Electronic Design

Tires Put Pressure On RF

Tire pressure monitoring systems (TPMS) soon will be required on most vehicles. Designed to meet these upcoming standards, Freescale Semiconductor's MPXY8300 can fit in each tire (Fig. 1). Typically, the chip and its battery would be mounted on the valve stem. A central control unit would receive information from these remote sensors and give drivers real-time information.

A TPMS can help extend the life of a tire and improve fuel efficiency if the driver keeps the tire pressure within specifications. Few people regularly check their tire pressure, which can vary significantly over the tire's lifetime. Now, the MPX8300 can do it for you.

The MPXY8300 uses Freescale's system-in-package (SiP), which includes an 8-bit microcontroller, a SmartMOS RF transmitter, and a pressure sensor. The SiP also can include an optional single Z-axis or dual Z- and X-axis accelerometer, which tracks rotation and position. Freescale has been working on this type of multichip packaging as more mixed technologies are brought together (see "Innovative Packaging Technology Aims At Miniaturized Devices" at www.electronicdesign.com, ED Online 13521). The RF transmitter utilizes different production processes than the microcontroller and the accelerometers.

The SiP exposes the pressure sensor while protecting other components from the harsh environment (Fig. 2). This selective encapsulation enables Freescale to contain all the parts on a single chip while significantly extending the lifetime of the SiP. Other external components include some capacitors, the antenna, and a lithium-ion battery.

According to Freescale, the battery will need to be changed after the two to four tire replacements that normally occur over a 10-year period. The long battery life is primarily due to the fact that the system will be sleeping most of the time. Sensor information will only be sent intermittently.

Half of the 32 kbytes of flash memory in the SiP's 8-bit S08 microcontroller is reserved for standard firmware, leaving plenty of headroom for customization. The system operates in sleep mode most of the time, since sensor information isn't continuously required. Typical applications wake the sensor up periodically, check it, and send the information to the central monitor.

Evaluation kits are available with a pair of battery-powered evaluation boards and a USB-based RF receiver. The RF support addresses a range of frequencies and modulation schemes because there is no single standard at this time. The MPXY8300 uses the standard Freescale BDM debug interface. Development tools include Code Warrior Studio.

The TPMS SiPs are designed for tires, but the peripheral complement makes them an interesting option for other applications, such as pointing devices. Expect to see more SiPbased solutions from Freescale as well as more uses for the TPMS platform. The chips are available with different pressure ranges as well as different accelerometer configurations. All are automotive-grade parts.

Freescale Semiconductor
www.freescale.com

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