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AN2579
Application note
LIS302DL 3-axis digital MEMS accelerometer
translates finger taps into actions
Introduction
This document is intended to provide application information for the LIS302DL low-voltage
3-axis digital output linear MEMS accelerometer housed in an LGA package.
The LIS302DL is an ultra-compact low-power 3-axis linear accelerometer that includes a
sensing element and an IC interface capable taking information from the sensing element
and providing the measured acceleration data to external applications via an I2C/SPI serial
interface.
The sensing element used to detect acceleration is manufactured using a dedicated process
developed by ST to produce inertial sensors and actuators in silicon.
The IC interface is instead manufactured using a CMOS process that allows a high level of
integration to design a dedicated circuit which is factory trimmed to better match the sensing
element characteristics.
The LIS302DL has a user-selectable full scale of ±2g and ±8g and is capable of measuring
accelerations with an output data rate of 100 Hz or 400 Hz. A self-test capability allows the
user to check that the system is operating correctly.
The device features two independent, highly programmable interrupt sources that can be
configured either to generate an inertial wake-up interrupt signal when a programmable
acceleration threshold is exceeded along one of the three axes, to detect a free-fall or to
recognize single/double click events.
Two independent pins can be configured to provide interrupt signals to connected devices.
The LIS302DL is available in a plastic SMD package and is designed to operate over a
temperature range extending from -40 °C to +85 °C.
The ultra small size and weight of the SMD package make it an ideal choice for handheld
portable applications such as cell phones and PDAs, or any other application where
reduced package size and weight are required.
January 2008
Rev 1
1/15
www.st.com
Contents
AN2579
Contents
1
2
3
4
2/15
Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
Single click . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2
Double click . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Register description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
CLICK_CFG (38h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2
CLICK_SRC (39h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3
CLICK_THSY_X (3Bh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4
CLICK_THSZ (3Ch) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5
CLICK_TimeLimit (3Dh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.6
CLICK_Latency (3Eh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.7
CLICK_Window (3Fh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8
CTRL_REG3 [Interrupt CTRL register] (22h) . . . . . . . . . . . . . . . . . . . . . . . 8
Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1
Playing with CLICK_TimeLimit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
Playing with CLICK_Latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
Playing with CLICK_Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
AN2579
1
Theory of operation
Theory of operation
The single click and double click recognition functions featured in the LIS302DL help to
create a man-machine interface with little software loading. The device can be configured to
output an interrupt signal on a dedicated pin when tapped in any direction.
If the sensor is exposed to a single input stimulus, it generates an interrupt request on
inertial interrupt pin INT1 and/or INT2. A more advanced feature allows the generation of an
interrupt request when a double input stimulus with programmable time between the two
events is recognized, enabling a mouse button-like functionality.
This function can be fully programmed by the user in terms of expected amplitude and
timing of the stimuli by means of the dedicated set of registers described in Chapter 2:
Register description.
The single and double click recognition works independently on the selected output data
rate.
1.1
Single click
If the device is configured for single click event detection, an interrupt is generated when the
input acceleration on the selected channel exceeds the programmed threshold, and returns
below it within a time window defined by the TimeLimit register.
If the LIR bit of the CLICK_CFG register is not set, the interrupt is kept high for the duration
of the Latency window. If the LIR bit is set, the interrupt is kept high until the CLICK_SRC
register is read.
Figure 1.
Single click event with non latched interrupt
(a)
(b)
In Figure 1(a) the click has been recognized, while in Figure 1(b) the click has not been
recognized because the acceleration goes under the threshold after the TimeLimit has
expired.
3/15
Theory of operation
1.2
AN2579
Double click
If the device is configured for double click event detection, an interrupt is generated when,
after a first click, a second click is recognized. The recognition of the second click occurs
only if the event satisfies the rules defined by the Latency and Windows registers.
In particular, after the first click has been recognized, the second click detection procedure
is delayed for an interval defined by the Latency register. This means that after the first click
has been recognized, the second click detection procedure will start only if the input
acceleration exceeds the threshold after the Latency window but before the Window has
expired [Figure 2(a)] or if the acceleration is still above the threshold after the Latency has
expired [Figure 3(b)].
