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Freescale Semiconductor Application Note

AN3468 Rev 1, 05/2009

The MMA745xL Digital Accelerometer
by: Kimberly Tuck Inertial Applications Engineer Tempe, AZ

The MMA745xL digital accelerometer is a 3 x 5 x 1 mm product that can communicate using both I2C and SPI. This device has both threshold and pulse detection interrupts. There are 2 sampling rates available at 125 Hz (using the 62.5Hz digital filter) and 250 Hz (using the 125 Hz digital filter). There is a self test function to verify the integrity of the MEMS sensor and the ASIC signal path. Figure 1 shows the simple evaluation board for the MMA745xL which is available online. This board contains the accelerometer with all pins mapped out to a 14 pin header. The image on the left shows the ground plane which ties thedigital and analog ground pins together. This is necessary to improve the noise performance based on internal trim and is the recommendation for this part. The image in the center is the front view of the board which shows all the other connections. This board can be wired to a Freescale MCU evaluation or demo board and programmed for various application functions which are described in detail below.Back View

Front View

Actual Image of the Board

Figure 1. MMA745xL Accelerometer Simple Evaluation Board

The MMA745xL is the first family of digital 3-axis consumer accelerometers that Freescale has designed. There are several sensing functions that accelerometers are capable of detecting. These are motion, freefall, shock,vibration, and tilt. The advantage of a digital accelerometer is that the A/D converter is implemented within the sensor and an MCU can be used to accompany the sensor that does not require the A/D converter. The sensor can connect directly to the SPI or I2C into the MCU. The disadvantage is that there are selected digital filters within the sensor which limits the sampling rate for certain applications.Motion Detection
Typically motion detection is used to identify if an object is in use or not based on change in acceleration output. The purpose of power cycling is to try to minimize the power consumption for the application. By moving into a standby (low power) mode when the device is not in use then switching to measurement mode (active mode) which is a full power mode, the overall powerconsumption can be decreased. The power consumption is the same regardless of the two sampling rate options in the active modes. The MMA745xL can be put into standby mode for a set period of time consuming only 5 μA. Then the device must be programmed to switch over to measurement mode to monitor the X, Y, Z outputs to determine if a significant change in acceleration has occurred. This can bedone using a timer function in the MCU. The device will not auto wake from standby mode. When in an active mode the device consumes 400-450 μA of current. The device can be power cycled between standby mode and measurement mode minimizing the overall current consumption for the application. This is useful in a wide variety of applications. It is particularly important in handheld devices whichrequire batteries to operate.

© Freescale Semiconductor, Inc., 2008-2009. All rights reserved.

Hints for a Power Cycling Algorithm During a power cycling routine the device will be set to standby mode until the MCU timer triggers. Then the MCU will switch the sensor into measurement mode and take one reading from X, Y and Z. If the RMS value is a certain threshold greater than 1g or less than 1gthen motion is detected and the device will stay in measurement mode. Otherwise the MCU will switch accelerometer back into standby mode. The timing will be somewhat dependant on the circumstances of the application and the tradeoff between the reaction time vs. the consumed current. Motion Detection using the MMA745xL Logic Interrupts The Level Detection mode can be used to detect motion with...
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