AN2960
Application note
LIS331DL: 3-axis - ±2 g /±8 g digital output
ultra compact linear accelerometer
Introduction
This document is intended to provide application information for the LIS331DL low-voltage
3-axis digital output linear MEMS accelerometer housed in an LGA package.
The LIS331DL 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 I
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 LIS331DL has a user-selectable full scale of ±2 g and ±8 g 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.
2
C/SPI serial
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 LIS331DL 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.
June 2009 Doc ID 15557 Rev 1 1/41
www.st.com
Contents AN2960
Contents
1 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Electrical connection and board layout hints . . . . . . . . . . . . . . . . . . . . . 7
2.1 Electrical connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Soldering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4 Digital interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1 I2C bus interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1 I2C operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.2 I2C subsequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2 SPI bus interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.1 Read & write protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.2 SPI read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2.3 SPI write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2.4 SPI Read in 3-wires mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5 Register description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1 Register address map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.1 Reserved registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.2 Registers loaded at boot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6 About control registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1 CTRL_REG1 (20h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2 CTRL_REG2 (21h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.3 CTRL_REG3 (22h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7 Data and status registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1 WHO_AM_I (0Fh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2 STATUS_REG (27h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.3 OUTX (29h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.4 OUTY (2Bh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2/41 Doc ID 15557 Rev 1
AN2960 Contents
7.5 OUTZ (2Dh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8 Free-fall and wake-up registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.1 HP_FILTER_RESET (23h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.2 FF_WU_CFG_1 (30h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.3 FF_WU_SRC_1 (31h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.4 FF_WU_THS_1 (32h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.5 FF_WU_DURATION_1 (33h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.6 FF_WU_CFG_2 (34h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.7 FF_WU_SRC_2 (35h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.8 FF_WU_THS_2 (36h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.9 FF_WU_DURATION_2 (37h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1 Start-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2 Reading acceleration data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2.1 Using the status register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2.2 Using the DataReady signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.3 Understanding acceleration data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.4 Interrupt generation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.5 Inertial wake-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.5.1 HP filter bypassed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.5.2 Using the HP filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.6 Free-fall detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.6.1 Roll function not used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.6.2 Roll function applied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.7 Output data rate selection and reading timing . . . . . . . . . . . . . . . . . . . . . 37
9.8 DataReady vs. interrupt signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Doc ID 15557 Rev 1 3/41
List of tables AN2960
List of tables
Table 1. Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 2. Serial interface pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 4. Registers address map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Table 5. Output data register content vs. acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Table 6. Output data rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Table 7. Timing value to avoid data loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Table 8. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4/41 Doc ID 15557 Rev 1
AN2960 List of figures
List of figures
Figure 1. Device block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. LIS331DL electrical connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 3. I
Figure 4. Read & write protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 5. SPI read protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 6. Multiple byte SPI read protocol (example using 2 bytes) . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 7. SPI write protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 8. Multiple bytes SPI write protocol (example using 2 bytes) . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 9. SPI read protocol in 3-wires mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 10. Digital processing chain block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Figure 11. FF_WU_CFG_1,2 high and low value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 12. DCRM bit function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 13. Interrupt generation block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Figure 14. Free-fall, wake-up interrupt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Figure 15. Reading timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Figure 16. Interrupt and DataReady signal generation block diagram. . . . . . . . . . . . . . . . . . . . . . . . . 39
Figure 17. DataReady signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2
C subsequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Doc ID 15557 Rev 1 5/41
Theory of operation AN2960
1 Theory of operation
The LIS331DL is an ultra compact, low-power, digital output 3-axis linear accelerometer
packaged in an LGA package. The complete device includes a sensing element and an IC
interface capable of taking information from the sensing element and providing a signal to
external applications through an I
A proprietary process is used to create a surface micro-machined accelerometer. The
technology allows the creation of suspended silicon structures which are attached to the
substrate at a few points, called “anchors”, and are free to move in the direction of the
sensed acceleration. To be compatible with traditional packaging techniques, a cap is
placed on top of the sensing element to avoid the blockage the moving parts during the
moulding phase of the plastic encapsulation.
When an acceleration is applied to the sensor, the proof mass displaces from its nominal
position, causing an imbalance in the capacitive half-bridge. This imbalance is measured
using charge integration in response to a voltage pulse applied to the sense capacitor.
In steady state, the nominal value of the capacitors are a few pico farads, and when an
acceleration is applied the maximum variation of the capacitive load is of a few femto farads.
The complete measurement chain is composed of a low-noise capacitive amplifier which
converts the capacitive unbalancing of the MEMS sensor into an analog voltage, and by
analog-to-digital converters.
The acceleration data may be accessed through an I
particularly suitable for direct interfacing with a microcontroller.
2
C/SPI serial interface (Figure 1).
2
C/SPI interface, making the device
Data synchronization in a digital system employing the device is made simpler through the
use of the DataReady signal (RDY), which indicates when a new set of measured
acceleration data is available, thus simplifying data synchronization in the digital system
employing the device.
The LIS331DL also features two independent, fully-programmable interrupt sources which
can be programmed to generate an interrupt signal when a programmable acceleration
threshold is exceeded along one of the three axes, or to detect a free-fall event.
The IC interface is factory calibrated for sensitivity (So) and zero-g level (Off).
The trimming values are stored in the device by a non-volatile structure. Any time the device
is turned on, the trimming parameters are loaded into the registers to be employed during
normal operation. This allows the user to use the device without further calibration.
Figure 1. Device block diagram
X+
I2C
SPI
CS
SCL/SPC
SDA/SDO/SDI
SDO
INT 1
INT 2
Y+
Z+
a
MUX
Z-
Y-
X-
REFERENCESELF TEST
CHARGE
AMPLIFIER
TRIMMING
CIRCUITS
A/D
CONVERTER
CONTROL LOGIC
CLOCK
CONTROL LOGIC
&
INTERRUPT GEN.
6/41 Doc ID 15557 Rev 1
AN2960 Electrical connection and board layout hints
2 Electrical connection and board layout hints
2.1 Electrical connection
The typical electrical connection of the LIS331DL is shown in Figure 2.
Figure 2. LIS331DL electrical connection
Vdd
Z
10µF
100nF
GND
Vdd_IO
SCL/SPC
1
TOP VIEW
SDO/SA0
SDA/SDI/SDO
13
X
INT 1
INT 2
95
Y
1
(TOP VIEW)
DIRECTION OF THE
DETECTABLE
ACCELERATIONS
CS
Digital signal from/to signal controller.Signal’s levels are defined by proper selection of Vdd_IO
AM03707v1
The LIS331DL is designed to operate with a supply voltage spanning from 2.16 V up to 3.6
V, while the serial interface can function down to 1.8 V.
The device core is supplied through the Vdd line (Vdd typ=2.5 V) while the I/O pads are
supplied through the Vdd_IO line. The typical current consumption in normal mode at 2.5 V
is 400 µA.
Both voltage supplies must be present at the same time to ensure proper behavior of the IC.
It is possible to remove Vdd, maintaining Vdd_IO, without blocking the communication bus.
