MMA8652FC datasheet(15/65 Pages) NXP

15 / 65 page

MMA8652FC

Sensors

Freescale Semiconductor, Inc.

5.5

Auto-WAKE/SLEEP mode

The MMA8652FC can be configured to transition between sample rates (with their respective current consumption) based on

four 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.

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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.

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SLEEP mode occurs after the accelerometer has not detected an interrupt for longer than the user-definable timeout 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/Freefall,

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

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 WAKE mode.

5.6

Freefall and motion detection

MMA8652FC has a flexible interrupt architecture for detecting either a Freefall or a Motion.

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Freefall can be enabled where the set threshold must be less than the configured threshold.

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Motion can be enabled where the set threshold must be greater than the configured threshold.

The motion configuration has the option of enabling or disabling a high-pass filter to eliminate tilt data (static offset); the freefall

configuration does not use the high-pass filter.

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. Usable threshold levels are typically between ±100 mg and

±500 mg.

5.6.2

Motion detection

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.

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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 > 2 g. This condition would need to occur for a

minimum of 100 ms to ensure that the event was not 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 (like 100 ms or

longer).

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To detect the direction of the motion, there is also directional data available in the source register. This is useful for

applications such as directional shake or flick, which assists with the algorithm for various gesture detections.

5.7

Transient detection

The MMA8652FC 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 cutoff frequency can be set to four different frequencies, which depends on the Output

Data Rate (ODR). A higher cutoff frequency ensures that 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 you to set the

threshold and debounce counter. The transient detection feature can be used in the same manner as the motion detection

feature, by bypassing the high-pass filter. There is an option in the configuration register to do this, which adds more flexibility to

accommodate various use cases.

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

due to gravity only. These functions benefit from acceleration data being filtered with 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 (which are dependent on dynamic acceleration data) when the static component has been removed.

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