ADXL366 datasheet(22/71 Pages) AD

22 / 71 page

Data Sheet

ADXL366

POWER SAVINGS FEATURES

analog.com

Rev. 0 | 22 of 71

The pedometer sensitivity default value is 400LSB, and it can

be modified by the user by writing the desired value to the

PEDOMETER_SENS_H register (Register 0x4C) and PEDOM-

ETER_SENS_L register (Register 0x4D). The scale factor for

these registers is 4000LSB/g.

EXTERNAL ADC INTERRUPT

The ADXL366 incorporates a 14-bit ADC for digitization of an

external analog input. An interrupt can be generated based on a

user set threshold of the external system battery supply, and the

ADC can be used to monitor the supply voltage. If the supply

voltage falls to less than the specified threshold, an interrupt can

be generated to alert the end user to charge or change the battery.

By using this function, the host processor does not need to use

another ADC to check the power supply periodically.

MOTION DETECTION

The ADXL366 features built-in logic that detects activity (presence

of acceleration greater than a threshold) and inactivity (lack of

acceleration greater than a threshold). Activity and inactivity events

can be used as triggers to manage the accelerometer mode of

operation or trigger an interrupt to a host processor.

Detection of an activity or inactivity event is indicated in the status

the activity status of the device, whether it is moving or stationary,

is indicated by the AWAKE bit (Bit 6 of the STATUS_COPY register,

Activity and inactivity detection can be used when the accelerome-

ter is in either measurement mode or wake-up mode.

Activity Detection

An activity event is detected when acceleration remains greater

than a specified threshold for a specified time period on any of the

axes. If any axis exceeds the threshold, an activity event occurs

(unless that axis is disabled).

Referenced and Absolute Configurations

Activity detection can be configured as referenced or absolute.

When using absolute activity detection, acceleration samples are

compared to a user set threshold to determine whether motion

is present. For example, if a threshold of 0.5g is set and the

acceleration on the z-axis is 1g for longer than the user defined

activity time, the activity status asserts.

In many applications, it is advantageous for activity detection to

be based not on an absolute threshold, but on a deviation from a

reference point or orientation, which is particularly useful because

it removes the effect on activity detection of the static 1g imposed

by gravity. When an accelerometer is stationary, its output can

reach 1g, even when it is not moving. In absolute activity, when the

threshold is set to less than 1g, activity is immediately detected in

this case.

The ADXL366 is in a referenced configuration when one or both

of the activity and inactivity enables are set to referenced mode (IN-

ACT_EN = 11 or ACT_EN = 11 in Register 0x27). In a referenced

configuration, activity is detected when acceleration samples are at

least a user set amount greater than an internally defined reference

for the user defined amount of time, as described in Equation 2.

ABS(Acceleration − Reference) > Threshold

(2)

Consequently, activity is detected only when the acceleration has

deviated sufficiently from the initial orientation. The reference for

activity detection is calculated when activity detection is engaged in

the following scenarios:

►

When the activity function is turned on and measurement mode

is engaged

►

If link mode is enabled, when inactivity is detected, and activity

detection begins

►

If link mode is not enabled, when activity is detected, and activity

detection repeats

The referenced configuration results in a sensitive activity detection

that detects even the most subtle motion events.

When using the referenced configuration, it is important to note

that the device still uses the absolute thresholds when it first en-

ters measurement mode, which becomes important if an inactivity

threshold of <1g is desired. In this case, the device must enter

measurement mode with a threshold of greater than 1g. The inac-

tivity threshold can then be lowered to the desired level (while still in

measurement mode), which allows the device to set the thresholds

around the 1g offset on the z-axis.

When in referenced mode, the activity reference values can be

read by properly setting the REF_READBACK bits (Bits[7:6]) in the

ACT_INACT_CTL register (Register 0x27):

1. Write b'01 to the REF_READBACK bits in the ACT_IN-

ACT_CTL register.

2. On the next ODR read, the activity reference values from

the XDATA_H register (Register 0x0E) to ZDATA_L register

(Register 0x13).

Fewer False Positives

Ideally, the intent of activity detection is to wake-up a system only

when motion is intentional, ignoring noise or small, unintentional

movements. In addition to being sensitive to subtle motion events,

the ADXL366 activity detection algorithm is designed to be robust in

filtering out undesired triggers.

The ADXL366 activity detection functionality includes a timer to

filter out unwanted motion and ensure that only sustained motion

is recognized as activity. The duration of this timer, as well as the

ADXL366 Datasheet(PDF) 22 Page - Analog Devices

ADXL366 Datasheet(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Datasheet Download

Link URL