ST LPS331AP Installation and operating instructions

November 2012 Doc ID 023639 Rev 3 1/27
AN4159
Application note
Hardware and software guidelines for use of the LPS331AP
Introduction
The LPS331AP is a digital MEMs pressure sensor with small package footprint and
enhanced digital features.
The official reference specification remains the datasheet.
Several demonstration systems supporting the LPS331AP are available.
www.st.com

Contents AN4159
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Contents
1 Pressure sensor demonstration boards . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Hardware (designing PCB schematics and layout) . . . . . . . . . . . . . . . . 6
2.1 LPS331AP device package, interconnect and polarization . . . . . . . . . . . . 6
2.1.1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3 Pin mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.0.1 Pressure sensor PCB layout and solder recommendations . . . . . . . . . . 8
4 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Power supply and sequencing, fail-safe I/Os . . . . . . . . . . . . . . . . . . . . . . 10
5 Using the device step-by-step, from basic to advanced . . . . . . . . . . . 11
5.1 First time bring-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1 One shot mode measurement sequence . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.2 Power optimization and estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Device register list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2 Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.1 One shot mode conversion time estimation . . . . . . . . . . . . . . . . . . . . . . 15
6.2.2 Reference S/W to get started with LPS331AP . . . . . . . . . . . . . . . . . . . 15
6.2.3 Pressure to altitude conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.4 Auto mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.5 SW filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.6 BOOT/SWRESET bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.7 Absolute accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.8 Self test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.9 Questions and answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

AN4159 List of figures
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List of figures
Figure 1. STEVAL-MKI120V1 LPS331AP adapter, STEVAL-MKI109V2 MEMS motherboard . . . . . . 4
Figure 2. STEVAL-MKI120V1 LPS331AP adapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. Pin mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 4. HCLGA-16L 3x3x1 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 5. LPS331AP electrical connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 6. Recommended land and solder mask design for LGA packages . . . . . . . . . . . . . . . . . . . . . 9
Figure 7. Overview first to second order compensation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 8. Comparison first and second order compensation at fixed pressure . . . . . . . . . . . . . . . . . 18
Figure 9. High temperature lead-free soldering profile 260 °C max. . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 10. Example of solder machine profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Pressure sensor demonstration boards AN4159
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1 Pressure sensor demonstration boards
UNICO/eMotion demonstration system: [STEVAL-MKI109V2] + [STEVAL-MKI120V1].
Figure 1. STEVAL-MKI120V1 LPS331AP adapter, STEVAL-MKI109V2 MEMS
motherboard
The STEVAL-MKI109V2 is a motherboard designed to provide users with a complete,
ready-to-use platform for the demonstration of STMicroelectronics’ MEMS products. The
board features a DIL24 socket to mount all available adapters for both digital and analog
output MEMS devices.
The motherboard includes a high-performance 32-bit microcontroller, which functions as a
bridge between the sensor and a PC, on which it is possible to use the downloadable
graphical user interface (GUI), or dedicated software routines for customized applications.
Figure 2. STEVAL-MKI120V1 LPS331AP adapter
The STEVAL-MKI120V1 adapter board is designed to facilitate the demonstration of the
LPS331AP product. The board offers an effective solution for fast system prototyping and
device evaluation directly within the user’s own application.
The STEVAL-MKI120V1 can be plugged into a standard DIL 24 socket. The adapter
provides the complete LPS331AP pinout and comes ready-to-use with the required
decoupling capacitors on the VDD power supply line.

AN4159 Pressure sensor demonstration boards
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The pinout of the adapter is fully compatible with all other available adapter boards, making
it possible to easily switch from one sensor to another during device evaluation without the
need for board redesign.

