ZSTAR3 from Freescale is a system designed to offer a multiple triple-axis reference design demo. ZSTAR3 is the natural evolution of the former model (the ZSTAR), enriched and extended with the adoption of the MMA7660FC digital accelerometer. Basically, the system consists of several accelerometer boards which communicate with a single USB node through a wireless ZigBee 2.4 GHz link. The USB node can then be connected to an ordinary PC equipped with a dedicated software application, able to control multiple nodes. The following picture shows the two basilar components of the ZSTAR3 family system: on the left there is a triaxial accelerometer demo board, whereas on the right there is the USB stick to be connected to a PC

The main components of a ZSTAR3 system are the following:

• accelerometer boards, either with a digital or analog device: in the first case the acceleration is measured in three axes using a pin compatible digital, whereas in the second case it makes use of an analog sensor. Digital accelerometer boards are based on the MMA7660FC or on the MMA7455L triple-axis digital accelerometers, whereas the analog one is based on the MMA7361L triple-axis analog accelerometer. These sensors are characterized by extremely small sizes, high sensitivity, selectable g-range for various kinds of applications, fast power-up response time. Sensor boards are powered by a coin-sized CR2032 3V battery
• a MC1321x 2.4 GHz low-power transceiver/modem, compliant to ZigBee specifications (IEEE 802.15.4), with a S08 microcontroller integrated on the same package. This solution is able to guarantee reduced PCB size and less need for external components. The transceiver/modem has 16 selectable channels, programmable output power, and 2V to 3.4V operating voltage with on-chip voltage regulators
• MC68HC908JW32 USB 2.0 full-speed 8-bit MCU. This architecture is available with a wide range of memory sizes, packages, and modules. Moreover, additional benefits are possible through the high level of code optimization of the C compiler.

The system can be powered directly by batteries, since operations involve low-power and low-current.The ZSTAR3 design demo board provides a valuable instrument to evaluate different types of accelerometers, which can be used in several target applications such as:

• Mobile phones
• Hand-held gaming devices
• Portable media players
• MP3 players
• PDAs
• Personal computer peripherals
• GPS navigation devices
• Remote controls

The block diagrams related to the sensor board and to the USB stick devices are shown in the picture shown below:

The MMA7660FC digital accelerometer is characterized by the following features:

• ±1.5g three-axis digital accelerometer with digital output (I2C interface) with a 6-bit resolution
• Small, low-profile 3mm x 3mm x 0.9mm DFN package
• low power current consumption specifications:
  • Off mode: 0.4 μA
  • Standby mode: 2 μA
  • Active mode: 47 μA at one sample per second
  • Configurable output data rate from 1 to 120 samples per second
• Configurable orientation detection for portrait/landscape capability
• Six orientations: left, right, up, down, back and front
• Gesture detection, including shake detection and tap detection
• Low voltage operation:
  • Analog voltage: 2.4V – 3.6V
  • Digital voltage: 1.71V – 3.6V
• Auto-Wake/Sleep feature for low power consumption
• Robust design with high shocks survivability (10000 g)
• Low cost device

Typical applications for the MMA7660FC include:

• Mobile Phone/PMP/PDA: Orientation Detection (Portrait/Landscape)
• Image Stability, Text Scroll, Motion Dialing, Tap to Mute
• Laptop PC: Anti-Theft
• Gaming: Motion Detection, Auto-Wake/Sleep For Low Power Consumption
• Digital Still Camera: Image Stability

The MMA7660FC digital accelerometer is all built in MEMS technology. Actually it is a capacitive MEMS sensor, since internally it consists of a MEMS capacitive sensing g-cell and a signal conditioning ASIC, both integrated on the same chip. The ASIC controller makes use of switched capacitor techniques to measure the g-cell capacitor, and derive the acceleration data from the difference between two capacitor values. Additional task performed by the ASIC involves signal conditioning, data filtering, and the conversion of acceleration data into a digital value which is proportional to the acceleration itself. The user can choose to enable or disable any of the following interrupts: Front/Back Interrupt, Up/Down/Left/Right Interrupt, Tap Detection Interrupt, GINT (real-time motion tracking), Shake on X-axis, Shake on Y-axis, and Shake on Z-axis. If an interrupt related to any of the three shake axes is enabled, an excessive agitation (i.e. greater than 1.3g) will cause an interrupt to be generated.

The MMA7660FC includes also a Tap Detection feature, useful for many different customer applications such as button replacement. For instance, a single tap could stop a song from playing and a double tap could start playing a song. This function is triggered by a fast transition that exceeds a user-defined threshold for a specified duration. The shake feature can be used as a button replacement to perform functions such as scrolling through images or web pages on a Mobile Phone/PMP/PDA. The customer can enable the shake interrupt on any of the 3 axes. The following picture shows a dialog part of the orientation demo application.

On the other hand, the main features owned by the MMA7455L digital accelerometer are the following:

• Digital output (I2C/SPI interface) for processor system performance
• Small, low-profile 3 mm x 5 mm x 1 mm 14-pin LGA package
• XYZ: three axes of sensitivity in one device (2g, 4g, 8g)
• Single or double click (pulse) recognition
• High sensitivity:
  • 64 LSB/g at 2g
  • 64 LSB/g at 8g in 10-bit mode

Reference

www.freescale.com/zstar

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