MMA8451QR1(2010) 데이터 시트보기 (PDF) - Freescale Semiconductor
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MMA8451QR1 Datasheet PDF : 53 Pages
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5.4 Low Power Modes vs. High Resolution Modes
The MMA8451Q can be optimized for lower power modes or for higher resolution of the output data. High resolution is
achieved by setting the LNOISE bit in Register 0x2A. This improves the resolution but be aware that the dynamic range is limited
to 4g when this bit is set. This will affect all internal functions and reduce noise. Another method for improving the resolution of
the data is by oversampling. One of the oversampling schemes of the data can activated when MODS = 10 in Register 0x2B
which will improve the resolution of the output data only. The highest resolution is achieved at 1.56 Hz.
There is a trade-off between low power and high resolution. Low Power can be achieved when the oversampling rate is
reduced. When MODS = 11 the lowest power is achieved. The lowest power is achieved when the sample rate is set to 1.56 Hz.
For more information on how to configure the MMA8451Q in Low Power mode or High Resolution mode and to realize the
benefits, refer to Freescale application note, AN4075.
5.5 Auto-WAKE/SLEEP Mode
The MMA8451Q 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. 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/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
Register 0x2C 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 AN4074, for more detailed information for configuring the Auto-WAKE/SLEEP.
5.6 Freefall and Motion Detection
MMA8451Q has flexible interrupt architecture for detecting either a Freefall or a Motion. Freefall can be enabled where the set
threshold must be less than the configured threshold, or motion can be enabled where the set threshold must be greater than
the threshold. The motion configuration has the option of enabling or disabling a high pass filter to eliminate tilt data (static offset).
The freefall does not use the high pass filter. For details on the Freefall and Motion detection with specific application examples
and recommended configuration settings, refer to Freescale application note AN4070.
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
±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. 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). There is also directional data available in the source register to detect the
direction of the motion. This is useful for applications such as directional shake or flick, which assists with the algorithm for various
gesture detections.
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