Wireless ICs in Automotive Security Applications
Let's start with a tough question: What is the difference between TPMS system and PKE system? Nothing! A TPMS system is placed inside the wheel and composed of a sensing device that measures temperature and pressure. When appropriate, for example before starting the engine, the car requests a measurement through a wireless link. If a pressure default occurs in one of the wheels, the car warns the driver through a signal on the dashboard. In PKE applications, the user carries a key fob device which contains an encoded identifier (ID). When the driver enters their car, the car requests the key fob ID through the wireless link. If the ID matches the ID stored by the car, the car doors will be unlocked. In both cases, a car requests security information from a remote device. In TPMS the data correspond to the pressure of the tires, whereas in PKE the data identity a person. In both cases, an "incorrect" response from the remote device (low pressure detected or a car owner ID mismatch) should inhibit the use of the car.
These similarities are reflected by comparable requirements and constraints. The system’s range is about 1 to 2 meters. The battery lifetime of the devices is critical (more than a few years), the response time must be very fast (hundreds of milliseconds) and the reliability must be very high. Consequently, in the last ten years we have observed a convergence of system architectures. The following schematic describes the two systems.
Here the elements drawn in black are common to both transponders while the blue elements are specific to each application (e.g. crypto engine and memory for RKE and sensor and interface for the TPMS).
Based on these block diagrams, the operating principle is simple to describe. A Low Frequency (LF) link, composed of an LF initiator in the car and an LF receiver in the remote device, is used to wake up the system with the lowest power budget. For this purpose, the receiver alternates between sleep and scanning periods. Once the presence of the LF field is detected, it checks that this LF field actually corresponds to a valid request through a predetermined preamble. The entire system should not awake from noise generated by other systems. The device controller is then interrupted and the treatment of the requests initiated. The difference between TPMS and PKE in this part of the architecture relates to the relative position between the car and the device. In the key fob case, there is a strong uncertainty about the device’s orientation. Three antennas are required to overcome any reception holes in the wake-up signal. Since the position of the TPMS device is known and fixed, only one antenna is needed to ensure reliable wake-ups. Once awake, the device controller treats the request and then extracts and processes the data containing the security information. The functional blocks of the device are powered and enabled only if they are involved in the treatment of the request. The TPMS sensor is powered and the acquisition chain measures the pressure inside the tire. The controller of the key fob reads the memory stored ID through the encryption engine. The transmission of these data is realized through a UHF RF link to ensure rapid communication. The device controller manages the framing according to the application’s link protocol. In the PKE application, the main constraint relates to convenience. The delay between the moment the driver pulls the door handle and the release of the door lock should be in the frame of a few hundreds of milliseconds. If this is not the case, the user will experience a "wall" effect. In TPMS, reliability is critical, as it could impact the power consumption by reissuing requests and answers. Therefore, the transmitter needs to be equipped with fast start up and high power efficiency. Once these tasks are completed, the TPMS or the key fob devices fall in scanning mode.
The main difference between both devices relates to an eventual backup mode. When the device runs out of battery power, the TPMS control of the car flags the absence of an answer, so a responsible driver would then visit the nearest garage. But lack of battery power does not impact the car’s operation. Alternatively, the key fob device offers a backup mode which allows the user to enter and start the car. In this case, the device behaves like a passive RFID tag requiring energy scavenging and power management. For these applications, embedded in the remote device must be a monitoring system which includes the battery data status.
As one can see in this description, both systems are built around an LF initiator coupled with an RF receiver on the car side, and an LF receiver and an RF transmitter on the remote device side. Considering Moore's law, power consumption and costs drive integration in the semiconductor business. The logical trend will be to integrate all of these commonalities into a single chip. Extrapolating one step further and considering a reduction in other embedded functions, for each application we anticipate that there will be a full integration of the device into one silicon piece. In the near future a portfolio of silicon vendors will contain one system on a chip (SOC) dedicated to TPMS and one SOC dedicated to PKE. Each IC will require only a few passive components, LF and RF antennas and a battery. On the car side, we anticipate that an LF initiator, LF demodulator and an RF receiver SOC will be embedded on a single chip which will be used for both TPMS devices and key fob monitoring.
For years Melexis has successfully provided automotive sensors, actuators, RFID, RF systems and application expertise. We intend on staying on the leading edge of PKE and TPMS applications by providing the next generation of innovative standard product ICs and IP blocks.