Design Article
Efficient analog front end drives passive-keyless-entry system
Youbok Lee and Willie Fitzgerald, Microchip Technology
12/14/2005 1:52 PM EST
Strategy Analytics, a global market research firm, predicts the total worldwide market for automotive semiconductors will be approximately $21.3 billion, with the Chinese market for automotive semiconductors being $1.4 billion, in 2011. In China, safety and security electronic control modules will utilize approximately 18% of the Chinese automotive semiconductors, according to Strategy Analytics.
Automotive wireless systems are continuing to unfold, from the well established application of remote keyless entry (RKE) to emerging uses from passive keyless entry and tire pressure monitoring to electronic toll collection and Bluetooth hands-free systems. These wireless connections are instrumental in advancing safety and security modules, which are establishing the desired feature sets for the driver. The emergence of additional dedicated short-range communication systems for automotive safety and security applications is only limited by the availability of cost-effective technology.
But beyond the constant pressures to reduce time-to-market cycles and increased functionality, designers face a range of challenges:
For example, let's examine a wireless system that typifies many of the challenges faced by today's system architect —the latest smart transponder that can both receive and transmit data. In this bidirectional communication system, the base station and transponder can communicate automatically, without a human interface. A low-cost, bi-directional communication transponder can be made that uses dual frequencies: 125 khz for receiving data and UHF (315, 433, 868, or 915 MHz) for transmitting data. The bidirectional communication range is typically less than about three meters, due to the non-propagating nature of the 125 khz signal. Since the transponder continues to include the push buttons for optional operations, it supports long unidirectional range (from the transponder to the base station) for transmitting push-button information.
In these applications, the base station transmits commands with a 125 khz frequency and looks for any responses in UHF from valid transponders in the field. The smart transponder is normally in the receiving mode and looks for any valid 125 khz base-station commands. The transponder transmits responses with UHF if any valid base station command is received. This is referred to as a passive-keyless-entry (PKE) system. The PKE system utilizes the 125 khz circuits for bidirectional communication. A low-cost, space-saving, power-conserving PKE transponder can be made by using an integrated system-on-chip, smart microcontrol unit (MCU) that includes both digital and low-frequency analog front-end sections.
PKE challenges
As designers gain more system experience, they are challenged to make the PKE transponder reliable and serve as a favorable cost-effective replacement option for the conventional RKE transponder—all while ensuring that certain system objectives are satisfied. The table below highlights key careabouts and solutions that face the system designer. Although the PKE transponder seems to require complex and expensive circuits, the challenges which the designer faces have been addressed by use of relatively simple, low-cost circuits that are centered around a smart PIC microcontroller (PIC16F639)—that includes all of the necessary functions to support requirements for secure bidirectional communications.
The figure above shows a smart PKE system. While it still has push buttons for optional operations, the main operation is accomplished without any human interaction. The bidirectional communication sequence for the PKE application is as follows:
One of the challenging tasks for design engineers is the cost-effective implementation of system performance enhancements, such as communication range, antenna orientation, small packages, encryption security, and low power consumption in both "key-on" and "key-off" conditions. Implementing a design to support an improvement in the range of the 125 khz base station command for reliable operations as well as maintaining long battery life in the transponder addresses key system enhancements.
Input sensitivity required for weak signal detection
In battery-powered transponder applications, the maximum communication distance with UHF is about 100 meters, but only a few meters for the low frequency (LF), 125 khz. Therefore, the communication range of the dual-frequency PKE transponder is limited by the range of the 125 khz base-station command. The 125 khz signal falls off very quickly over distance, due to the non-propagating nature of the low-frequency signal. For example, assuming the base station outputs around 300 Vpp of antenna voltage, the voltage picked up by the transponder's coil antenna at about three meters away is only about 3 mVpp, which is within a noise level of the application environment. Detecting the weak signal is a challenging performance-oriented issue for system designers.
For increasing the range of the 125 khz base-station command, two possible solutions should be considered: Increasing the base station's transmitter power, or increasing the input sensitivity of the transponder. The transmitter's maximum power is generally defined by government regulations. Therefore, assuming that the base station is outputting the maximum power within the allowable limits, the second option of increasing the input signal detection sensitivity is the only effective choice. In order to achieve a three-meter, bidirectional communication range, the transponder's input sensitivity needs to be about 3 mVpp.
The antenna directionality problem
Any radio signal radiated from an antenna element propagates with a certain directional angle, and exhibits higher directionality (or narrower radiation angle) when excited by a good antenna. The low-frequency (125 khz) signal that is radiated from an LC resonant circuit is not directional as much as the high frequency signal is, but it still has directional field components. With the given design conditions of the transponder, the communication range (or induced voltage) of the low-frequency signal is dependent on how well the base station and the transponder antennas are coupled inductively. The best mutual coupling occurs when the two antennas are oriented face to face.