Adequate power supply decoupling is required to ensure IC performance. Optimum
decoupling is achieved by using two capacitors of different types that target different kinds of
noise on the power supply leads. To attenuate high frequency transients, spikes, or digital
hash on the line, the use of one 100 nF ceramic or polyester capacitor is recommended,
which must be placed as close as possible to the device Vdd lead. For filtering lowerfrequency noise signals, a larger aluminum capacitor of 10 µF or greater should be placed
near the device in parallel with the former capacitor. It is recommended to place these
capacitors as near as possible to pin 14 of the device.
The functionality of the device and the measured acceleration data are selectable and
accessible through the I
2
C/SPI interface. When using the I2C, the CS must be tied high
while the SDO allows the user to select between two device addresses in cases where two
Doc ID 15557 Rev 1 7/41
Electrical connection and board layout hints AN2960
sensors must be connected on the same bus. Whenever a single sensor is present on the
2
same I
C bus, it is recommended either to connect SDO to Vdd_IO, or to leave it floating.
2.2 Soldering information
The LGA-14 package is lead-free and green package qualified for soldering heat resistance
according to JEDEC J-STD-020C. Land pattern and soldering recommendations are
available upon request.
8/41 Doc ID 15557 Rev 1
AN2960 Absolute maximum ratings
3 Absolute maximum ratings
Stresses above those listed as “absolute maximum ratings” may cause permanent damage
to the device. These are stress ratings only and functional operation of the device under
these conditions is not implied. Exposure to maximum rating conditions for extended periods
may affect device reliability.
Table 1. Absolute maximum ratings
Symbol Ratings Maximum Value Unit
Vdd Supply voltage
Vdd_IO I/O pin supply voltage
(1)
(1)
-0.3 to 6 V
-0.3 to 6 V
Vin
Input voltage on any control pin
(CS, SCL/SPC, SDA/SDI/SDO, CK)
-0.3 to Vdd_IO +0.3 V
3000 g for 0.5 ms
A
POW
Acceleration (any axis, powered, Vdd=2.5V)
10000 g for 0.1 ms
3000 g for 0.5 ms
A
T
UNP
T
STG
Acceleration (any axis, unpowered)
10000 g for 0.1 ms
Operating temperature range -40 to +85 °C
OP
Storage temperature range -40 to +125 °C
ESD Electrostatic discharge protection Class 1: 0 - 2 kV HBM
1. Supply voltage on any pin should never exceed 6.0 V
Warning:This is an ESD sensitive device. Improper
handling can cause permanent damage to the part.
Warning:This is a mechanical shock-sensitive device,
improper handling can cause permanent damage to the
part.
Doc ID 15557 Rev 1 9/41
Digital interfaces AN2960
4 Digital interfaces
The registers embedded in the LIS331DL may be accessed through I2C and SPI serial
interfaces. The latter may be SW configured to operate in either 3-wire or 4-wire interface
mode.
The serial interfaces are mapped onto the same pads. To select/exploit the I
CS line must be tied high (i.e. connected to Vdd_IO).
Table 2. Serial interface pin description
Pin Name Pin description
2
C interface, the
CS
SCL/SPC
SDI/SDA/SDO
SPI chip select (CS)
I2C/SPI selector (1: I2C mode; 0: SPI enabled)
SPI CK line (SCL)
2
I
2
I
SPI data in (SDI)
SPI data out (SDO) -for 3-wire SPI mode
2
SDO
I
SPI data out (SDO) - for 4-wire SPI mode
4.1 I2C bus interface
The LIS331DL I2C is a bus slave. The I2C is employed to write/read the data to/from the
registers.
Relevant I
Table 3. Terminology
2
C terminology is provided in Ta b l e 3 :
Term Description
Transmitter The device which sends data to the bus
Receiver The device which receives data from the bus
C clock line (SPC)
C serial data (SDA)
C less significant bit of device address
Master
The device which initiates a transfer, generates clock signals and terminates
a transfer
Slave The device addressed by the master
There are two signals associated with the I2C bus: the serial clock line (SCL) and the serial
data line (SDA). The latter is a bidirectional line used for sending and receiving data to/from
the interface. Both lines are connected to Vdd_IO through a pull-up resistor embedded in
the LIS331DL. When the bus is free, both the lines are high.
2
The I
10/41 Doc ID 15557 Rev 1
C interface is compliant with fast mode (400 kHz) I2C standards as well as normal
mode.
AN2960 Digital interfaces
4.1.1 I2C operation
The transaction on the bus is started through a START (ST) condition, which is defined as a
HIGH to LOW transition on the data line, while the SCL line is held HIGH. After the START
condition has been generated by the master, the bus is considered busy. The next byte of
data transmitted contains the address of the slave in the first 7 bits and the 8th bit instructs
whether the master is receiving data from the slave or transmitting data to the slave (SAD
subsequence). When an address is sent, each device in the system compares the first
seven bits after a start condition with its own address. If they match, the device considers
itself addressed by the master.
The slave address (SAD) associated with the LIS331DL may be selected from the two
predefined values 0011100b or 0011101b depending on the logic level present on the SDO
pin. Specifically, if the SDO pin is either connected to Vdd_IO or left unconnected, the slave
address is 0011101b. Otherwise, when it is connected to GND, the slave address is
0011100b. When placing two sensors on the same bus is not required, it is recommended to
use slave address 0011101b by either connecting the SDO pin to Vdd_IO, or leaving it
floating.
Data transfer with acknowledge is mandatory. The transmitter must release the SDA line
during the acknowledge pulse. The receiver must then pull the data line LOW so that it
remains stable low during the HIGH period of the acknowledge clock pulse. A receiver that
has been addressed is obligated to generate an acknowledge after each byte of data has
been received.
2
The I
C embedded in the LIS331DL behaves as a slave device and the following protocol
must be adhered to. After the start condition (ST), a slave address is sent. Once a slave
acknowledge (SAK) has been returned, an 8-bit sub-address will be transmitted: the 7 LSb
represents the actual register address, while the MSB enables address auto-increment. If
the MSb of the SUB field is 1, the SUB (register address) will be automatically incremented
to allow multiple data read/write. Otherwise, if the MSB of the SUB field is ‘0’, the SUB will
remain unchanged and multiple read/write on the same address can be performed.
The slave address is completed with a read/write bit. If the bit is ‘1’ (read), a repeated
START (SR) condition must be issued after the two sub-address bytes; if the bit is ‘0’ (Write)
the master transmits to the slave with unchanged direction.
Transfer when master is writing one byte to slave
Master ST SAD + W SUB DATA SP
Slave SAK SAK SAK
Transfer when master is writing multiple bytes to slave:
Master ST SAD + W SUB DATA DATA SP
Slave SAK SAK SAK SAK
Transfer when master is receiving (reading) one byte of data from slave:
Master ST SAD + W SUB SR SAD + R NMAK SP
Slave SAK SAK SAK DATA
Doc ID 15557 Rev 1 11/41
Digital interfaces AN2960
Transfer when master is receiving (reading) multiple bytes of data from slave
Master ST SAD + W SUB SR SAD + R MAK
Slave SAK SAK SAK DATA
Master MAK NMAK SP
S la ve DATA DATA
Data are transmitted in byte format (DATA). Each data transfer contains 8 bits. The number
of bytes per transfer is unlimited. Data is transferred with the most significant bit (MSb) first.
If a receiver cannot receive another complete byte of data until it has performed some other
function, it can hold the clock line SCL low to force the transmitter into a wait state. Data
transfer continues only when the receiver is ready for another byte and releases the data
line. If a slave receiver does not acknowledge the slave address (i.e. it is not able to receive
because it is performing some other real-time function) the data line must be left high by the
slave. The master can then abort the transfer. A low to high transition on the SDA line while
the SCL line is HIGH is defined as a STOP condition. Each data transfer must be terminated
by the generation of a STOP (SP) condition.