Hardware (designing PCB schematics and layout) AN4159
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2 Hardware (designing PCB schematics and layout)
2.1 LPS331AP device package, interconnect and polarization
2.1.1 Typical application circuit
Figure 3. Pin mapping
Figure 4. HCLGA-16L 3x3x1 mm
!-V

AN4159 Pin mapping
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3 Pin mapping
Table 1. LPS331AP pin mapping details
Pin # Name Class What to do
1 VDD_IO IO supply Power supply for I/Os (1.8 ~ 3.3 V)
2 NC Don’t connect
3 NC Don’t connect
4SCL (I2C)
SPC (SPI)
In (open-drain)
In (open-drain)
Ex ternal pull-up n eeded
(2.2 kΩ~ 10 kΩto VDD_IO)
---
5 GND Connect to PCB ground
6
SDA (I2C)
SDI (SPI 4W)
SDIO (SPI 3W)
I2C (open-drain)
SPI 4-wire data input
SPI 3-wire data bi-dir
External pull-up needed (2.2 kΩ~
10 kΩto VDD_IO)
7
SA0 (I2C)
SDO (SPI 4W)
NC (SPI 3W)
I2C slave address select
SPI 4-wire data output
Not connect SPI 3-wire
High (VDD_IO): 0xBA/BB I2C
slave address (best)
Low (GND_IO): 0xB8/B9 I2C slave
address
8CS SPI slave select and
I2C/SPI mode selection
1 = VDD_IO: I2C mode
0 = GND: SPI mode
9 INT1 Interrupt 1 / data ready Leave unconnected if unused
10 GND Connect to PCB ground
11 INT2 Interrupt 2 / data ready Leave unconnected if unused
12 GND_IO Connect to PCB ground
13 GND Connect to PCB ground
14 VDD Core supply voltage
1.8 ~ 3.3 V very clean supply
(put 10 µF and 100 nF decoupling
caps near device)
15 VDD 1.8 ~ 3.3 V
16 GND Connect to PCB ground

Pin mapping AN4159
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Figure 5. LPS331AP electrical connection
Key notes:
●SDA and SCL pull-up resistors should be connected to VDD_IO
●VDD_IO should be same or lower than VDD (use level shifters otherwise)
●If VDD_IO is higher than VDD, high non-destructive current may occur
●If there is choice and interface is I2C, use SA0 = VDDIO slave address by default.
3.0.1 Pressure sensor PCB layout and solder recommendations
The LPS331AP has an opening on top of the package, sensor performance can be
compromised by:
●Mechanical stress coming from PCB board
– The whole package surface + air should have minimum temperature gradient
– Avoid placement in long and narrow PCB area, warp free area
●Temperature gradients (non-uniform/rapidly changing temperature around sensor)
●Strong electrical field / light source
●Localized air pressure stability (unwanted fast air pressure variation, fans)
●Dust and water exposure/condensation (GORE-TEX®protection, etc.)

AN4159 PCB design rules
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4 PCB design rules
The pressure sensor senses mechanical stress coming from the PCB board, hence it
should be kept minimal.
PCB land and solder masking general recommendations are shown below. Refer to the
LPS331AP datasheet for pad count, size and pitch.
●It is recommended to open a solder mask external to PCB land
●The area below the sensor (on the same side of the board) must be defined as keepout
area. It is strongly recommended not to place any structure in top metal layer
underneath the sensor.
●Traces connected to pads should be as much symmetric as possible. Symmetry and
balance for pad connection will help component self alignment and will lead to a better
control of solder paste reduction after reflow
●For better performances over temperature, it is strongly recommended not to place
large insertion components like buttons or shielding boxes at distances less than 2 mm
from the sensor
●Pin #1 indicator must be left unconnected to ensure proper device operation
Figure 6. Recommended land and solder mask design for LGA packages
●A = Clearance from PCB land edge to solder mask opening ≥0.25 mm to ensure that
solder mask is opened externally to device area
●B = PCB land length = LGA solder pad length + 0.1 mm
●C = land width = LGA solder pad width + 0.1 mm
●D = Solder mask opening length = PCB land length + 0.3 mm: design 0.05 mm inside
and 0.25 mm outside
●E = Solder mask opening width = PCB land width + 0.1 mm

