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ADXL313WACPZ

Digital Accelerometer / 32-Lead Lead Frame Chip

Analog Devices 

Data Sheet

ADXL313

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

reference 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

difference 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

considered inactive and the inactivity interrupt is triggered.

ACT_x and INACT_x Bits

A setting of 1 enables x-, y-, 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 inactiv-

ity detection, all participating axes are logically AND’ed, causing

the inactivity function to trigger only if all participating axes are

below the threshold for the specified period of time.

Register 0x2C—BW_RATE (Read/Write)

D7 D6 D5 D4

D3 D2 D1 D0

0 0 0 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

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

Table 5 and Table 6 for details). The default value is 0x0A, which

translates to a 100 Hz output data rate. Select an output data

rate 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.

Register 0x2D—POWER_CTL (Read/Write)

D7

D6

D5

D4

0

I2C_DISABLE Link

AUTO_SLEEP

D3

D2

D1

D0

Measure

Sleep

Wake-up

I2C_Disable Bit

The ADXL313 is capable of communicating via SPI or I2C

transmission protocols. Typically, these protocols do not

overlap; however, situations may arise where SPI transactions

can imitate an I2C start command. This causes the ADXL313 to

respond unexpectedly, causing a communications issue with

other devices on the network. To ensure that the ADXL313 does

not interpret SPI commands as an I2C start condition, assert the

I2C_Disable bit.

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 part be

placed into standby mode and then set back to measurement

mode with a subsequent write. This is done 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 ADXL313 to switch to sleep mode when inactivity is detected

(that is, when acceleration is below the THRESH_INACT value for

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

disables automatic switching to sleep mode. See the description of

the sleep bit in the Sleep Bit section for more information.

When clearing the AUTO_SLEEP bit, it is recommended that the

part be placed into standby mode and then set back to measure-

ment mode with a subsequent write. This is done to ensure that

the device is properly biased if sleep mode is manually disabled;

otherwise, the first few samples of data after the AUTO_SLEEP

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

was asleep when the bit was cleared.

Measure Bit

A setting of 0 in the measure bit places the part into standby mode,

and a setting of 1 places the part into measurement mode. The

ADXL313 powers up in standby mode with minimum power

consumption.

Sleep Bit

A setting of 0 in the sleep bit puts the part into the normal mode

of operation, and a setting of 1 places the part into sleep mode.

Sleep mode suppresses DATA_READY (see Register 0x2E,

Register 0x2F, and Register 0x30), stops transmission of data to the

FIFO, and switches the sampling rate to one specified by the

wake-up bits. In sleep mode, only the activity function can be used.

When clearing the sleep bit, it is recommended that the part be

placed into standby mode and then set back to measurement

mode with a subsequent write. This is done to ensure that the

device is properly biased if sleep mode is manually disabled;

otherwise, the first few samples of data after the sleep bit is

cleared may have additional noise, especially if the device was

asleep when the bit was cleared.

Rev. 0 | Page 19 of 28

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