In order to read multiple bytes, it is necessary to assert the most significant bit of the subaddress field. In other words, SUB(7) must be equal to 1 while SUB(6-0) represents the
address of the first register to read.
In the communication format presented, MAK signifies master acknowledge and NMAK is
no master acknowledge.
4.1.2 I2C subsequences
In order to better define subsequences and to clarify line SCL and SDA behavior, a
description containing the discrete values of SCL and SDA follow. These simple
subsequences are used to implement the complex commands described in the following
paragraph.
12/41 Doc ID 15557 Rev 1
AN2960 Digital interfaces
Figure 3. I2C subsequences
ST: START condition
SCL 1111
SDA 1100
SR: Repeated START condition
SCL 1111
SDA 1100
SAD: Slave address (binary address: abcdefgh. In LIS331DL “abcdef” = “001110”)
SCL 00110 0110 0110 0110 0110 0110 0110 0110
SDA 0aaaa bbbb cccc dddd eeee f f f f gggg hhhh
Bit g=0 if SDO connected to GND, g=1 if SDO pad connected to Vdd
Bit h=0 => write, h=1 => read
SAK: Slave acknowledge (Z means high impedance)
SCL 0011
SDA (force) ZZZZ Check that SDA is 0 after SCL=1
SDA (re ad) XX00
SUB: Sub a
DATA (M aster): send DATA byte (binary number: abcdefgh)
DATA (Slave): read DATA byte (binary number: abcdefgh)
ddress (binary address: a0cdefgh)
SCL 00110 0110 0110 0110 0110 0110 0110 0110
SDA 0aaaa 0000 cccc dddd eeee f f f f gggg hhhh
Bit a=0 => do not increment address in multiple mode
a=1 => increment address in multiple mode
SCL 00110 0110 0110 0110 0110 0110 0110 0110
Saaaabbbbcccc dddd eeee f f f f gggg hhhh
SCL 00110 0110 0110 0110 0110 0110 0110 0110
SDA (force) ZZZZ ZZZZ ZZZZ ZZZZ ZZZZ ZZZZ ZZZZ ZZZZ
SDA (re ad) a b c d e f g h
SP: STOP condition
SCL 1111
SDA 0011
MAK: Master Acknowledge
S
CL 0011
NMAK: No Master Acknowledge
SDA 0000
SCL 0011
SDA ZZZZ or 1111
Doc ID 15557 Rev 1 13/41
Digital interfaces AN2960
4.2 SPI bus interface
The LIS331DL SPI is a bus slave. The SPI allows writing and reading to the registers of the
device.
The serial interface interacts with the outside world through 4 wires: CS, SPC, SPDI and
SPDO.
4.2.1 Read & write protocol
Figure 4. Read & write protocol
CS
SPC
SDI
DI7 DI6 DI5 DI4 DI3 DI2 DI1 DI0
DO7 DO6 DO5 DO4 DO3 DO2 DO1 DO0
SDO
RW
MS
AD5 AD4 AD3 AD2 AD1 AD0
CS is the chip select and is controlled by the SPI master. It goes low at the start of the
transmission and returns high at the end. SPC is the serial port clock and is controlled by
the SPI master. It is stopped high when CS is high (no transmission). SDI and SDO are,
respectively, the serial port data input and output. These lines are driven at the falling edge
of SPC and should be captured at the rising edge of SPC.
Both the read register and write register commands are completed in 16 clock pulses or in
multiples of 8 in cases of multiple byte read/write. Bit duration is the time between two falling
edges of SPC. The first bit (bit 0) starts at the first falling edge of SPC after the falling edge
of CS while the last bit (bit 15, bit 23, etc.) starts at the last falling edge of SPC just before
the rising edge of CS.
bit 0: RW
bit. When 0, the data DI(7:0) is written into the device. When 1, the data DO(7:0)
from the device is read. In the latter case, the chip drives SDO at the start of bit 8.
bit 1: MS
bit. When 0, the address remains unchanged in multiple read/write commands.
When 1, the address is auto-incremented in multiple read/write commands.
bit 2-7: address AD(5:0). This is the address field of the indexed register.
bit 8-15: data DI(7:0) (write mode). This is the data that is written to the device (MSb first).
bit 8-15: data DO(7:0) (read mode). This is the data that is read from the device (MSb first).
In multiple read/write commands, further blocks of 8 clock periods are added. When the MS
bit is 0, the address used to read/write data remains the same for every block. When the MS
bit is 1, the address used to read/write data is incremented at every block.
The function and the behavior of SDI and SDO remain unchanged.
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AN2960 Digital interfaces
4.2.2 SPI read
Figure 5. SPI read protocol
CS
SPC
SDI
RW
MS
AD5 AD4 AD3 AD2 AD1 AD0
SDO
DO7 DO6 DO5 DO4 DO3 DO2 DO1 DO0
The SPI Read command is performed with 16 clock pulses. A Multiple byte read command
is performed by adding blocks of 8 clocks pulses to the previous one.
bit 0: READ bit. The value is 1.
bit 1: MS
bit. When 0, do not increment the address. When 1, increment the address in
multiple readings.
bit 2-7: address AD(5:0). This is the address field of the indexed register.
bit 8-15: data DO(7:0) (read mode). This is the data that is read from the device (MSb first).
bit 16-... : data DO(...-8). Further data in multiple byte reading.
Figure 6. Multiple byte SPI read protocol (example using 2 bytes)
CS
SPC
SDI
SDO
4.2.3 SPI write
Figure 7. SPI write protocol
RW
AD5 AD4 AD3 AD2 AD1 AD0
MS
CS
DO7 DO6 DO5 DO4 DO3 DO2 DO1 DO0
DO15 DO14 DO13 DO12 DO11 DO10 DO9 DO8
SPC
SDI
RW
AD5 AD4 AD3 AD2 AD1 AD0MS
The SPI Write command is performed with 16 clocks pulses. A Multiple byte write command
DI7 DI6 DI5 DI4 DI3 DI2 DI1 DI0
is performed by adding blocks of 8 clocks pulses to the previous one.
Doc ID 15557 Rev 1 15/41
Digital interfaces AN2960
bit 0: Write bit. The value is 0.
bit 1: MS bit. When 0, do not increment the address. When 1, increment the address in
multiple writings.
bit 2 -7: address AD(5:0). This is the address field of the indexed register.
bit 8-15: data DI(7:0) (write mode). This is the data that is written to the device (MSb first).
bit 16-... : data DI(...-8). Further data in multiple byte writing.
Figure 8. Multiple bytes SPI write protocol (example using 2 bytes)
CS
SPC
SDI
RW
MS
AD5 AD4 AD3 AD2 AD1 AD0
DI7 DI6 DI5 DI4 DI3 DI2 DI1 DI0 DI1 5 DI14 DI13 DI12 DI11 DI10 DI9 DI8
4.2.4 SPI Read in 3-wires mode
3-wires mode is entered by setting the bit SIM (SPI serial interface mode selection) to 1 in
CTRL_REG2.
