AN1024 PKE System Design Using the PIC16F639 base station unit. Furthermore, due to antenna Author: Youbok Lee, Ph.D. orientation properties, the input signal level at the Microchip Technology Inc. transponder becomes considerably weaker if the antenna is not oriented face-to-face with the base station antenna. INTRODUCTION The most probable source of PKE operation failure is Hands-free Passive Keyless Entry (PKE) is quickly due to a weak input signal level at the transponder. becoming mainstream in automotive remote keyless Therefore, for a reliable hands-free PKE application, it entry applications and is a common option on new is necessary to make the input signal strong enough automobile models. Instead of pressing a transmitter (above input sensitivity level) in any condition within the button to unlock or lock a car door, it is possible to gain desired communication range. vehicle access simply having a valid transponder in In order to make the PKE system reliable, the system your possession. designer must consider four important parameters: Hands-free PKE applications require bidirectional 1. Output power of the base station command, communication between the base station and 2. Input sensitivity of the transponder, transponder units. The base unit inside the vehicle 3. Antenna directionality, and transmits a Low-Frequency (LF) command that searches for a transponder in the field. Once located, 4. Battery life of the transponder. the transponder in the vehicle owner’s possession then The PIC16F639 is a microcontroller (MCU) with a automatically responds to the base unit. The base unit three-channel analog front-end. The device’s analog then unlocks the car doors, if a valid authentication front-end features are controlled by the MCU firmware. response is received. Because of its easy-to-use features, the device can be In typical PKE applications, the base station unit is used for various smart low-frequency sensing and designed to output the maximum power that is allowed bidirectional communication applications. by electromagnetic field radiation rules that are This application note provides design circuit examples mandated by government agencies. When it operates of the smart PKE transponder using the PIC16F639 DC of power source, the maximumwith a 9 to 12 V MCU. The MCU firmware examples for the circuits PP. Becauseattainable antenna voltage is about 300 V shown in this application note are also available. The of the non-propagating property of the low-frequency given circuit and MCU firmware examples can be easily (125 kHz) signal, the signal level becomes only about modified for users specific applications. PP when it is received by a typical key foba few mV transponder approximately two meters away from the