Automotive Application - Keyless Entry System

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Remote keyless entry systems, now fairly commonplace devices, provide car owners with a degree of convenience, making the task of physically inserting a key unnecessary. A passive keyless entry (PKE) system uses advanced technology to take vehicle accessibility to a higher level. With PKE, car owners have simply to pull the door handle while the key fob is on their person; no searching through pockets or purses, no button pressing necessary.

Bi-directional communication makes this possible. When the door handle is pulled (or some similar trigger is activated) the base unit located within the vehicle emits a short range, low frequency signal – polling for a nearby key fob transponder. If present, the key fob recognizes the signal and responds with a UHF (ultra-high frequency) signal, transmitting a code to be authenticated by the base unit encryption firmware, such as Microchip Technology’s KeeLoq®. Finally, if a proper code is verified, the base unit instructs the vehicle’s central body controller to unlock the door. Of course, all of this takes place within a fraction of a second, without any additional action from the user.

Major subsystems include:

Transponder: Power efficiency is one of the most important factors in designing a key fob/transponder; such devices are expected to operate for years without battery replacement. The microcontroller is selected to be very low power with an especially energy-efficient sleep state. The analog front-end (AFE) is essentially an LC resonant circuit which wakes the key fob MCU when energized by the LF signal sent from the base unit (located in the vehicle). The microcontroller remains in its wake state only long enough to respond to the base unit, transmitting its encrypted codes over the UHF band.

Base Unit: The base unit consists of a microcontroller, an LF transmitter, and a UHF reciever. As part of an automotive system, the unit should also conserve energy when possible. Both the MCU and the RF modules remain in a sleep state until externally triggered by the car’s owner, such as by pulling on the door handle.

Power Management: Remote keyless entry is only a small fraction of a vehicle’s electronics system, which often includes stepper motors, one or more MCUs, external memory, CAN/LIN networks, LED backlighting, etc. – all of which may operate at different voltage levels. With so many different power rails, careful consideration is required when designing for efficiency, compactness, low cost, and low EMI. Low-dropout (LDO) regulators can be particularly useful for instances of heavy loading, such as when engaging an electric starter.

This design is for reference only. The design, as well as the products suggested, has not been tested for compatibility or interoperability.