For hands-free PKE applications, the transponder can be placed in any direction inside a person's pocket. Therefore, the best chance that the transponder antenna is faced to the fixed base station antenna orientation is approximately 30% (x, y, z directions). This chance increases to approximately 100% if the transponder has three orthogonally placed antennas. Each antenna is placed to x, y, and z directions. By using three orthogonally placed antennas, the transponder can pick up the base-station signal at any given direction.
Wake-up filter for battery power savings
The PIC16F639's operation can be managed effectively for battery power savings. The micrcontroller must also operate with minimum circuits during inactive mode. The PIC16F639 in this transponder includes both low-frequency (LF) front-end and digital sections. The LF front-end section is consistently searching for input signals. On the other hand, the digital section is in sleep mode to save battery power, and wakes up only when a valid base-station command is received. This can be achieved by using a specific wake-up filter in the LF front-end section. The LF detection circuit is programmed to make its output available only for the input signal with a predefined header.
The wake-up filter is used to prevent the digital section from waking up because of noises or unwanted input signals. As a result, the filter can save operating currents and battery lifetime.
Power management
In addition to the specific filters, the PIC16F639 features proprietary nanoWatt Technology, which gives the system designer greater control of the on-chip peripherals, including the 8 MHz internal oscillator with several software-selectable speed options down to 32 khz. The extremely low sleep current consumption, combined with a fast start-up internal oscillator, supports low-power-consumption design. Periodic wake-up mechanisms include low-power real-time clock operation, ultra-low-power wake-up, and an extended low-power watchdog timer. With these extensive power-management features, designers are able to implement power-saving in the application's software and gain tighter control of overall power consumption at reduced cost.
Small packages
The degree of integration between the MCU and analog front end was carefully evaluated to ease implementation and flexibility while maintaining a small footprint. A "dual-die in a single package approach" was chosen, which supports future migrations to different MCUs, based on application requirements. The two functional dice are internally bonded via an serial peripheral interface.
The PIC microcontroller family is supported by an extensive range of package options. Devices from 6 pins to 80 pins are available. Package options equal to or less than 20 pins are excellent choices for space-constrained applications in wireless access systems. The combination of small form factor, advanced on-chip peripheral integration, and cost efficiency offers the system designer a foundation for creating enhanced systems that meet the challenges of wireless system implementation.
Encryption support
The patented KEELOQ global-standard cryptographic technology provides cost-effective, authentication, keyless entry, and other remote access control systems, as noted in the figure below. KEELOQ utilizes an industry-proven code hopping encoding methodology. The code changes when the encoder device is activated, and the code is securely transmitted. With an implementation based on an encoder and decoder pair, the encoder is in the remote, and transmits a rolling code ID number and counter value. The decoder is in the receiver and decodes the message sent by the encoder remote. It stores the Identification numbers and counter values of the remotes that it has "learned". The decoder allows access only to "learned" remotes. KEELOQ encryption is a highly secure algorithm that is achieved by means of a complex equation and randomizer with a 32-bit result. For parking-lot entry applications, a person can drive onto the parking lot without stopping, because the system recognizes the PKE transponder within an active zone of about three meters.
Conclusions
The designer of wireless secure access systems for vehicle's of tomorrow may encounter his or her share of challenges. Cost-effective microcontrollers offer a proven, reliable building block for wireless systems within the vehicle. The implementation of a low-cost bidirectional communication transponder utilizing an integrated system-on-chip solution is an example of a wireless system that delivers enhanced safety and security functions to the driver. While the PKE transponder receives low-frequency base station commands and responds with encrypted data via an UHF transmitter, it can operate without any human intervention. A PKE fob transponder located in the driver's pocket can lock or unlock entrance doors automatically, without any human activation.
Some of the driver's increasing demands for safety and security features can be addressed by such wireless secure access systems. Beyond consumer preferences, government legislation and the automakers themselves are pushing current and future initiatives to increase safety and security innovations within the automobile. These advancements are expected to be made without sacrificing consumer confidence. The convergence of individual sub-systems is launching the next evolutionary step in passenger safety and security by utilizing enhanced wireless secure access systems to deliver a competitive advantage for the automaker.
Youbok Lee is a technical staff engineer in the Security, Microcontroller and Technology Division, and Willie Fitzgerald is product marketing director, Automotive Products Group, for Microchip Technology.
