ADXL346ACCZ-R2 datasheet(30/40 Pages) AD

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ADXL346

Data Sheet

INVALIDATES DOUBLE TAP IF

SUPRESS BIT IS SET

TIME WINDOW FOR SECOND

TAP (WINDOW)

LATENCY

TIME (LATENT)

TIME LIMIT

FOR TAPS

(DUR)

Figure 47. Double-Tap Event Invalid Due to High g Event

When the Suppress Bit Is Set

A double-tap event can also be invalidated if acceleration above

the threshold is detected at the start of the time window for the

second tap (set by the window register (Address 0x23)). This results

in an invalid double tap at the start of this window, as shown in

Figure 48. Additionally, a double-tap event can be invalidated if

an acceleration exceeds the time limit for taps (set by the DUR

the end of the DUR time limit for the second tap event, also

shown in Figure 48.

INVALIDATES DOUBLE TAP

AT START OF WINDOW

TIME WINDOW FOR

SECOND TAP (WINDOW)

LATENCY

TIME

(LATENT)

INVALIDATES

DOUBLE TAP AT

END OF DUR

TIME LIMIT

FOR TAPS

(DUR)

TIME LIMIT

FOR TAPS

(DUR)

TIME LIMIT

FOR TAPS

(DUR)

Figure 48. Tap Interrupt Function with Invalid Double Taps

Single taps, double taps, or both can be detected by setting the

respective bits in the INT_ENABLE register (Address 0x2E).

Control over participation of each of the three axes in single-tap/

double-tap detection is exerted by setting the appropriate bits in

the TAP_AXES register (Address 0x2A). For the double-tap

function to operate, both the latent and window registers must

be set to a nonzero value.

Every mechanical system has somewhat different single-tap/

double-tap responses based on the mechanical characteristics of

the system. Therefore, some experimentation with values for the

DUR, latent, window, and THRESH_TAP registers is required.

In general, a good starting point is to set the DUR register to a

value greater than 0x10 (10 ms), the latent register to a value greater

than 0x10 (20 ms), the window register to a value greater than

0x40 (80 ms), and the THRESH_TAP register to a value greater

than 0x30 (3 g). Setting a very low value in the latent, window, or

THRESH_TAP register may result in an unpredictable response

due to the accelerometer picking up echoes of the tap inputs.

After a tap interrupt has been received, the first axis to exceed

the THRESH_TAP level is reported in the ACT_TAP_STATUS

overwritten with new data.

IMPROVED TAP DETECTION

Improved tap detection is enabled by setting the improved tap

bit of the TAP_AXES register (Address 0x2A). When improved

tap detection is enabled, the filtered output data corresponding to

the output data rate set in the BW_RATE register (Address 0x2C)

is processed to determine if a tap event occurred. In addition, an

ac-coupled differential measurement is used. This results in the

timing values and threshold values for improved tap detection

being different from those used for normal tap detection.

When improved tap detection is used, new values must be

determined based on test results. In general, no timing values

(in the DUR, latent, or window registers) should be set that are

less than the time step resolution set by the output data rate.

The threshold value for improved tap detection can typically be

set much lower than the threshold for normal tap detection.

The value used depends on the value in the BW_RATE register

and should be determined through system testing. Refer to the

Threshold section for more details.

TAP SIGN

A negative sign is produced by experiencing a negative accel-

eration, which corresponds to tapping on the positive face of the

device for the desired axis. The positive face of the device is the

face such that movement in that direction is positive acceleration.

For example, tapping on the face that corresponds to the +X

direction, labeled as front in Figure 49, results in a negative sign

for the x-axis. Tapping on the face labeled as left in Figure 49

results in a negative sign for the y-axis, and tapping on the face

labeled top results in a negative sign for the z-axis. Conversely,

tapping on the back, right, or bottom side results in positive

signs for the corresponding axes.

FRONT

(+X)

LEFT

(+Y)

TOP

(+Z)

Figure 49. 3D Orientation with Coordinate System

Rev. C | Page 30 of 40

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