PCB design rules AN4159
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Stencil design and solder paste application
The soldering paste thickness and pattern are important for the proper pressure sensor
mounting process.
●Stainless steel stencils are recommended
●Stencil thickness of 90 - 150 µm (3.5 - 6 mils) is recommended for screen printing
●The final soldering paste thickness should allow proper cleaning of flux residuals and
clearance between sensor package and PCB
●Stencil aperture should have a rectangular shape with dimensions up to 25 µm (1 mil)
smaller than PCB land
●The openings of the stencil for the signal pads should be between 70 - 80% of the PCB
pad area
●Optionally, for better solder paste release, the aperture walls should be trapezoidal and
the corners rounded
●The fine IC leads pitch requires accurate alignment of the stencil to the PCB. The
stencil and printed circuit assembly should be aligned to within 25 µm (1 mil) prior to
application of the solder paste
Process consideration
In case of use of no self-cleaning solder paste, proper board washing after soldering must
be done to remove any possible source of leakage between pads due to flux residues.
The PCB soldering profile depends on the number, size and placement of components on
the board. The soldering profile should be defined by experience more than the pressure
sensor soldering profile only.
No solder material reflow on the side of the package is allowed since LGA packages show
metal trace out of package side.
Solder heat resistance and environmental specification:
The second level interconnect category on ST ECOPACK® lead-free package is marked on
the inner box label, in compliance with JEDEC Standard JESD97. Soldering conditions
maximum ratings are also marked on the same label.
LGA packages for pressure sensor are qualified for soldering heat resistance according to
JEDEC J-STD-020, in MSL3 condition.
4.1 Power supply and sequencing, fail-safe I/Os
There are 2 voltage supplies, VDD_IO and VDD in LPS331AP.
VDD is the core voltage supply to the internal circuits, power on reset, and sensor.
VDD_IO is the supply for the I2C blocks and interface signals.
The operating voltage for both VDD_IO and VDD is from 1.71 V to 3.6 V.
In order to prevent possible leakage in the operational condition, it is necessary to ensure
VDD_IO <= VDD.

AN4159 Using the device step-by-step, from basic to advanced
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5 Using the device step-by-step, from basic to
advanced
5.1 First time bring-up
1. Start by using the same supply for VDD_IO and VDD to check the device functionality.
2. Make sure of the supply impedances and check all the pins voltages in static condition.
3. The device is a simple slave with 1 byte sub-address which bit 7 should always be '1'
(bit 7 = 1 enables I2C sub-address multi-byte auto-increment = use it always!)
Use an oscilloscope to check that the device acknowledge its slave address.
Use a normal I2C bus speed (<400 kHz)
Make sure there are no other slaves with same address on the I2C bus lines.
The maximum I2C speed is driven by the slowest slave on the same I2C bus.
4. Read the Chip ID (sub address 0x8F [=0x0F]) byte: 0xBB should be read
5.1.1 One shot mode measurement sequence
5. Power down the device (clean start)
– LPS331AP_WriteByte(LPS331AP_CTRL_REG1_ADDR,0x00); // @0x20=0x00
6. Set the pressure sensor to higher-precision
– LPS331AP_WriteByte(LPS331AP_RES_ADDR,0x7A);// @0x10=0x7A
– Please note that 0x7A is forbidden in auto-mode (25Hz,25Hz): use 0x79 instead.
7. Turn on the pressure sensor analog front end in single shot mode
– LPS331AP_WriteByte(LPS331AP_CTRL_REG1_ADDR,0x84); // @0x20=0x84
8. Run one shot measurement (Temperature and Pressure), self clearing bit when done.
– LPS331AP_WriteByte(LPS331AP_CTRL_REG2_ADDR,0x01); // @0x21=0x01
9. Wait until the measurement is completed: Wait that reading (@0x21=0x00)
10. Read the Temperature measurement (2 bytes to read)
– LPS331AP_Read((u8*)pu8, LPS331AP_TEMP_OUT_ADDR, 2); // @0x2B~2C
– Temp_Reg_s16 = ((u16) pu8[1]<<8) | pu8[0]; // make a SIGNED 16 bit variable
– Temperature_DegC = 42.5 + Temp_Ref_s16 / (120*4); // scale and offset
●Read the Temperature-compensated Pressure measurement
– LPS331AP_Read((u8*) pu8, LPS331AP_PRESS_OUT_ADDR, 3); // reading
auto-incremented @0x28/29/2A
– LPS331AP_pressure_hw_reg_u32 = ((u32)pu8[2]<<16)|((u32)pu8[1]<<8)|pu8[0];
// make a unsigned 32 bit variable
– LPS331AP_pressure_mb = LPS331AP_pressure_hw_reg_u32 / 4096; // scale
11. Check the temperature and pressure values make sense
– Reading fixed 760 mb, means the sensing element is damaged.
Example of register measurements and conversion:
P = 0x3FF58D means 1023.347 mb
T = 0xE07C means 42.5 + (-8068/120) = -25.7 °C