Figure 9. SPI read protocol in 3-wires mode
CS
SPC
SDI
RW
MS
AD5 AD4 AD3 AD2 AD1 AD0
DO7 DO6 DO5 DO4 DO3 DO2 DO1 DO0
The SPI Read command is performed with 16 clock pulses:
bit 0: READ bit. The value is 1.
bit 1: MS bit. When 0, do not increment the address. When 1, increment the address in
multiple readings.
bit 2-7: address AD(5:0). This is the address field of the indexed register.
bit 8-15: data DO(7:0) (read mode). This is the data that is read from the device (MSb first).
The Multiple read command is also available in 3-wires mode.
16/41 Doc ID 15557 Rev 1
AN2960 Register description
5 Register description
5.1 Register address map
The table below provides a listing of the registers embedded in the device.
Table 4. Registers address map
Name Type
Register
address
Default Comment
Hex Binary
Reserved (do not modify) 00-0E Reserved
Who_Am_I r 0F 00 1111 00111011 Dummy register
Reserved (do not modify) 10-1F Reserved
CTRL_REG1 rw 20 10 0000 00000111
CTRL_REG2 rw 21 10 0001 00000000
CTRL_REG3 rw 22 10 0010 00000000
HP_filter_reset r 23 10 0011 dummy Dummy register
Reserved (do not modify) 24-26 Reserved
Status_Reg r 27 10 0111 00000000
-- r 28 10 1000 Not used
Out_X r 29 10 1001 output
-- r 2A 10 1010 Not used
Out_Y r 2B 10 1011 output
-- r 2C 10 1100 Not used
Out_Z r 2D 10 1101 output
Reserved (do not modify) 2E-2F Reserved
FF_WU_CFG_1 rw 30 11 0000 00000000
FF_WU_SRC_1(ack1) r 31 11 0001 00000000
FF_WU_THS_1 rw 32 11 0010 00000000
FF_WU_DURATION_1 rw 33 11 0011 00000000
FF_WU_CFG_2 rw 34 11 0100 00000000
FF_WU_SRC_2 (ack2) r 35 11 0101 00000000
FF_WU_THS_2 rw 36 11 0110 00000000
FF_WU_DURATION_2 rw 37 11 0111 00000000
Reserved (do not modify) 38-3F Reserved
Doc ID 15557 Rev 1 17/41
Register description AN2960
5.1.1 Reserved registers
Registers marked as reserved must not be changed. Random changes of the content of
these registers may cause permanent damage to the device.
5.1.2 Registers loaded at boot
The LIS331DL is factory-trimmed. The content of the registers loaded at boot must not be
changed. Their content is automatically restored when the device is powered up.
18/41 Doc ID 15557 Rev 1
AN2960 About control registers
6 About control registers
6.1 CTRL_REG1 (20h)
Control register #1.
DR PD FS ST P ST M Zen Yen Xen
DR
PD
FS
STP, STM
Zen
Ye n
Xen
Data rate selection. Default value: 0
(0: 100 Hz output data rate; 1: 400 Hz output data rate)
Power-down control. Default value: 0
(0: power-down mode; 1: active mode)
Full scale selection. Default value: 0
(0: +/- 2 g; 1: +/- 8 g)
Self test enable. Default value: 00
(00: normal mode; 10: self test P; 01 self test M; 11: forbidden)
Z-axis enable. Default value: 1
(0: Z-axis disabled; 1: Z-axis enabled)
Y-axis enable. Default value: 1
(0: Y-axis disabled; 1: Y-axis enabled)
X-axis enable. Default value: 1
(0: X-axis disabled; 1: X-axis enabled)
The DR bit allows the selection of the data rate at which acceleration samples are produced.
The default value is 0, which corresponds to a data rate of 100 Hz. By changing the content
of DR to 1, the selected data rate is set equal to 400 Hz.
The PD
bit allows the device to exit power-down mode. The device is in power-down mode when
PD= “0” (default value after boot). The device is in normal mode when PD is set to 1.
DR (Data rate) PD (Power-down) Status
0 0 Power-down (default)
0 1 Active (100 Hz output)
1 0 Power-down
1 1 Active (400 Hz output)
The STP and STM bits are used to activate the self-test function. When one of the bits is set
to 1, a change to the device outputs occurs (refer to datasheet for specification), allowing
checking of the functionality of the whole measurement chain. STP and STM move the
output in opposite directions.
The Zen bit enables the generation of a DataReady signal for the Z-axis measurement
channel when set to 1. The default value is 1.
The Yen bit enables the generation of a DataReady signal for the Y-axis measurement
channel when set to 1. The default value is 1.
The Xen bit enables the generation of a DataReady signal for the X-axis measurement
channel when set to 1. The default value is 1.
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About control registers AN2960
6.2 CTRL_REG2 (21h)
Control register #2.
SIM BOOT -- FDS
HP
FF_WU2
HP
FF_WU1
HP coeff2 HP coeff1
SIM
BOOT
FDS
HP FF_WU2
HP FF_WU1
HP coeff2
SPI serial interface mode selection. Default value: 0
(0: 4-wire interface; 1: 3-wire interface)
Reboot memory content. Default value: 0
(0: normal mode; 1: reboot memory content)
Filtered data selection. Default value: 0
(0: internal filter bypassed; 1: data from internal filter sent to output register)
High-pass filter enabled for free-fall/wakeup # 2. Default value: 0
(0: filter bypassed; 1: filter enabled)
High-pass filter enabled for free-fall/wake-up #1. Default value: 0
(0: filter bypassed; 1: filter enabled)
High-pass filter cut-off frequency configuration. Default value: 00
HP coeff1
The SIM bit selects the SPI serial interface mode. When SIM is ‘0’ (default value), the 4-wire
interface mode is selected. The data coming from the device are sent to the SDO pad. In 3wire interface mode, the output data are sent to the SDA/SDI pad.
The BOOT bit is used to refresh the content of internal registers stored in the Flash memory
block. At device power-up, the content of the Flash memory block is transferred to the
internal registers related to the trimming functions, to permit good device behavior. If for any
reason the content of the trimming registers is changed, this bit can be used to restore the
correct values. When the BOOT bit is set to ‘1’, the content of the internal Flash is copied to
the corresponding internal registers and used to calibrate the device. These values are
factory-trimmed, are different for every accelerometer and normally should not be changed.
At the end of the boot process, the BOOT bit is set again to ‘0’.
The FDS bit enables (FDS=1) or bypasses (FDS=0) the high-pass filter in the signal chain of
the sensor.
The HP FF_WU[2:1] bits enable (HP FF_WU=1) or bypass (HP FF_WU=0) the high-pass
filter in interrupt generation blocks.
The HP coeff[2:1] bits are used to configure the high-pass filter cut-off frequency ft,
accordingly to the following table:
HPcoeff2,1
00 2 8
01 1 4
10 0.5 2
11 0.25 1
20/41 Doc ID 15557 Rev 1
ft (Hz)
(DR=100 Hz)
ft (Hz)
(DR=400 Hz)
AN2960 About control registers
Figure 10 shows the block diagram of the digital processing chain and the related control
signals.
Figure 10. Digital processing chain block diagram
Regs Array
Offset
Adjustment
Block
Digital Processing Chain
6.3 CTRL_REG3 (22h)
Control register #3.
IHL PP_OD I2CFG2 I2CFG1 I2CFG0 I1CFG2 I1CFG1 I1CFG0
IHL
Interrupt active high, low. Default value 0.
