Tire-pressure monitoring (TPM) is among the more useful, albeit humble, of wireless applications. Although application-specific integrated circuits (ASICs) have been commonly used for TPM functions, several integrated solutions from Atmel (San Jose, CA) can simplify the addition of wireless TPM functions to automotive telematics systems. These ICs are available with adequate on-board memory to support tire identification for production, service, and control purposes. This level of integration even helps keep vehicle tires in balance.

Most tire air-pressure losses occur from slow leaks in which the pressure may escape over hours, days, or even months; only a few percent of air-pressure losses are due to immediate incidents because of contact with road hazards. Slow leaks may be caused by different factors such as natural leakage, permeation, or seasonal climatic changes. Furthermore, slight damage to a tire by a road hazard that punctures small holes in the tire have been detected as a typical reason of slow air-pressure losses. When a tire is used while significantly under-inflated, its sidewalls flex more and the air temperature inside increases, making the tire more prone to failure. In addition, significantly under-inflated tires lose lateral traction, making handling more difficult. High-quality tires with optimum air pressure are equivalent to a life-insurance policy for drivers, passengers, and the vehicles themselves. Tire-related problems account for many vehicle breakdowns. Most drivers do not regularly monitor their tires, inviting dangerous conditions.

To identify slow pressure leaks and to guarantee that the driver will be warned, two automotive TPM methodologies have evolved during the past 15 years: indirect sensing systems and direct sensing systems. In the first type, the tire pressure is evaluated based on the rotational speed of a vehicle's wheel relative to the other wheels. These systems—usually linked to an automatic braking system (ABS)—compare the rotation of the wheels to determine if a wheel is under-inflated. In direct sensor systems, encapsulated tire module boards contain pressure and temperature sensors to perform an on-going series of measurements on the tire pressure and temperature. These modules are mounted in the wheel and transmit an RF signal containing the tire-pressure and temperature information to a central receiver (Rx) in the vehicle.

Early TPM systems were introduced with the indirect sensing method. The differential speed detection of the indirect pressure sensing system has several inherit weaknesses, including the fact that the loss of tire pressure cannot be detected for all four tires at a similar rate. Even monitoring two tires on the same side or axis can result in ambiguous results. In addition, the correlation of tire temperature to air pressure is not included in the indirect method, and drop in tire pressure of less than 15 percent cannot be detected with the indirect method. A system calibration time of several minutes and up to hours would be needed to teach the variables associated with distinct tire types at differential driving preconditions. The detection of under-inflated tires itself requires several minutes. The detection and determination of which tire is under-inflated is an unsolved problem. An incorrect indication of under-inflated tires has been found in certain circumstances.

Direct-sensor TPM systems have been developed to overcome the limitations of indirect sensing systems. Dedicated tire-pressure monitoring is now possible, even when all four wheels are involved. The tire-pressure monitoring can also operate when the vehicle is stationary. Smooth pressure decrease can be monitored easily. Advances in direct-sensor circuitry has enabled these modules to run under battery power with low current consumption, and new technologies that use RF energy supplied from the car body module, thus avoiding a battery supply, are in development.