Using the device step-by-step, from basic to advanced AN4159
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100 °C = 0x6BD0, 0 °C = 0xB050
5.1.2 Power optimization and estimation
●Pavg = bit [3..0] of I2C register @0x10 (LPS331AP_RES_ADDR)
●Tavg = bit [6..4] of I2C register @0x10 (LPS331AP_RES_ADDR)
Stand-by: The minimum power consumption is when the device is in PD=0 AND the
interrupt pins are not configured to drain current.
Operating: The power consumption depends on HW averaging and AutoRefresh frequency.
In one shot mode, the device automatically goes stand by when measurement is completed.
The right compromise between consumption, update speed and software filtering has to be
tuned application by application.
●Icc = [1 µA/Hz + 48 nA/Hz*Pavg] * ODRP + 32 nA/Hz * Tavg * ODRT
ODRP= ODR Pressure (Hz)
ODRT= ODR Temperature (Hz)
Pavg=nPAve= Pressure Average (1,2,4…512)
Tavg=nTAve= Temperature Average (1,2,4..128)
●Examples 1:
ODRT=ODR Temperature=25 Hz
ODRP=ODR Pressure=25 Hz
Pavg=Pressure average=512
Tavg=Temperature Average=64
Icc=690 µA (Vdd independent at first approximation)
Table 2. Resolution mode
T_RES=bit[6..4]@0x10 Tavg P_RES=bit[3:0]@0x10 Pavg
0101
1212
2424
3838
416416
532532
664664
71287128
8 256
9 384
A 512

AN4159 Using the device step-by-step, from basic to advanced
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●Examples 2:
ODRT=ODR Temperature=1 Hz
ODRP=ODR Pressure=1 Hz
Pavg=Pressure average=512
Tavg=Temperature Average=64
Icc=27.6 µA
●When using one shot mode, the current consumption is equivalent to the current
consumption at 1 sample/Hz, during the conversion time. (see conversion time)

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6 Software
6.1 Device register list
6.2 Hints
●(AVGT,AVGP) = (128,512) (RES=0x7A) is not supported with 25/25 Hz ODR.
– We recommend to use 0x79 as default value to try in a new system
– Symptom of wrong configuration can be pressure stuck to 760 mb
●If the I2C line have glitches and slaves are holding SDA low, a simple I2C error
recovery would be to send 9 stop bits (which will flush the bus), this is usually done by
S/W using the I2C GPIOs resources of the host device. A good practice would be to
check that SDA is high prior to generating a START bit.
Table 3. Registers address map
Name Type
Register address
Default Comment
Hex Binary
Reserved (do not modify) 00-07 0D - 0E Reserved
REF_P_XL R/W 8 1000 0
REF_P_L R/W 9 1001 0
REF_P_H R/W 0 A 1010 0
WHO_AM_I R 0 F 1111 10111011 Dummy register
RES_CONF R/W 10 10000 11111010
Reserved (do not modify) 11-1F Reserved
CTRL_REG1 R/W 20 010 0000 0
CTRL_REG2 R/W 21 010 0001 0
CTRL_REG3 R/W 22 010 0010 0
INT_CFG_REG R/W 23 100011 0
INT_SOURCE_REG R 24 100100 0
THS_P_LOW_REG R/W 25 100101 0
THS_P_HIGH_REG R/W 26 100110 0
STATUS_REG R 27 010 0111 0
PRESS_POUT_XL_REH R 28 010 1000 output
PRESS_OUT_L R 29 010 1001 output
PRESS_OUT_H R 2A 010 1010 output
TEMP_OUT_L R 2B 010 1011 output
TEMP_OUT_H R 2C 010 1100 output
Reserved (do not modify) 2D-2F Reserved