(0: active high; 1: active low)
HP Filter
HP_FILTER_RESET
CTRL_REG3(FDS)
0
Interrupt Generator
(FF_WU 1)
1
CTRL_REG2(HP FF_WU1)
0
Interrupt Generator
(FF_WU 2)
1
CTRL_REG2(HP FF_WU2)
0
Output regs
1
SRC reg 1
SRC reg 2
PP_OD
2
I
CFG2,
2
CFG0
I
I1CFG2,
I1CFG0
Push-pull/open drain selection on interrupt pad. Default value 0.
(0: push-pull; 1: open drain)
Interrupt 2 configuration bits. Default value 000.
(see table below)
Interrupt 1 configuration bits. Default value 000.
(see table below)
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About control registers AN2960
The IHL bit selects the polarity of the interrupt signal. When IHL is ‘0’ (default value) any
interrupt event is signalled with a logical 1.
The PP_OD bit defines whether the interrupt pad should operate in Push-pull or in Open
Drain mode. The latter is specifically intended for wired-OR connection of multiple interrupt
signals on the same interrupt line. The default value is ‘0’ which corresponds to Push-pull
mode.
2
The I
CFG[2:0] and I1CFG[2:0] bits select which signal should be sent out from the INT2
and INT1 interrupt pads as described in the following table:
I2CFG2 I2CFG1 I2CFG0
I1CFG2 I1CFG1 I1CFG0 INT1 pin
0 0 0 GND
001 FF_WU 1
010 FF_WU 2
0 1 1 FF_WU1 or FF_WU2
100 DataReady
111 --
INT2 pin
Two completely independent interrupt blocks are available in the LIS331DL: FF_WU1 and
FF_WU2. They can be configured using the registers described in Section 8.
HP filtered data can be selected for further processing by the Interrupt Generator block by
setting the desired values for the HP FF_WU1 and HP FF_WU2 bits in CTRL_REG2. The
default value is '0' and corresponds to the use of non-filtered data. The output of the
Interrupt Generator block is used to load the FF_WU_SRC_1 and FF_WU_SRC_2
registers.
22/41 Doc ID 15557 Rev 1
AN2960 Data and status registers
7 Data and status registers
7.1 WHO_AM_I (0Fh)
Device identification register.
00111011
This register contains a device identifier which, for the LIS331DL, is set to 3Bh.
7.2 STATUS_REG (27h)
Data output status register.
ZYXOR ZOR YOR XOR ZYXDA ZDA YDA XDA
X, Y and Z axes data overrun. Default value: 0
ZYXOR
ZOR
(0: no overrun has occurred;
1: new data has overwritten the previous data before it was read)
Z-axis data overrun. Default value: 0
(0: no overrun has occurred;
1: new data for the Z-axis has overwritten the previous data)
Y-axis data overrun. Default value: 0
YOR
(0: no overrun has occurred;
1: new data for the Y-axis has overwritten the previous data)
X-axis data overrun. Default value: 0
XOR
(0: no overrun has occurred;
1: new data for the X-axis has overwritten the previous data)
ZYXDA
X, Y and Z-axis new data available. Default value: 0
(0: a new set of data is not yet available; 1: a new set of data is available)
Z-axis new data available. Default value: 0
ZDA
(0: new data for the Z-axis is not yet available;
1: new data for the Z-axis is available)
Y-axis new data available. Default value: 0
YDA
(0: new data for the Y-axis is not yet available;
1: new data for the Y-axis is available)
X-axis new data available. Default value: 0
XDA
(0: new data for the X-axis is not yet available;
1: new data for the X-axis is available)
ZYXOR is set to one whenever new acceleration data is produced before completing the
retrieval of the previous set. When this occurs, the content of at least one acceleration data
register (i.e. OUTX, OUTY, OUTZ) has been overwritten. ZYXOR is cleared when the
acceleration data (OUTX, OUTY, OUTZ) of all the active channels are read.
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Data and status registers AN2960
ZOR is set to 1 whenever a new acceleration sample related to the Z-axis is generated
before the retrieval of the previous sample. When this occurs, the previous sample is
overwritten. ZOR is cleared anytime the OUTZ register is read.
YOR is set to 1 whenever a new acceleration sample related to the Y-axis is generated
before the retrieval of the previous sample. When this occurs, the previous sample is
overwritten. YOR is cleared anytime the OUTY_H register is read.
XOR is set to 1 whenever a new acceleration sample related to the X-axis is generated
before the retrieval of the previous sample. When this occurs, the previous sample is
overwritten. XOR is cleared anytime the OUTX_H register is read.
The ZYXDA bit signals that a new sample for all the enabled channels is available. ZYXDA is
cleared when the acceleration data (OUTX, OUTY, OUTZ) of all the enabled channels are
read.
ZDA is set to 1 whenever a new acceleration sample related to the Z-axis is available. ZDA
is cleared anytime the OUTZ register is read. In order to trigger, the ZDA bit requires the Zaxis to be enabled (bit Zen=1 in CTRL_REG1).
YDA is set to 1 whenever a new acceleration sample related to the Y-axis is available. YDA
is cleared anytime the OUTY register is read. In order to trigger, the YDA bit requires the Yaxis to be enabled (bit Yen=1 in CTRL_REG1).
XDA is set to 1 whenever a new acceleration sample related to the X-axis is available. XDA
is cleared anytime the OUTX register is read. In order to trigger, the XDA bit requires the Xaxis to be enabled (bit Xen=1 in CTRL_REG1).
7.3 OUTX (29h)
X-axis output register.
XD7 XD6 XD5 XD4 XD3 XD2 XD1 XD0
7.4 OUTY (2Bh)
Y-axis output register.
.
YD7 YD6 YD5 YD4 YD3 YD2 YD1 YD0
7.5 OUTZ (2Dh)
Z-axis output register.
ZD7 ZD6 ZD5 ZD4 ZD3 ZD2 ZD1 ZD0
.
24/41 Doc ID 15557 Rev 1
AN2960 Free-fall and wake-up registers
8 Free-fall and wake-up registers
The following sections describe the registers that are involved in the generation of the
interrupt signals associated with inertial wake-up and free-fall events.
8.1 HP_FILTER_RESET (23h)
Dummy register. A reading at this address forces the high-pass filter to recover
instantaneously the dc level of the acceleration signal provided to its inputs. After the above
reading, the output of the high-pass filter is zero.
XXXXXXXX
8.2 FF_WU_CFG_1 (30h)
Free-fall and wake-up configuration register for interrupt source 1.
.
AOI LIR ZHIE ZLIE YHIE YLIE XHIE XLIE
AOI
And/Or combination of interrupt events. Default value: 0
(0: OR combination of interrupt events; 1: AND combination of interrupt events)
Latch interrupt request to FF_WU_SRC reg with the FF_WU_SRC reg cleared by
LIR
reading FF_WU_SRC_1 reg. Default value: 0
(0: interrupt request not latched; 1: interrupt request latched)
Enable interrupt generation on Z high event. Default value: 0
ZHIE
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
Enable interrupt generation on Z low event. Default value: 0
ZLIE
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
Enable interrupt generation on Y high event. Default value: 0
YHIE
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
Enable interrupt generation on Y low event. Default value: 0
YLIE
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
Enable interrupt generation on X high event. Default value: 0
XHIE
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
XLIE
Enable interrupt generation on X low event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
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Free-fall and wake-up registers AN2960
The AOI bit allows selection of wake-up (OR combination of interrupt events) or free-fall
(AND combination of interrupt events) detection
LIR defines whether the configured interrupt event should be latched by the device once it
has occurred. The interrupt request is cleared by reading the related source register
(FF_WU_SRC_1).
