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Wireless ASICs enable tire pressure monitoring

Posted: 01 Nov 2006 ?? ?Print Version ?Bookmark and Share

Keywords:tire pressure monitoring system? tpms critical specification requirement? wireless car? wireless ASIC? Craig Christensen?

In recent years, the automotive electronics industry has been able to help make significant improvements in vehicle safety. One application that looks to ramp up on a mass scale is the tire pressure monitoring system (TPMS).

The high incidence of serious and fatal accidents due directly to tire blowouts has created an acute need for a low-cost, low-power, reliable small-form-factor method of monitoring vehicle tire pressures. Over the next few years, the arrival of such technology that also integrates low-data-rate wireless capability will see TPMS propagate through OEM vehicle offerings.

Many new high-end vehicles list tire pressure monitoring as part of standard equipment. In the past, this was typically achieved by enhancing the capabilities of the antilock braking system using differential (wheel-to-wheel) rotational speed measurement as a criterion to check if one tire was under-inflated. The accuracy and response time of this type of system was not high enough to satisfy legislation requirements. It was also incapable of providing a reliable warning if all the tires were under-inflated, as it relied on a differential measurement. On such systems, user intervention was required to reset the system after any tire maintenance period.

In a system that monitors pressure, motion, temperature and battery voltage at each tire, information is processed and data transmitted from each wheel to the vehicle's central controller where, if necessary, the driver is warned of an unsafe condition. The information provided to the driver can be as simple as an alarm or as complex as a dashboard digital display of each individual tire's pressure. Some systems may also include the capability of a requested read back or pressure-on-demand.

The critical specification requirements of a TPMS are current consumption, size, reliability and cost. The device must also be capable of meeting various worldwide regulatory requirements, with major mandates established by the FCC and the European Telecommunications Standards Institute.

Power consumption
Current consumption is one of the primary concerns for a tire-mounted TPMS sensor/transmitter. To keep the sensor/transmitter module as small and lightweight as possible, severe restrictions are being imposed on battery size. As the battery size is reduced, the battery capacity also decreases, thus reducing the total available energy. Long battery life is a desirable feature, with vehicle manufacturers often specifying a minimum 10-year life from batteries with capacities as low as 220mA-hr. This equates to 87,600hrs for the TPMS sensor/transmitter, an average 2.5?A of continuous current consumption.

To maximize battery life, it is necessary to heavily duty cycle power to various portions of the device. As part of this power management, devices may include an "inactive" operating mode and an "active" mode. The active mode would be triggered by car motion and increase the repetition rate for the pressure readings by up to 100 times compared to the inactive mode. The highest current consumption mode of a TPMS application is during RF transmissionupwards of five times higher than the pressure measurement processing mode. Additional power savings can be achieved if the pressure measurements are transmitted infrequently or if only a significant decrease in pressure is measured and then broadcast.

Circuit solution is based on a system that monitors pressure, motion, temperature and battery voltage at each tire.

The RC oscillator for the wake-up timer is one section of circuitry required to operate continuously. Another continuous current drain is the leakage of the device. The average continuous current for these two loads will require approximately 400nA. The current consumption of the circuit in inactive mode, which is expected to account for approximately 90 percent of the 10-year lifetime, may be as low as 500nA average current. This would allow the average current in the active mode to be as high as approximately 2.8?A, which is still a very small number.

By quickly ramping up to their desired operating points, further power savings can be made in several circuits. One circuit that typically requires significant startup time is the crystal oscillator. For situations such as this, the AMIS quick-start crystal oscillator IP can be beneficial. This self-calibrating circuitry reduces the start time of the oscillator to between 5?s and 10?s, compared to the 5-10ms required for a typical crystal oscillator.

Specific low-power ADCs, AMIS' "sniff mode" IP fast wake-up capability and the ability to use on-chip intelligence to reduce the number of RF transmissions can also minimize power consumption.

TPMS module reliability can be improved and board or module space reduced, if design efforts also focus on minimizing the number of external components. Many IP blocks have been specifically designed with this goal in mind. It is normally possible to integrate a number of external components, such as the crystal load capacitors, PLL filter components and temperature sensor.

With any RF application, multipath fading can cause severe problems in obtaining reliable wireless communications. Because vehicles are in an ever-changing environment, there are likely to be significant problems caused by multipath in TPMS applications. To reduce these problems, dual-antenna diversity receivers could be used, such as in the AMIS-52100. The diversity functions operate autonomously, requiring no external controller or RF switches.

Critical criterion
Cost remains an essential criterion to vehicle manufacturers as they strive to meet the mandatory TPMS requirements. There is a large economic benefit to be gained by coupling the TPMS central controller with the RKE system already present in most car models. The use of mature, high-availability processes and IP can also help reduce cost. For example, AMIS uses proven 0.35?m mixed-signal technology. An economical EEPROM module can also be included in the specification, allowing the storage of some calibration and tire serial or position numbers.

To minimize the system component count, the integration of the sensorswhich are likely to be silicon-based MEMSwith the interface/transmitter IC in a multichip package is recommended. Integration of a sensor on the same die as the interface/transmitter IC is not likely to be economically viable for several reasons. Even though CMOS processes in larger geometries are relatively inexpensive, the process for the manufacture of sensors is much less complex and expensive than CMOS processing. Sensor elements also tend to be physically large. These sensor characteristics make them an unlikely candidate for co-processing (i.e. integration) with the CMOS ASIC.

The available microelectronics to meet such TPMS challenges will further enhance automotive safety in an economical manner for OEMs and car buyers alike.

- Craig Christensen
Wireless System Architect, AMI Semiconductor Inc.

- Hervi Branquart
Automotive Strategic Market Manager, AMI Semiconductor Inc.

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