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●The serial bus power consumption can be reduced by reducing the data traffic:
– Using built-in H/W averaging instead of S/W averaging
– Reduce the polling rate when waiting for the one shot measurement completion
(2.1.1 #9)
– The INT pin can be used as event to minimize serial bus polling
– Always set the PD bit to zero before changing the register settings
6.2.1 One shot mode conversion time estimation
●Typical conversion time ≈62.5*(Pavg+Tavg) + 1545 µs
– ex: Tavg = 128; Pavg = 512; Typ. conversation time ≈41545 µs (or 24 Hz => this
configuration is not compatible with 25 Hz)
– ex: Tavg = 128; Pavg = 384; Typ. conversation time ≈33545 µs
– The formula is accurate within +/- 3% at room temperature
6.2.2 Reference S/W to get started with LPS331AP
There is a list of C source files that provide the baseline in initializing, configuring and
performing measurements with the pressure sensor.
6.2.3 Pressure to altitude conversion
The simplest and widely used barometer (altitude) formula used in most watches come from
the US Standard Atmosphere, for example the 1976 edition.
Example of the source code is shown below:
void From_Pressure_mb_To_Altitude_US_Std_Atmosphere_1976_ft(double*
Pressure_mb, double* Altitude_ft) {
//=(1-(A18/1013.25)^0.190284)*145366.45
*Altitude_ft = (1-pow(*Pressure_mb/1013.25,0.190284))*145366.45;
}
void From_ft_To_m(double* ft, double* m) {
//=D18/3.280839895
*m = *ft/3.280839895;
}
Terminology is very delicate for altitude. There is not one altitude, there are many.
For example, the GPS altitude is different than the pressure altitude or density altitude.
Aircraft flying between airports are using the ISA altitude which is based on yearly/earth
wide sea level pressure average value (as shown above) to avoid plane collision: it is the
altimeter reference.
When aircraft get near the airport, local equivalent sea level and airfield temperature are
wirelessly shared from airport to plane to compute local instant airfield altitude. This is
known as QNH, using METAR data.

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6.2.4 Auto mode
Some applications have an enable/measure/disable scheme. In such cases, it can be useful
to use the auto mode (CTRL1_REG ODN[1:0]). This way, the device runs measurements at
fixed time interval, and the application can get the latest measurement without latency.
6.2.5 SW filtering
If higher precision is required, or if the air pressure/flow is unstable, a S/W filter could be
implemented on the sensor measurements.
While a classical moving average can be implemented as a simple solution it has some
drawback on the RAM usage and response time.
For more advanced filtering, pressure variation limiter can be implemented: using a fast
measuring rate (12 or 25 Hz rate), decide what is the maximum pressure variation between
2 successive measurement. Use this variation limit to dampen the high-frequency noise.
For indoor navigation when sub meter detection is required, a special recursive filter could
be used. Tailoring the S/W filter for the sensor characteristic and the application requirement
is usually a good move. Sensor fusion is the most advanced filtering which uses all available
information and uses them to reduce the positioning noise.
6.2.6 BOOT/SWRESET bit
The boot bit can be used when the device is enabled to reinitialize the volatile registers from
internal non-volatile trimming memory. Follow this sequence:
1. Turn on BOOT bit
2. Wait BOOT bit self-clear (with S/W timeout)
3. Wait an additional 5 msec
4. Configure the registers
5. Power up the device for normal use
The SWRESET bit resets all the other registers to their reset values and stops any internal
state machines.
6.2.7 Absolute accuracy
The devices are trimmed at final test with a typical absolute accuracy of +/-2mb with no
external S/W pressure temperature compensation over the temperature range 0°C - 65°C,
after quadratic compensation
Reflow soldering may cause an additional spread of the device population, the spread is
PCB construction, assembly and layout specific.
Beware that normal sockets and board warping can cause absolute accuracy errors.
If very good absolute accuracy is needed, a 1 point calibration in the production line could
be implemented. Use a high precision barometer, and store in the application a S/W
compensation offset, which could also take into account ageing test results.
LPS331AP devices may have higher absolute accuracy by implementing an extra S/W
pressure temperature compensation algorithm, which is called “quadratic pressure
compensation in temperature”, and is described in this chapter.

