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Requirements for Designing Next-generation Key Fobs

Fri, 01/28/2011 - 5:49am
Paul Lepek, Atmel Corporation

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.

Atmel-Table 1

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 contactless passive transponder (LF field) then unlocked the steering column and enabled the engine start.

Only more recently fobs began to penetrate convenience, general utilities, secure communications, and secure access ID applications (even extending to payment systems and e-ticketing). These functions were not developed before due to a lack of hardware and software resources, primarily because of the fob’s physical size and power consumption. Recently, however, it has become possible to overcome these shortcomings by incorporating much larger user and program memories, and the use of faster, more compact, and ultra-low power processors without increasing the cost of production. Additionally, the integration of flexible, reconfigurable, and secure authentication peripherals can be made feasible. These types of peripherals include crypto units, secure key management features, and integration of smart cards useful for payment, user ID, and cipher-based authentication systems. An automotive key fob can therefore be used not only to interact with the vehicle but also to gain entry to a park garage, ski lift or to purchase train tickets. This can deliver considerable benefits when goods are purchased using one of the major credit card networks. The secure user memory of peripherals can also be used to store personal and secure information as personal data ID and provide transit information for e-ticketing.

System-in-fob Hardware Resources
• Ultra-low-power 8-bit MCU 
• Large Flash program and EEPROM data memory (memory segmentation with locks) 
• RF communication interfaces 
    - Infrared (IrDA) IF 
    - Immobilizer IF at 125 kHz 
    - Passive entry IF at 125 kHz (RX only) 
    - Smart card IF at 13.5 MHz 
    - RKE IF at 315, 413, 868, 915 MHz (frequency hopping) 
• Power management (optional battery charge) 
• Hardware cryptological unit (AES-128) 
• Integrated proximity coupling smart card (ISO 14443) 
• Cyclic Redundancy Check (CRC) block 
• Serial interfaces (SPI, SSI) 
• ISP/debug (dW) 
• Analog comparator 
• Flexible GP timers and WDT 
• Oscillators: RTC, INTRC (125 kHz, 4 MHz)

The heart of a modern key is an ultra-low power microcontroller with sufficient program and data memories. Typical program memory can range from 8 KB to 16 KB and beyond with its data memory ranging from 1 KB to 2 KB depending on application requirements. Because of secure application support, the key fob’s program and data memories must have provisions for memory segmentation and locks in both memory blocks. For example, the application firmware resident in program memory can be divided into separate memory sectors (e.g., application and immobilizer sections). Also, data memory may have its own partitioning which can allow for soft and hard memory locks when it comes to releasing user-sensitive device-stored data (e.g., authorization password or secure key). While the microcontroller core executes application firmware, the secure user and key data is stored in the on-chip nonvolatile memories (EEPROM).

The core uses various wireless communication peripheral interfaces to communicate with the infrared transceiver (IrDA), the LF transponder (125 kHz), the 3D LF receiver (125kHz), the smart card (13.5 MHz) and the RKE transceiver (315, 413, 868, 915 MHz). Flexible serial interfaces can be shared such as SPI or Serial Synchronous Interface (SSI) to enable data exchange with every communication peripheral. An integrated data integrity check module, based on a Cyclic Redundancy Check (CRC) checksum algorithm, supports validation of received data.

A unique feature of the immobilizer transponder interface is that it is closely bound to the power management unit which is used to provide power supply voltage, VDD, while exchanging LF data with the immobilizer in passive mode. In this mode all other communication with the key fob is disabled to support batteryless operation. Some key fobs may also support the battery charge feature which is integrated into the power management module for recharging the battery with the engine running.

While in secure smart card mode the device can also operate in passive mode and exchange as well as encrypt and decrypt proprietary data using its own crypto module with the reader at FC = 13.5 MHz. It can use its own device memory or the fob’s internal nonvolatile memory.

Integrated crypto modules can support many different cipher algorithms—the most popular being the 256-bit block cipher known as the Advanced Encryption Standard (AES) based on the Rijndael algorithm (AES-256/128 has a key strength of 128 bits which can easily be expanded to 196 or 256 bits).

The fob must also contain analog peripherals such as internal oscillators where F = 125 kHz and 4 MHz to generate its internal clock signals used for the transponder front end and the microcontroller core, respectively, with low frequency deviation across VDD and temperature. The supplied analog comparator can facilitate detection of VDD drops and prevent data corruption during nonvolatile data memory writes in passive mode. 

Figure 1.  Fob Floor Plan

Wireless Data Communication Interfaces
An entry/immobilizer system consists of at least two communication partners where one side is on the vehicle and the other on the key fob. Depending on the link type, there are several possible communication interfaces available, including: 
• IrDA link for convenience and comfort applications; 
• PE/PEG link to enter and start engine, including LF downlinks and UHF uplinks; 
• Immobilizer link to start engine and emergency vehicle entry, including LF down- and uplinks; 
• Chip card as user ID, authorization, authentication at pay stations, including HF down- and uplinks; 
• RKE as entry authentication, including UHF down- and uplinks;

The summary of the communication channels and a brief overview is shown in table 2.

