Passive access control system RF design considerations (1)

Introduction

Passive control (Passive Entry, PE) systems in automotive comfort and security is leading a new trend of development.

Although this technology a few years ago have come out, but until recently began rapidly popular, primarily because of system integration, so that the system costs can be drastically reduced.

Is complete, the remote access without a key (Remote Keyless Entry, RKE) system is an interactive, that is, the user must press the key to open the door; and passive access control system is passive, meaning they do not need the user to make any interactive action you can open the door.

When the user is ready to enter the vehicle by pulling the door handle of actions triggered PE system launch Low frequency, low frequency LF) signal. A few milliseconds key card receives a signal, and LF received packets are encrypted and then transmitted by radio frequency (RF) channel to send an encrypted signal to vehicles for confirmation.

Passive access control system can also contain a passive engine features, i.e. passive access start (Passive Entry Go, PEG).

As long as the system recognizes the key card inside the vehicle, the driver sits on the driving seat LF circuit is triggered. During the verification to confirm and complete position measurement, just press the start key to start the engine.

Both are key card to receive the plain text data, and take advantage of powerful hardware cryptographic modules (such as AES-28 module) to be encrypted, and then encrypt the data returned to the vehicle to do the validation.

PE key card uses a small lithium battery for data reception, encryption and transmission power.

Passive access control system specifically designed for the key card is to ensure as far as possible, long battery life. If the batteries are exhausted, key card will enter an emergency mode by LF coil magnetic field energy sufficient to realize the work without the battery. You need to put the key card is placed in the position of the door near the coil. In this case, the system will only communicate by LF channel.

Typical PE system

A typical PE system is composed of automotive interior parts and a key card system, both as a communication peers, build two communication link is: (1) LF uplink: vehicle to key card, (2) ultra-high frequency (UHF) downlink: key card to the vehicle (see Figure 1).

Figure 1 passive access control system module diagram

Car parts

When you pull the car door handle, vehicle antenna drive will produce LF field.

This change to activate the Central dashboard controller, start the LF communication request key card. Usually every door is installed in the LF antennas, antenna coil drive unit, driven by (one antenna drive unit is capable of driving a multiple antenna coil, for example, ATA5279 can drive up to six different antenna coil). The system uses a UHF receiver module to receive from key card issued by the RF data to support the RF link. The received data is encrypted before being sent back to the dashboard controller, and then decrypted by software (AES-128).

Key card

In any PE system, key card must be able to measure the LF signal on three orthogonal axes (X, y, and z-direction), and on the intensity of use UHF transmitter RF channel, through the information back to the vehicle to determine the location of the key fob.

This signal strength information (also called remote signal strength indicator, i.e. RSSI) and 3D LF receiver connected to the three orthogonal antenna coil collection. Any digital data, such as wake-up data mode (preamble, ID), the system command or as a protocol load of plain text data password, will be received and transmitted to the key card in the microcontroller (MCU) processing (return packets, encryption). In order to save energy, LF receiver with a dedicated control logic, to very low power consumption to analyze and detect wake-up signal, it is therefore not required to fully awaken the entire system, which can greatly extend the key fob battery life. Key card data flow in and out through a small 8-bit MCU for ultra low power consumption (e.g. ATtiny44) to control. The received data can be encrypted through the software or by a powerful encryption capabilities of the hardware cryptographic modules (such as AES-128) for encryption. For improved security, an encryption mechanism is also used in hardware and embedded in the MCU. The encrypted data is transferred to the UHF transmitter, and to a very high baud rate to vehicle emissions.

The battery is fully drained, launch the transponder can be used as a battery-powered devices to work, then called the emergency mode.

In this mode, only orthogonal coil and magnetic coupling a LF, obtain enough energy to charge the forms stored in an external capacitor. Link through emitting LF transponder and the base station to open the door, and is used as an anti-theft locking device that prevents the engine (see Figure 1, where x axis circle the equivalent of a 3D LF receiver coils and one emergency/anti-theft locking transceiver antenna). Analog front end (AFE) module is used for communication, and LF power management (PM) module to manage farm power, that is stored in an external capacitor charge of Cbuf. In emergency mode, RSSI measurement, 3D RF LF data reception and transmission are disabled.

Table 1: PE system basic parameters key fob

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Figure 2 3D LF receiver wake-up

Figure 3 RSSI position measurement

Receive LF signal

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