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Requirements for designing next-generation key fobs.

Today's key fobs can be generally subdivided into two different functional categories. The first includes Remote Keyless Entry (RKE) devices which require some sort of human intervention or a physical interface of the user to the key fob (e.g., key push) in order for the fob to produce the desired function effect such as unlocking a door or opening a sunroof. The second group of devices provides similar functionality but also features an added level of comfort by performing the same function without physical intervention by the user. Instead of the pushbutton or touch sensor interface, a Passive Entry (PE) identifies the user (and the key fob) as a legitimate entity and automatically triggers authentication or issues a request (e.g., passive door unlock, trunk release, etc.).

Both systems are based on a preprogrammed key fob device ID and authentication protocols which include an encryption stage for authorizing the issue of key fob commands to the vehicle. In this way the key can be identified by the vehicle and vice versa before any action is executed. All RKE-based systems require key fobs to support RF links which fall into Industrial, Scientific and Medical (ISM) frequency bands (i.e., 0- 135 kHz, 13.56 MHz, 315/433 MHz, 869 MHz, and 915 MHz). However, for a PE system the LF downlink is used by the key fob to compute a Received Signal Strength Indicator (RSSI) value and thus the fob's physical coordinates in relation to the vehicle while the RF link is used to execute the authentication protocol with the vehicle. RKE and PE system fobs are designed to be powered by a small coin battery intended to last for the life of the vehicle.

Moreover, all key fobs support engine immobilizer system authentication. To prevent theft, every automobile uses an immobilizer system which authenticates engine starts. In this case, the key fob acts as a passive authentication tag similar to the RFID tag but with a larger feature set. Most automotive key fobs use Near Field Communication (NFC) transponders which communicate with the engine controller. The transponder is integrated into the key and is a passive device. It does not need a battery for operation, but instead uses a magnetic field generated by the LF vehicle coil. It also transmits the device ID and executes a special immobilizer protocol for its authentication, but all communication takes place via an LF field generated by the vehicle.
                                          Physical Interface (3)
                                          (Button, Touch, etc.)

Class       Immobilizer    Battlery/   Remote  Remote  Accessories
                (1)      Recharge (2)  Start   Entry

Basic            x            -          -       -          -

RKE (std.)       x           x/          x       -          -

RKE (ext.)       x           x/x         x       -          x

PE (std.)        x           x/x         x       x          x

PE (ext.)        x           x/x         x       x          x

   Class      Passive     Personalization Settings  Time/Data Logging
            (Entry/Go)                                  In Fob (4)

Basic            -                   x                       x

RKE (std.)       -                   x

RKE (ext.)       -                   x                       x

PE (std.)       x/x                  x

PE (ext.)       x/x                  x                       x

(1) Immobilizer support includes secure fob and vehicle authentication
via the LF Field using an integrated LF transponder.
(2) The fob includes a built-in battery with the option of recharging
the battery via the LF field.
(3) Remote Start and Remote Entry are controlled via RF uni- or
bidirectional link. Control of accessories can either be done using RF
or IrDA links.
(4) Data can be logged such as time stamp data, last device ID, last
vehicle service date, and much more either via LF or RF links.

Expanding Fob Applications

Originally key fobs were designed for only one purpose: to unlock the door and start the engine with the metal key. Later, remote keyless entry was used to remotely (HF field) unlock the door. The integrated con-tactless passive transponder (LF field) then unlocked the steering column and enabled the engine start.

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By Paul Lepek, Atmel Corporation,
COPYRIGHT 2011 Advantage Business Media
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2011 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Automotive
Author:Lepek, Paul
Publication:ECN-Electronic Component News
Date:Jan 1, 2011
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