Table 2.  Modern Key Fob Communication Links – Overview

Secure and Reconfigurable Firmware
Application firmware which supports the complete functionality and feature set is the fundamental building block of the key fob. It may consist of many different modules and must encompass all functional and likely operating scenarios, including battery failure which comprises emergency or passive operation mode. To improve reliability it is a common practice to keep both application and immobilizer programs separate and in two distinct program spaces. While the immobilizer firmware supports distinct engine starts, the application software controls all other fob functionalities including RKE, convenience or user-ID applications. The immobilizer/emergency functionality is required to take priority over any other function, which is the equivalent of an override which suspends any function currently in progress when the LF field is detected at the transponder LF coil. Figure 2 depicts a flow diagram and interaction between application and immobilizer firmware. 

Figure 2. Immobilizer and application firmware interaction flow chart.

All data communication is fully supported in the fob’s firmware. Various communication protocols including unilateral, bilateral for immobilizer, PE/PEG and RKE systems can be fully configured by the application software. Based on protocol topology, the application software controls dedicated peripherals by enabling them, and reading data during RX phases, and writing data during TX phases of the protocol as soft triggers (e.g., immobilizer and PE applications) or hard triggers (e.g., RKE or IrDA applications using a push button interface).

Figure 3. Software Partitioning as a bottom-up approach. A major advantage to a next-generation key fob is its in-field programmability, which can be very helpful in the event of a firmware or user data upgrade or programming. The fob can be initially configured using its dedicated general-purpose software via the LF field while the final test is performed at the factory. The user data can be added later by the Tier1 or OEM without modifying the original configuration. Even while in the field the fobs can be reprogrammed with new application and user data via one of the communication interfaces at a later date. Of course, in this case only a single functionality can be enabled while using memory locks to provide security. This is especially useful when used as a pay token in e-commerce or e-ticketing environments. 

Figure 4. Immobilizer couplingTransponder LF Field Coupling
Transponder-to-base-station coupling still remains the most challenging aspect for key fobs. The proper transponder coupling can be achieved when sufficient energy is transferred from the base station to the transponder for the transponder to communicate with the base station. During design, the L-C tank must be carefully selected for optimum energy and communication performance. Figure 4 shows a typical transponder coupling at 125kHz vs. operating distance from the base station coil.

Figure 5 shows one complete challenge-response authentication protocol which could be used in a passive automotive immobilizer application. The LF field voltage (green) is enabled for 160 ms. The field is damped (2.2 V) during RX state and then switched to the undamped level (6 V) during TX state. The charge storage capacitor voltage (yellow) which provides VDD to the transponder is immediately charged to 2.2 V during the RX data stage. The transponder encrypts (AES-128) received plain text data (128-bit challenge) and transits the response. In many immobilizer systems the system authentication time is a major concern. To minimize authentication time, the number of bits transmitted can be reduced without compromising system security. It is common for authentication time TAUT < 130 ms at BR = ~3.9 kBaud. 

Figure 5. Immobilizer protocol execution scope shot/power analysis.

The figure below shows field voltage and VCC traces as the key fob is energized by the field and begins to receive a BPLM data stream. Field gaps are visible which separate field ‘On’ interval which is decoded by the fob using a dedicated hardware peripheral. The fob’s MCU is in sleep mode 95 percent of the time to save power consumption. 

Figure 6. Transponder at power-up.

RF Communication Links
The RKE bidirectional link and the PE high-speed communication channel are both supported using HF communication links. In comparison to LF links, HF links are superior in their operating range (up to several hundred meters) and baud rate (up to 80 kBaud can be achieved). RF transceivers currently available on the market use N-fractional PLL frequency tuning techniques where the carrier frequency can be selected in firmware by the MCU. Some devices allow large tuning frequency variations, permitting more design flexibility. The transceiver’s operating range remains a key performance parameter. To extend operating range, it is common for the transmitter power to be as high as 12.5 dBm and the receiver sensitivity to be less than -100 dBm.

Antenna design is also a determining factor providing additional performance gain. Although whip antennas add additional performance gain, small loop antennas printed on the PCB are usually chosen for use in automotive key fobs.

Power consumption is another critical factor on the receiver as well as on the transmitter side. Power consumption can be reduced by selecting higher data baud rates. Choosing ASK modulation tends to lower operating current since the power amplifier is momentarily disabled during modulation.

Table 3 shows some typical RF transceiver parameters. 

Table 3.  RF Transceiver Parameters


Figure 7. Typical receiver and transmitter characteristics at FC = 433 MHz (typ.).

A snapshot of the transmitter spectrum at 433 MHz during transmission of an RKE message to the vehicle is shown below. The transmitter carrier frequency, the span, power output, and device setting are configured using the serial interface by shifting configuration data (in this case the 32-bit configuration word) into the RF transmitter via the MCU when the user presses the open-door button. 

References:
1. ISO 14223 – Advanced Transponders Standard
2. ISO 10536 – Close Coupling Smart Cards Standard
3. ISO 14443 – Proximity Coupling Smart Cards Standard
4. ISO 15693 – Vicinity Coupling Smart Cards Standard
5. ISO 18000 – Item Management Standard


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.
5 Assumes the fob coil is placed in the center of the base station coil where the coupling is best

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