XHIE (YHIE, ZHIE) set an interrupt event to occur when the measured acceleration data on
the X (Y, Z) channel is higher than the threshold set in the FF_WU_THS_1 register.
XLIE (YLIE, ZLIE) set an interrupt event to occur when the measured acceleration data on
the X (Y, Z) channel is lower than the threshold set in the FF_WU_THS_1 register.
The threshold module used by the system to detect free-fall or inertial wake-up events is
defined by FF_WU_THS_1. The threshold value is expressed over 7 bits as an unsigned
number and X, (Y, Z) high is true when the unsigned acceleration value of the X (Y, Z)
channel is higher than or equal to FF_WU_THS_1. Similarly, X, (Y, Z) low is true when the
unsigned acceleration value of the X (Y, Z) channel is lower than FF_WU_THS_1. Refer to
Figure 11 for more details.
Figure 11. FF_WU_CFG_1,2 high and low value
+ Full Scale
X (Y, Z) high
Threshold module
Positive
acceleration
X (Y, Z) low
X (Y, Z) high
8.3 FF_WU_SRC_1 (31h)
Free-fall and wake-up source register for interrupt 1. Read only register.
.
x IA ZHZLYHYLXHXL
Interrupt active. Default value: 0
IA
ZH
ZL
YH
YL
XH
XL
(0: no interrupt has been generated;
1: one or more interrupt event has been generated)
Z high. Default value: 0
(0: no interrupt; 1: ZH event has occurred)
Z low. Default value: 0
(0: no interrupt; 1: ZL event has occurred)
Y high. Default value: 0
(0: no interrupt; 1: YH event has occurred)
Y low. Default value: 0
(0: no interrupt; 1: YL event has occurred)
X high. Default value: 0
(0: no interrupt; 1: XH event has occurred)
X low. Default value: 0
(0: no interrupt; 1: XL event has occurred)
0 g level
Threshold module
- Full Scale
Negative
acceleration
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AN2960 Free-fall and wake-up registers
This register keeps track of the acceleration event triggering (or that has triggered, if the LIR
bit in FF_WU_SRC_1 reg is set to 1) the interrupt signal. In particular, IA is equal to 1 when
the combination of acceleration events specified in FF_WU_CFG_1 register is true. This bit
is used for the generation of the interrupt signal associated with free-fall/wake-up events.
X, (Y, Z) high is true when the module of the acceleration value of the X (Y, Z) channel is
higher than the preset threshold which is defined as the concatenation of FF_WU_THS_1.
Similarly, X, (Y, Z) low is true when the module of the acceleration value of the X (Y, Z)
channel is lower than FF_WU_THS_1.
Reading at this address clears the FF_WU_SRC_1 register and the FF_WU1 interrupt and
allows the refreshment of data in the FF_WU_SRC_1 register itself, if the latched option was
chosen.
8.4 FF_WU_THS_1 (32h)
Free-fall, wake-up threshold for interrupt 1.
DCRM THS6 THS5 THS4 THS3 THS2 THS1 THS0
DCRM
Duration counter reset mode selection. Default value: 0
(0: counter reset; 1: counter decremented)
THS6, THS0 Free-fall / wake-up threshold: default value: 000 0000
The DCRM bit allows selection of the way in which the duration counter is reset. When
DCRM is ‘0’, the duration counter is reset immediately whenever the internal inertial event
programmed by the user is not active (Figure 12 part B) while it is decremented when
DCRM is set to 1 (Figure 12 part C). The latter configuration allows the filtering out of
spurious spikes which might impair the recognition and validation of inertial events.
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Free-fall and wake-up registers AN2960
Figure 12. DCRM bit function
Duration
Free-fall event
Duration THS
Counter value
Free-fall interrupt
Duration
Free-fall event
Duration THS
B
Counter value
Free-fall interrupt
Duration
Free-fall event
Duration THS
Counter value
Free-fall interrupt
8.5 FF_WU_DURATION_1 (33h)
Set the minimum duration of the free-fall, wake-up event that must be recognized by the
LIS331DL.
D7 D6 D5 D4 D3 D2 D1 D0
D7, D0 Duration value. Default value: 0000 0000
D7, D0 define the minimum duration of the programmed inertial event such as free-fall and
wake-up that must be recognized by the device. Duration increments and maximum value
depend on the ODR chosen. For an output data rate of 400 Hz, the register allows setting a
duration spanning from 0 to 637.5 ms in increments of 2.5 ms. Conversely, when the output
data rate is set to 100 Hz, it is possible to define an event duration spanning from 0 to 2.55
seconds in increments of 10 ms. The counter used to implement the duration function is
blocked when the LIR bit in the configuration register is set to one and the interrupt event
has occurred.
C
28/41 Doc ID 15557 Rev 1
AN2960 Free-fall and wake-up registers
8.6 FF_WU_CFG_2 (34h)
Free-fall and wake-up configuration register for interrupt source 2.
AOI LIR ZHIE ZLIE YHIE YLIE XHIE XLIE
AOI
LIR
ZHIE
ZLIE
YHIE
YLIE
XHIE
XLIE
And/Or combination of interrupt events. Default value: 0
(0: OR combination of interrupt events; 1: AND combination of interrupt events)
Latch interrupt request to FF_WU_SRC reg with the FF_WU_SRC reg cleared by
reading FF_WU_SRC_1 reg. Default value: 0
(0: interrupt request not latched; 1: interrupt request latched)
Enable interrupt generation on Z high event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
Enable interrupt generation on Z low event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
Enable interrupt generation on Y high event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
Enable interrupt generation on Y low event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
Enable interrupt generation on X high event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value higher than preset threshold)
Enable interrupt generation on X low event. Default value: 0
(0: disable interrupt request;
1: enable interrupt request on measured accel. value lower than preset threshold)
Two structures can be used to generate interrupts. They have exactly the same
configuration capability and are completely independent.
The interrupt result of each structure can be sent with indifference to the INT1 or INT2 pins
using the CTRL_REG3 register.
Doc ID 15557 Rev 1 29/41
Free-fall and wake-up registers AN2960
8.7 FF_WU_SRC_2 (35h)
Free-fall and wake-up source register for interrupt 2. Read-only register. Refer to
FF_WU_SRC_1 for details.
x IA ZHZLYHYLXHXL
Interrupt active. Default value: 0
IA
ZH
ZL
YH
YL
XH
XL
(0: no interrupt has been generated;
1: one or more interrupt event has been generated)
Z high. Default value: 0
(0: no interrupt; 1: ZH event has occurred)
Z low. Default value: 0
(0: no interrupt; 1: ZL event has occurred)
Y high. Default value: 0
(0: no interrupt; 1: YH event has occurred)
Y low. Default value: 0
(0: no interrupt; 1: YL event has occurred)
X high. Default value: 0
(0: no interrupt; 1: XH event has occurred)
X low. Default value: 0
(0: no interrupt; 1: XL event has occurred)
Reading at this address clears the FF_WU_SRC_2 register and the FF_WU2 interrupt and
allows the refreshment of data in the FF_WU_SRC_2 register if the latched option was chosen.
8.8 FF_WU_THS_2 (36h)
Free-fall, wake-up threshold for interrupt source 2.
