ADXL314 datasheet(20/27 Pages) AD

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Data Sheet

ADXL314

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Rev. A | 20 of 27

The TIME_INACT register is eight bits and contains an unsigned

time value representing the amount of time that acceleration must

be less than the value in the THRESH_INACT register for inactivity

to be declared. The scale factor is 1 sec/LSB. Unlike the other

interrupt functions, which use unfiltered data (see the Threshold

section), the inactivity function uses filtered output data. At least

one output sample must be generated for the inactivity interrupt to

be triggered. This results in the function appearing unresponsive

if the TIME_INACT register is set to a value less than the time

constant of the output data rate. A value of 0 results in an interrupt

when the output data is less than the value in the THRESH_INACT

Table 18. Register 0x27—Bits[D7:D4]

ACT ac/dc

ACT_X enable

ACT_Y enable

ACT_Z enable

Table 19. Register 0x27—Bits[D3:D0]

INACT ac/dc

INACT_X enable

INACT_Y enable

INACT_Z enable

ACT AC/DC and INACT AD/DC Bits

A setting of 0 selects dc-coupled operation, and a setting of 1

enables ac-coupled operation.

In dc-coupled operation, the current acceleration magnitude is

compared directly with THRESH_ACT and THRESH_INACT to

determine whether activity or inactivity is detected.

In ac-coupled operation for activity detection, the acceleration value

at the start of activity detection is taken as a reference value. New

samples of acceleration are then compared to this reference value

and, if the magnitude of the difference exceeds the THRESH_ACT

value, the device triggers an activity interrupt.

Similarly, in ac-coupled operation for inactivity detection, a refer-

ence value is used for comparison and is updated whenever

the device exceeds the inactivity threshold. After the reference

value is selected, the device compares the magnitude of the differ-

ence between the reference value and the current acceleration

with THRESH_INACT. If the difference is less than the value in

THRESH_INACT for the time in TIME_INACT, the device is consid-

ered inactive and the inactivity interrupt is triggered.

ACT_x Enable Bits and INACT_x Enable Bits

A setting of 1 enables x-axis, y-axis, or z-axis participation in

detecting activity or inactivity. A setting of 0 excludes the selected

axis from participation. If all axes are excluded, the function is

disabled. For activity detection, all participating axes are logically

OR’ed, causing the activity function to trigger when any of the

participating axes exceeds the threshold. For inactivity detection,

all participating axes are logically AND’ed, causing the inactivity

function to trigger only if all participating axes are less than the

threshold for the specified period of time.

Table 20. Register 0x2C

LOW_POWER

Rate

LOW_POWER Bit

A setting of 0 in the LOW_POWER bit selects normal operation,

and a setting of 1 selects reduced power operation, which has

somewhat higher noise (see the Power Modes section for details).

Rate Bits

The rate bits select the device bandwidth and output data rate (see

Table 8 and Table 9 for details). The default value is 0x0A, which

translates to a 100 Hz output data rate. An output data rate must

be selected that is appropriate for the communication protocol and

frequency selected. Selecting too high of an output data rate with a

low communication speed results in samples being discarded.

Table 21. Register 0x2D

Link

AUTO_SLEEP

Measure

Sleep

Wakeup

Link Bit

A setting of 1 in the link bit with both the activity and inactivity

functions enabled delays the start of the activity function until

inactivity is detected. After activity is detected, inactivity detection

begins, preventing the detection of activity. This bit serially links the

activity and inactivity functions.

When this bit is set to 0, the inactivity and activity functions are

concurrent. Additional information can be found in the Link Mode

section.

When clearing the link bit, it is recommended that the device be

placed into standby mode and then set back to measurement

mode with a subsequent write. This recommendation is advised to

ensure that the device is properly biased if sleep mode is manually

disabled. Otherwise, the first few samples of data after the link bit

is cleared may have additional noise, especially if the device was

asleep when the bit was cleared.

AUTO_SLEEP Bit

If the link bit is set, a setting of 1 in the AUTO_SLEEP bit sets

the ADXL314 to switch to sleep mode when inactivity is detected

(that is, when acceleration has been less than the THRESH_INACT

value for at least the time indicated by TIME_INACT). A setting of 0

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