening system engine start control tire pressure monitoring system LF start sensor

(b) Security industry:

Long-distance access control parking space control automatic door switch

Smart wireless car communication can be achieved using a two-way communication method. A low-cost two-way communication transceiver can be implemented using an integrated system-on-chip (SoC) intelligent microcontroller (MCU). Battery-free operation can also be achieved by adding a simple voltage charging circuit to the transceiver and generating a DC voltage using the input low frequency controller command.

Figure 7. Multiple application examples of passive remote-controlled door open (PKE) transceivers. One transceiver can be used for many different usage control applications.

Not only in the automotive market, in fact, every day in daily life, security and privacy have become a growing concern for consumers. Government regulations, consumer and car manufacturer interactions are working together to drive innovative efforts that improve car safety without sacrificing comfort. The ever-increasing communications applications in the car enable security and privacy features to be integrated into a wider range of automotive platforms. Evolving wireless communication technologies can integrate independent subsystems in a car. This also heralds the next revolution in the safety of car occupants and the secrecy of car systems.

Figure 8. Example of tire pressure monitoring sensor application: (a) The LF controller sends the LF command to the pressure monitoring module in the tire; (b) The high input sensitivity intelligent MCU receives the LF command and sends the tire pressure data through the UHF transmitter. Give the controller.

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