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Automotive security apps go wireless

Posted: 01 Nov 2006 ?? ?Print Version ?Bookmark and Share

Keywords:wireless automotive security application? automotive security app? wireless car? wireless security application? Youbok Lee?

With consumer demand for safety and security as a catalyst for automotive electronics expansion, carmakers are challenged to implement cost-effective, performance-oriented electronic control modules. Automotive safety and security systems provide a synergistic bridge between the automakers' product-differentiation goals and consumer needs.

Automotive wireless systems are continuing to unfoldfrom the well-established application of remote keyless entry to emerging ones, such as Bluetooth hands-free systems. Wireless connections are instrumental in advancing safety and security modules. The emergence of additional dedicated short-range communication systems for automotive safety and security applications is limited only by the availability of cost-effective technology.

But beyond the constant pressures to reduce time-to-market cycles and to increase functionality, designers face many challenges. These include cost-effective performance enhancements, power consumption, system size and encryption security.

Smart transponder
Let's examine a wireless system that typifies many of the challenges faced by today's system architect: a smart transponder that can receive and transmit data. In this bidirectional communication system, the base station and transponder can communicate automatically without a human interface. A low-cost, bidirectional communication transponder can be made to use dual frequencies: 125kHz for receiving data and UHF (315-, 433-, 868- or 915MHz) for transmitting data. The bidirectional communication range is typically less than about 3m, due to the non-propagating nature of the 125kHz signal. Since the transponder continues to include the pushbuttons for optional operations, it supports long unidirectional range (from the transponder to the base station) for transmitting pushbutton information.

The base station transmits commands with 125kHz 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 125kHz 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 uses the 125kHz circuits for bidirectional communication. A low-cost, space-saving, power-conserving PKE transponder can be made by using an integrated SoC, smart MCU that includes both digital and low-frequency analog front-end sections.

As designers gain more system experience, they are challenged to make the PKE transponder reliable enough to serve as a cost-effective replacement for the conventional remote-keyless-entry transponder while ensuring that certain system objectives are satisfied. Although the PKE transponder seems to require complex and expensive circuits, the challenges are addressed by using relatively simple, low-cost circuits centered on a smart PIC MCU (PIC16F639) that includes all the functions to support secure bidirectional communications.

No human interaction
The smart PKE system still has pushbuttons for optional operations, but the main operation is accomplished without any human interaction. The bidirectional communication sequence is as follows:

  • The base station transmits commands at 125kHz frequency.

  • The transponder receives those commands via the three orthogonally placed 125kHz LC resonant antennas.

  • If the command is valid, the transponder transmits responses (encrypted data) via a UHF transmitter. If the data is correct, the base station receives the responses and activates switches.

Smart passive-keyless-entry system needs no human interaction.

One of the challenges 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 "key-on" and "key-off" conditions. Improving the range of the 125kHz base-station command for reliable operations and maintaining long battery life in the transponder address key system enhancements.

In battery-powered transponder applications, the maximum communication distance with UHF is about 100m, but only a few meters for the low frequency, 125kHz. Thus, the communication range of the dual-frequency PKE transponder is limited by the range of the 125kHz base-station command. The 125kHz signal falls off very quickly over distance, due to the non-propagating nature of the low-frequency signal.

Antenna directionality
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 (125kHz) signal radiated from an LC resonant circuit is not as directional as the high-frequency signal, but 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. Hence, the best chance that the transponder antenna is faced to the fixed base-station antenna orientation is approximately 30 percent. This chance increases to approximately 100 percent if the transponder has three orthogonally placed antennas. In that case, the transponder can pick up the base-station signal at any given direction.

Besides specific filters, the PIC16F639 features proprietary nanoWatt Technology. It gives the system designer greater control of the on-chip peripherals, including the 8MHz internal oscillator with several software-selectable speed options down to 32kHz. The extremely low sleep-current consumption combined with a fast-startup internal oscillator supports low-power-consumption design. Periodic wake-up mechanisms include low-power real-time clock operation, ultralow-power wake-up and an extended low-power watchdog timer. With these extensive power-management features, designers can implement power saving in the application's software and gain tighter control of overall power consumption at reduced cost.

Small footprint
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 supporting future migrations to different MCUs was chosen based on application requirements. The two functional dice are internally bonded via a serial peripheral interface.

The designers of wireless secure-access systems for the vehicles of tomorrow may encounter their share of challenges. Cost-effective MCUs offer a proven, reliable building block for wireless systems within the vehicle. The implementation of a low-cost bidirectional communication transponder using an integrated SoC 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 a 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.

- Youbok Lee
Technical Staff Engineer, Security, Microcontroller and Technology Division
Microchip Technology Inc.

- Willie Fitzgerald
Product Marketing Director, Automotive Products Group
Microchip Technology Inc.

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