FXOS8700CQ datasheet(20/86 Pages) FREESCALE

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7.4

Auto-Wake/Sleep mode

FXOS8700CQ can be configured to transition between sample rates (with their respective current consumptions) based on the

status of the embedded interrupt event generators in 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 event 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 when none of the enabled interrupt event functions has detected an interrupt within the user defined time-

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

mode to save power during this period of inactivity. Please refer to AN4074 for more detailed information on configuring the Auto-

Wake/Sleep function.

7.5

Hybrid mode

FXOS8700CQ uses a single common Analog-to-Digital Converter (ADC) for both the accelerometer and magnetometer. When

operating in hybrid mode (M_CTRL_REG1[m_hms] = 2’b11), both the accelerometer and magnetometer sensors are actively

measured by the ADC at an ODR equal to one half of the setting made in CTRL_REG1[dr] when operating in accelerometer-only

mode (M_CTRL_REG1[m_hms] = 2’b00 (default)) or magnetometer-only mode (M_CTRL_REG1[m_hms] = 2’b01). While the

ODR is common to both sensors when operating in hybrid mode, the OSR settings for each sensor are independent and may be

set via CTRL_REG2[mods] for the accelerometer and M_CTRL_REG1[m_os] for the magnetometer, respectively.

7.6

Accelerometer Freefall and Motion event detection

FXOS8700CQ integrates a programmable threshold based acceleration detection function capable of detecting either motion or

freefall events depending upon the configuration. For further details and examples on using the embedded freefall and motion

detection functions, please refer to Freescale application note AN4070.

7.6.1

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. Typically, the usable threshold ranges are between

±100 mg and ±500 mg.

7.6.2

Motion detection

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

set threshold for a set amount of time, 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. The timing value is set by a configurable debounce counter. The debounce counter acts like

a filter to indicate whether the condition exists for longer than a set amount 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 that generated the interrupt. This is useful for applications

such as directional shake or flick detection, and can also assist gesture detection algorithms by indicating that a motion gesture

has started.

7.7

Transient detection

FXOS8700CQ integrates an acceleration transient detection function that incorporates a high-pass filter. Acceleration data goes

through the high-pass filter, eliminating the DC tilt offset and low frequency acceleration changes. The high-pass filter cutoff can

be set by the user to four different frequencies which are dependent on the selected Output Data Rate (ODR). A higher cutoff

frequency ensures that DC and slowly changing acceleration data will be filtered out, allowing only the higher frequencies to pass.

The transient detection feature can be used in the same manner as the motion detection by bypassing the high-pass filter. There

is an option in the configuration register to do this. This adds more flexibility to cover the various customer use cases.

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 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 dynamic acceleration. The transient

detection function can be routed to either interrupt pin through bit 5 in CTRL_REG5 register (0x2E). Registers 0x1D – 0x20 and

0x79 – 0x7C are the dedicated transient detection configuration registers. The source register contains directional data to

determine the direction of the transient acceleration, either positive or negative. For further information of the embedded transient

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

application note AN4071.

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