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.
Next: Meeting the challenges
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