Once the second click detection procedure is initiated, the second click will be recognized
with the same rule as the first: the acceleration must return below the threshold before the
TimeLimit has expired.
Appropriately defining the Latency window is important to avoid unwanted clicks due to
spurious bouncing of the input signal.
Figure 2.
Single and double click recognition
(a)
(b)
Figure 2 illustrates a single click event (a) and a double click event (b). The device is able to
distinguish between (a) and (b) by changing the settings of the CLICK_CFG register from
single to double click recognition.
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AN2579
Theory of operation
Figure 3.
Double click recognition
(a)
(b)
In Figure 3(a) the double click event has been correctly recognized, while in Figure 3(b) the
interrupt has not been generated because the input acceleration exceeds the threshold after
the Window interval has expired.
5/15
Register description
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2
Register description
2.1
CLICK_CFG (38h)
Table 1.
-
Register
LIR
Table 2.
Double_Z
Single_Y
Double_X
Single_X
Description
Latch interrupt request to CLICK_SRC reg with the CLICK_SRC reg
refreshed by reading CLICK_SRC reg. Default value: 0
(0: interrupt request not latched; 1: interrupt request latched)
Double_Z
Enable interrupt generation on double click event on Z axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Single_Z
Enable interrupt generation on single click event on Z axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Double_Y
Enable interrupt generation on double click event on Y axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Single_Y
Enable interrupt generation on single click event on Y axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Double_X
Enable interrupt generation on double click event on X axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Single_X
Enable interrupt generation on single click event on X axis. Default value: 0
(0: disable interrupt request; 1: enable interrupt request)
Truth table
Double_Z / Y / X
Single_Z / Y / X
Click output
0
0
0
0
1
Single
1
0
Double
1
1
Single or double
CLICK_SRC (39h)
Table 4.
X
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Double_Y
LIR
Table 3.
2.2
Single_Z
Register
IA
Double_Z
Single_Z
Double_Y
Single_Y
Double_X
Single_X
AN2579
Register description
Table 5.
2.3
IA
Interrupt active. Default value: 0
(0: no interrupt has been generated;
1: one or more interrupt events have been generated)
Double_Z
Double click on Z axis event. Default value: 0
(0: no interrupt; 1: Double Z event has occurred)
Single_Z
Single click on Z axis event. Default value: 0
(0: no interrupt; 1: Single Z event has occurred)
Double_Y
Double click on Y axis event. Default value: 0
(0: no interrupt; 1: Double Y event has occurred)
Single_Y
Single click on Y axis event.Default value: 0
(0: no interrupt; 1: Single Y event has occurred)
Double_X
Double click on X axis event. Default value: 0
(0: no interrupt; 1: Double X event has occurred)
Single_X
Single click on X axis event. Default value: 0
(0: no interrupt; 1: Single X event has occurred)
IA
Interrupt active. Default value: 0
(0: no interrupt has been generated;
1: one or more interrupt events have been generated)
CLICK_THSY_X (3Bh)
Table 6.
THSy3
Table 7.
2.4
Description
Register
THSy2
THSy1
THSy0
THSx3
THSx2
THSx1
THSx0
Description
THSy3, THSy0
Click threshold on Y axis. Default value: 0000
THSx3, THSx0
Click threshold on X axis. Default value: 0000
CLICK_THSZ (3Ch)
Table 8.
Register
X
Table 9.
X
X
X
THSz3
THSz2
THSz1
THSz0
Description
THSz3, THSz0
Click threshold on Z axis. Default value: 0000
From 0.5g(0001) to 7.5g(1111) with increments of 0.5g.
The THSx, THSy and THSz registers define the threshold which is used by the system to
start the click detection procedure. The threshold value is expressed over 4 bits as an
unsigned number.
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Register description
2.5
AN2579
CLICK_TimeLimit (3Dh)
Table 10.
Dur7
Register
Dur6
Dur5
Dur4
Dur3
Dur2
Dur1
Dur0
From 0 to 127.5 msec in increments of 0.5 msec.
Dur7 through Dur0 define the maximum time interval that can elapse between the start of
the click detection procedure (the acceleration on the selected channel exceeds the
programmed threshold) and when the acceleration goes back below the threshold.
