MMA8450QT datasheet(14/57 Pages) FREESCALE

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MMA8450Q

5.5

Auto-Wake/Sleep Mode

The MMA8450Q can be configured to transition between sample rates (with their respective current consumption) based on

five of the interrupt functions of the device. The advantage of using the Auto-Wake/Sleep is that the system can automatically

transition to a higher sample rate (higher current consumption) when needed but spends the majority of the time in the Sleep

Mode (lower current) when the device does not require higher sampling rates. Auto-Wake refers to the device being triggered by

one of the interrupt functions to transition to a higher sample rate. This may also interrupt the processor to transition from a sleep

mode to a higher power mode.

Sleep Mode occurs after the accelerometer has not detected an interrupt for longer than the user definable time-out period.

The device will transition to the specified lower sample rate. It may also alert the processor to go into a lower power mode to save

on current during this period of inactivity.

The Interrupts that can wake the device from sleep are the following: Tap Detection, Orientation Detection, Motion/Freefall1,

Motion/Freefall2, and Transient Detection. The FIFO can be configured to hold the data in the buffer until it is flushed if the FIFO

Gate bit is set in Register 0x3A but the FIFO cannot wake the device from sleep.

The interrupts that can keep the device from falling asleep are the same interrupts that can wake the device with the addition

of the FIFO. If the FIFO interrupt is enabled and data is being accessed continually servicing the interrupt then the device will

remain in the wake mode. Refer to AN3921, for more detailed information for configuring the Auto-Wake/Sleep and for application

examples of the power consumption savings.

5.6

Freefall and Motion Detection

MMA8450Q has flexible interrupt architecture for detecting Freefall and Motion with the two Motion/Freefall interrupt functions

available. With two configurable interrupts for Motion and Freefall, one interrupt can be configured to detect a linear freefall while

the other can be configured to detect a spin motion. The combination of these two events can be routed to separate interrupts or

to the same interrupt pin to detect tumble which is the combination of spin with freefall. For details on the advantages of having

the two embedded functions of Freefall and Motion detection with specific application examples with recommended configuration

settings, refer to Freescale application note AN3917.

5.6.1

Freefall Detection

The detection of “Freefall” involves the monitoring of the X, Y, and Z axes for the condition where the acceleration magnitude

is below a user specified threshold for a user definable amount of time. Normally the usable threshold ranges are between

±0 mg and ±500 mg.

5.6.2

Motion Detection

There are two programmable functions for motion (MFF1 and MFF2). Motion is configured using the high-g mechanism.

Motion is often used to simply alert the main processor that the device is currently in use. When the acceleration exceeds a set

threshold the motion interrupt is asserted. A motion can be a fast moving shake or a slow moving tilt. This will depend on the

threshold and timing values configured for the event. The motion detection function can analyze static acceleration changes or

faster jolts. For example, to detect that an object is spinning, all three axes would be enabled with a threshold detection of > 2g.

This condition would need to occur for a minimum of 100 ms to ensure that the event wasn't just noise. The timing value is set

by a configurable debounce counter. The debounce counter acts like a filter to determine whether the condition exists for

configurable set of time (i.e., 100 ms or longer).

5.7

Transient Detection

The MMA8450Q has a built in high pass filter. Acceleration data goes through the high pass filter, eliminating the offset (DC)

and low frequencies. The high pass filter cut-off frequency can be set by the user to four different frequencies which are

dependent on the Output Data Rate (ODR). A higher cut-off frequency ensures the DC data or slower moving data will be filtered

out, allowing only the higher frequencies to pass. The embedded Transient Detection function uses the high pass filtered data

allowing the user to set the threshold and debounce counter.

Many applications use the accelerometer’s static acceleration readings (i.e., tilt) which measure the change in acceleration

due to gravity only. These functions benefit from acceleration data being filtered from a low pass filter where high frequency data

is considered noise. However, there are many functions where the accelerometer must analyze dynamic acceleration. Functions

such as tap, flick, shake and step counting are based on the analysis of the change in the acceleration. It is simpler to interpret

these functions dependent on dynamic acceleration data when the static component has been removed. The Transient Detection

function can be routed to either interrupt pin through bit 5 in CTRL_REG5 Register (0x3C). Registers 0x2B – 0x2E are the

dedicated Transient Detection configuration registers. For details on the benefits of the embedded Transient Detection function

along with specific application examples and recommended configuration settings, please refer to Freescale application note,

AN3918.

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