The
DCRM THS6 THS5 THS4 THS3 THS2 THS1 THS0
DCRM
THS6, THS0 Free-fall / wake-up threshold: default value: 000 0000
The DCRM bit and THS6 to THS0 behave exactly like the bit in FF_WU_THS_1 but they are
applied to interrupt source 2.
Duration counter reset mode selection. Default value: 0
(0: counter reset; 1: counter decremented)
30/41 Doc ID 15557 Rev 1
AN2960 Free-fall and wake-up registers
8.9 FF_WU_DURATION_2 (37h)
Set the minimum duration of the free-fall, wake-up event that must be recognized by the
LIS331DL.
D7 D6 D5 D4 D3 D2 D1 D0
D7, D0 Duration value. Default value: 0000 0000
Refer to FF_WU_DURATION_1 for details.
Doc ID 15557 Rev 1 31/41
Application information AN2960
9 Application information
9.1 Start-up sequence
Once the device is powered up, it automatically downloads the calibration coefficients from
the embedded Flash to the internal registers. When the boot procedure is complete (after
approximately 3 ms) the device automatically enters power-down mode.
To turn-on the device and gather acceleration data, it is necessary to write 47h in the
CTRL_REG1. With this command the three acceleration channels (i.e. X, Y and Z-axis) are
enabled and the output data rate is set to 100 Hz.
9.2 Reading acceleration data
9.2.1 Using the status register
The device is provided with a STATUS_REG which should be polled to check when a new
set of data is available. The reading procedure should be the following:
1 read STATUS_REG
2 if STATUS_REG(3)=0 then goto 1
3 if STATUS_REG(7)=1 then some data have been overwritten
4 read OUTX
5 read OUTY
6 read OUTZ
7 data processing
8 goto 1
The check performed at step 3 permits understanding of whether the reading rate is
adequate compared to the data production rate. In case one or more acceleration samples
have been overwritten by new data because of a too slow reading rate, the bit
STATUS_REG(7) is set to 1.
The overrun bits are automatically cleared when all the data present inside the device have
been read and new data have not been produced in the meantime.
9.2.2 Using the DataReady signal
The device may be configured to generate one HW signal (DataReady) on either pin 9 or 11,
to flag that a new set of measurement data is available for reading. This signal corresponds
to the ZYXDA bit present in the STATUS_REG. The polarity of the signal is defined through
the IHL bit present in CTRL_REG3 and is deasserted when the acceleration data of all the
enabled channels have been read. To enable the DataReady signal on pin 11
(corresponding to INT1), it is necessary to set the I1CFG[2:0] bit present in CTRL_REG3 to
100. Conversely, to enable the DataReady signal on pin 9 (corresponding to INT2) it is
necessary to set the I
2
CFG[2:0] bit present in the CTRL_REG3 to 100.
32/41 Doc ID 15557 Rev 1
AN2960 Application information
9.3 Understanding acceleration data
The measured acceleration data are sent to the OUTX, OUTY and OUTZ registers. The
acceleration values are expressed as a 2’s complement number. When the full-scale is set
to 2 g, each LSB corresponds to 18 mg.
The table below provides a few basic examples of the data that is read in the data registers
when the device is subject to a given acceleration. The values listed in the table are given
under the hypothesis of perfect device calibration (i.e. no offset, no gain error, etc.) and
rounded to the closest integer.
Table 5. Output data register content vs. acceleration
FS bit = 0 FS bit = 1
Acceleration values
Output register content
0 g 00h 00h
350 mg 14h 05h
1 g 38h 0Eh
2 g 6Fh 1Ch
-350 mg ECh FBh
-1 g C8h F2h
-2 g 91h E4h
Doc ID 15557 Rev 1 33/41
Application information AN2960
9.4 Interrupt generation description
The LIS331DL provides two fully-programmable interrupt sources which may be configured
to trigger different inertial events. Among them, it is worth mentioning, is the recognition of
free-fall and wake-up events. When an interrupt condition is verified, the interrupt signal is
asserted and by reading either the FF_WU_SRC_1 or FF_WU_SRC_2 registers, it is
possible to understand which condition has occurred.
The block diagram of the interrupt block is shown below.
Figure 13. Interrupt generation block diagram
CTRL_REG2(FDS)
ADC
HP filter
100/400Hz
Ph_Gen
0
1
0
FF_WU1
1
CTRL_REG2(HP FF_WU1)
CTRL_REG1(DR,PD)
0
FF_WU2
1
CTRL_REG2(HP FF_WU2)
Figure 14. Free-fall, wake-up interrupt generator
THS reg
Accel_X
Accel_Y
|b|>a?
X_en
|b|>a?
Y_en
Output Reg
SRC1
CFG 1
SRC2
CFG 2
WU
Serial
Interface
(SPI
2
C)
& I
0
1
|b|>a?
Accel_Z
FF or WU interrupt generation is selected for each interrupt generation channel through the
AOI bit in the corresponding FF_WU_CFG register. If the AOI bit is ‘0’, signals coming from
the comparators are put in logical OR. Depending on the values written in the
FF_WU_CFG_1 and FF_WU_CFG_2 registers, each time the value of at least one of the
enabled axes exceeds the threshold written in the module in the corresponding
FF_WU_THS_1 and FF_WU_THS_2 registers, a FF, WU interrupt is generated. Otherwise,
if the AOI bit is ‘1’, signals coming from the comparators go into a “NAND” port. In this case,
an interrupt signal is generated whenever the acceleration signal of all the enabled axes is
below the threshold written in the FF_WU_THS_1 or FF_WU_THS_2 registers.
34/41 Doc ID 15557 Rev 1
Z_en
FF
FF_WU_CFG(AOI)
AN2960 Application information
The FF_WU_CFG(LIR) bit permits the determination of whether or not the interrupt request
should be latched. If the LIR bit is ‘0’ (default value), the interrupt signal goes high when the
interrupt condition is satisfied and comes back low immediately if the interrupt condition is
no longer verified. Otherwise, if the LIR bit is ‘1’, whenever an interrupt condition is applied
the interrupt signal remains high even if the condition returns to a non-interrupt status, until
a reading to the FF_WU_SRC register is performed.
The remaining bits of the FF_WU_CFG register allow the decision as to which axis the
interrupt should be performed on and in which direction the threshold must be exceeded to
generate the interrupt request.
9.5 Inertial wake-up
9.5.1 HP filter bypassed
This paragraph provides a basic algorithm which shows the practical use of the inertial
wake-up feature. In particular, with the code below, the device is configured to recognize
when the absolute acceleration along either X or Y axis exceeds a preset threshold (180 mg
is used in the example). The event which triggers the interrupt is latched inside the device
using FF_WU interrupt source 1 and its occurrence is signalled through the use of the INT2
pin.
1 write C7h into CTRL_REG1 // Turn-on the sensor and set ODR=400Hz
2 write 00h into CTRL_REG2 // Default value: high-pass filter bypassed
3 write 08h into CTRL_REG3 // FF_WU1 interrupt sent to INT2 pin
4 write 0Ah into FF_WU_THS_1 reg // Set wake-up threshold = 180 mg
5 write 00h into FF_WU_DURATION_1 reg // No filtering/confirmation on the event
6 write 4Ah into FF_WU_CFG_1 // Configure desired wake-up event
7 poll INT2 pin; if INT2=0 then goto 7 // Poll INT2 pin waiting for the wake-up event
// Return the event that has triggered the
8 read FF_WU_SRC_1 reg
interrupt
// Clear interrupt request
(Wake-up event has occurred; insert your code
9
here)
// Event handling
10 goto 7
Doc ID 15557 Rev 1 35/41
Application information AN2960
9.5.2 Using the HP filter
The code provided below gives a basic routine which shows the practical use of the inertial
wake-up feature performed on high-pass filtered data. In particular, the device is configured
to recognize when the high-frequency component of the acceleration applied along either
the X, Y or Z axis exceeds a preset threshold (180 mg is used in the example). The event
which triggers the interrupt is latched inside the device using FF_WU interrupt source 1 and
its occurrence is signalled through the usage of the INT1 pin.