2.6
CLICK_Latency (3Eh)
Table 11.
Lat7
Register
Lat6
Lat5
Lat4
Lat3
Lat2
Lat1
Lat0
From 0 to 255 msec in increments of 1 msec.
Lat7 through Lat0 define the time interval that starts after the first click detection where the
click detection procedure is disabled, in cases where the device is configured for double
click detection.
2.7
CLICK_Window (3Fh)
Table 12.
Win7
Register
Win6
Win5
Win4
Win3
Win2
Win1
Win0
From 0 to 255 msec in increments of 1 msec.
Win7 through Win0 define the maximum interval of time that can elapse after the end of the
latency interval in which the click detection procedure can start, in cases where the device is
configured for double click detection.
2.8
CTRL_REG3 [Interrupt CTRL register] (22h)
Table 13.
IHL
Table 14.
8/15
Register
PP_OD
I2CFG2
I2CFG1
I2CFG0
I1CFG2
I1CFG1
Description
IHL
Interrupt active high/low. Default value 0.
(0: active high; 1: active low)
PP_OD
Push-pull/open drain selection on interrupt pad. Default value 0.
(0: push-pull; 1: open drain)
I1CFG0
AN2579
Register description
Table 14.
Description (continued)
I2CFG2
I2CFG1
I2CFG0
Data signal on Int2 pad control bits. Default value 000.
(see table below)
I1CFG2
I1CFG1
I1CFG0
Data signal on Int1 pad control bits. Default value 000.
(see table below)
Table 15.
Truth table
I1(2)_CFG2
I1(2)_CFG1
I1(2)_CFG0
Int1(2) Pad
0
0
0
GND
0
0
1
FF_WU_1
0
1
0
FF_WU_2
0
1
1
FF_WU_1 or FF_WU_2
1
0
0
Data ready
1
1
1
Click interrupt
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Examples
3
AN2579
Examples
The following figures show the click interrupt generation in different conditions. The
illustrations have been captured on a PC running the EK302DL GUI interface. The content
of the LIS302DL registers have been modified via the dedicated panel of the software
interface that allows the user to evaluate all the different settings and features of the click
embedded function. In the following examples, only the Y axis has been enabled for the click
interrupt generation.
3.1
Playing with CLICK_TimeLimit
Figure 4 shows an acquisition carried out with CLICK_TimeLimit = 02h (1 ms). With this
setting, the single click recognition window is short and often the acceleration does not
return below the threshold in time.
In Figure 5 an acquisition done with CLICK_TimeLimit = FFh (127 ms) is shown. With this
setting the single click recognition window is longer, and it is easier for the event to be
recognized.
Figure 4.
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Short TimeLimit
AN2579
Examples
Figure 5.
Long TimeLimit
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Examples
3.2
AN2579
Playing with CLICK_Latency
Figure 6 illustrates an acquisition done with CLICK_Latency = 06h (6 ms). With this setting
the device recognizes as a click nearly every acceleration peak. In Figure 7 an acquisition
carried out with CLICK_Latency = ffh (255 ms) is displayed. With this setting the device
recognizes as a click one peak in every two.
12/15
Figure 6.
Short Latency
Figure 7.
Long Latency
AN2579
3.3
Examples
Playing with CLICK_Window
In cases of double click recognition, the CLICK _Latency + CLICK_Window defines the
maximum distance between two consecutive clicks to be recognized as double click event.
By fixing the latency to avoid spurious bouncing of the signal, one can play with the
CLICK_Window as with the “double-click speed” settings of the mouse properties on the
PC.
Figure 8 shows an acquisition done with CLICK_Window = 10h (10 ms). With this setting the
two consecutive peaks of acceleration are too far apart and the second one occurs outside
of the Window.
In Figure 9 an acquisition carried out with CLICK_Window = ffh (255 ms) is shown. With this
setting the device correctly generates the double click interrupt after the second acceleration
peak.
Figure 8.
Short Window
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Revision history
AN2579
Figure 9.
4
Revision history
Table 16.
14/15
Long Window
Document revision history
Date
Revision
17-Jan-2008
1
Changes
Initial release
AN2579
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