1 write C7h into CTRL_REG1 // Turn-on the sensor and set ODR=400 Hz
2 write 04h into CTRL_REG2
3 write 01h into CTRL_REG3 // FF_WU1 interrupt sent to INT1 pin
4 write 0Ah into FF_WU_THS_1 reg // Set wake-up threshold = 180 mg
5 write 00h into FF_WU_DURATION_1 reg // No filtering/confirmation on the event
6 read HP_FILTER_RESET register
7 write 6Ah into FF_WU_CFG_1 // Configure desired wake-up event
8 poll INT1 pin; if INT1=0 then goto 7 // Poll INT1 pin waiting for the wake-up event
// High-pass filter enabled on FF_WU1, fcutoff= 8Hz
// Dummy read to force the HP filter to
// actual acceleration value
// (i.e. set reference acceleration/tilt value)
(Wake-up event has occurred; insert your code
9
here)
// Event handling
// Return the event that has triggered the
10 read FF_WU_SRC_1 reg
interrupt
// Clear interrupt request
11 (Insert your code here) // Event handling
12 goto 8
At step 6, a dummy read at the HP_FILTER_RESET register is performed to set the
current/reference acceleration/tilt state against which the device performed the threshold
comparison.
This read may be performed any time required to set the orientation/tilt of the device as a
reference state without waiting for the filter to settle.
36/41 Doc ID 15557 Rev 1
AN2960 Application information
9.6 Free-fall detection
9.6.1 Roll function not used
This paragraph provides the basics for the use of the free-fall detection feature. The SW
routine that configures the device to detect free-fall events and to signal them is as follows:
1 write 47h into CTRL_REG1 // Turn-on the sensor and set ODR=100 Hz
2 write 00h into CTRL_REG2 // Default value: high-pass filter bypassed
3 write 08h into CTRL_REG3 // FF_WU1 interrupt sent to INT2 pin
4 write 14h into FF_WU_THS_1 reg // Set the free-fall threshold
5 write 09h into FF_WU_DURATION_1 reg // Set minimum event duration to 90 ms
6 write D5h into FF_WU_CFG_1
// Configure free-fall recognition and latch
// interrupt request
7 poll INT2 pad; if INT2=0 then goto 7 // Poll INT2 pin waiting for the free-fall event
(Free-fall event has occurred; insert your code
8
here)
// Event handling
9 read FF_WU_SRC_1 register // Clear interrupt request
10 goto 7
The code sample exploits a threshold set at 350 mg for free-fall recognition and the event is
notified by the hardware signal INT2 pin. At step 5, the FF_WU_DURATION_1 register is
configured to ignore events that are shorter than 9/ODR=9/100~90 ms (ODR=output data
rate) in order to avoid false detections.
Once the free-fall event has occurred, a dummy read at FF_WU_SRC_1 reg clears the
request and the device is ready to recognize other events.
9.6.2 Roll function applied
The roll function can be added to free-fall condition recognition using both interrupt sources.
Interrupt signals can be sent to two different pins (INT1, INT2) or can be sent to the same
pin so that the first condition verified controls the pin (logic “OR” of interrupt signals).
9.7 Output data rate selection and reading timing
The output data rate is user-selectable through the DR bit stored in the CTRL_REG1 (20h)
register. At power-on-reset, DR is reset to 0, thus providing a default output data rate set to
100 Hz.
The selectable output data rates are given in Tabl e 6 below:
Table 6. Output data rate
DR Output data rate
0 100 Hz
1 400 Hz
Doc ID 15557 Rev 1 37/41
Application information AN2960
The output data rate precision is related to internal oscillator or to external clock precision,
and an error of +/- 10% should be taken into account.
The output data rate precision is related to internal oscillator or to external clock precision,
and an error of +/- 10% should be taken into account.
A minimum reading period 150 µs shorter than the output data rate period is defined to
avoid the loss of any data produced. During this time period the reading of the data must be
performed and the DataReady signal can be used as a trigger to begin the reading
sequence. At the end of the complete sequence, the DataReady signal goes down and the
following rising edge advises that new data are available. If this minimum reading frequency
is not observed, it is possible to lose some data and the DataReady signal loses its function
as a trigger signal. The status register can be used to infer whether an overrun condition has
occurred.
Figure 15. Reading timing
T0
T2T1
New data available
DataReady
Table 7. Timing value to avoid data loss
Time Description Min. Typ. Max.
T0 Data rate 2.5 ms - 10 ms
T1 Reading period - T0-T2
T2 New data generation 150 µs -
38/41 Doc ID 15557 Rev 1
AN2960 Application information
9.8 DataReady vs. interrupt signal
The device includes two pins which can be activated to generate either the DataReady or
the interrupt signal. The functionality of the pins is selected by setting the interrupt
configuration bits located in CTRL_REG3(2-0) for interrupt pin 1 (INT1) and in
CTRL_REG3(5-3) for interrupt pin 2 (INT2), in accordance with the block diagram given in
Figure 16.
The DataReady signal stored in STATUS_REG(3), which indicates when a new set of
acceleration data is ready, is made available by setting the cfg bit to 100. The interrupt
source 1 signal is sent to the pin when the cfg bit is set to 001 and the interrupt source 2
when the cfg bit is set to 010. A logic “OR” combination of interrupt source 1 and 2 can be
sent to an interrupt pin by writing 011 in the cfg bit.
Figure 16. Interrupt and DataReady signal generation block diagram
Interrupt Generator
(FF_WU_1, 2)
Free-Fall,
Wake-Up
Interrupt
Generator
Free-Fall,
Wake-Up
Interrupt
Generator
IA
(FF_WU_SRC_1)
IA
(FF_WU_SRC_2)
DataReady
Signal
Generator
Clock (100/400Hz)
Counter
ff_wu_duration
Clock (100/400Hz)
Counter
ff_wu_duration
1
2
DataReady signal
Latch
FF_WU_CFG1(LIR)
Latch
FF_WU_CFG2(LIR)
0
0 0 0
0 0 1
INT1 signal
0 1 0
0 1 1
0
1
0
1
1 0 0
1 1 1
CTRL_REG3(2,1,0)
0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 1 1
CTRL_REG3(5,4,3)
INT2 signal
The DataReady signal rises to 1 when a new set of acceleration data has been generated
and is available for reading. The signal is reset after all the enabled channels are read
through the serial interface.
Figure 17. DataReady signal
ACCEL DATA Accel. Sample #(N) Accel. Sample #(N+1)
RDY
DATA READ
XYZXYZ
Doc ID 15557 Rev 1 39/41
Revision history AN2960
10 Revision history
Table 8. Document revision history
Date Revision Changes
12-Jun-2009 1 Initial release.
40/41 Doc ID 15557 Rev 1
AN2960
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