AN4159 Software
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General presentation of the concept
A linear pressure temperature compensation is implemented in the ASIC, using 3
embedded sensor calibration point information.
A second order of accuracy can be achieved (less pressure variation when temperature
changes) by implementing a quadratic compensation (second order polynomial
approximation) on top of the piecewise linear (PWL) one, by S/W, using the same calibration
data built-in the sensor registers.
Here we describe the basic quadratic concept, assuming the T0 = 10 °C:
Figure 7. Overview first to second order compensation
●Input parameters: TCV1, TCV2, TCS1, TCS2, Dgain
●Constants parameter: T1=480*(10-42.5)LSB; T2=0 LSB, T3=480*(70-42.5) LSB;
P0=1000mbar.
Step 1: pressure calibration points calculation
Equation 1

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Step 2: linear system resolution for Quadratic law determination
Equation 2
Step 3: linear system resolution for pwl compensation
Equation 3
Coefficient straight line equation:
Equation 4
Since d1=d2=c, the linear system resolution became:
Equation 5
Step 4: output pressure correction
Equation 6
Figure 8. Comparison first and second order compensation at fixed pressure
y1=e1*T+d1; y2=e2*T+d2

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All the needed coefficients to compute the quadratic pressure in the microcontroller are
available within the sensor registers. A reference C source code (using floating point or non-
floating point arithmetic) is available.
6.2.8 Self test
Devices can be self tested to a certain degree:
●If the I2C slave address is not acknowledged, the power supply, I2C lines and pull-ups
are missing or the wrong slave address has been selected
●If the device respond and the pressure is always fixed and out of range (e.g. 760 mb),
check that if you are using the auto reporting of the ODR, make sure that Tave and
Pave is in line with the specs and if u are in one shot mode, waiting for the data ready
bit flag before to do a read pressure, if all configurations are in line and the pressure is
still fixed at 760mb, the sensing element (PZT membrane/bonding wires between
sensing element and ASIC) is damaged. Do a registers dump for us to do a diagnostic.
This feature is available on FROG. LGA socket to UNICO adapter is available,
especially for failure analysis preliminary report.
●A very big or “negative” pressure could come from bonding wire being cut by
mishandling of the device (bonding between sensing element and ASIC)
●An example of C type self-test function is available
6.2.9 Questions and answers
Case 1:
“We are testing the barometer board through 4 wire SPI interface.
After setting CTRL_REG1 (0x20) to 0xF0, we are polling STATUS_REG (0x27) and
PRESS_OUT_XL/L/H, TEMP_OUT_L/H every second.
The value of STATUS_REG is 0x11 for every read.
The temperature values are updating correctly; it’s about 22.5 °C after the conversion, and
goes up if I put my hand on the board.
However, the pressure value is always constant at 0x2f8000 (PRESS_OUT_H &
PRESS_OUT_L & PRESS_OUT_XL); it does not seem to work properly.
Are there any additional configurations required?”
●RES register was set to @0x10 = 0x7 A while ODR was set to (25 Hz, 25 Hz). Solved
by changing RES to 0x79 or ODR to (12.5 Hz, 12.5 Hz).

Software AN4159
20/27 Doc ID 023639 Rev 3
Case 2:
“The measured pressure altitude by the sensor is different than my portable GPS
Barometer”
●Absolute pressure sensor measures the air pressure at the sensing point. The base
rule of thumb is that the pressure drops by 1 mbar every 8.3 meters.
●The “sea level pressure average” is 1013.25 mbar (standard altitude measurement
done in most of digital barometer wrist watches)
●The “instant sea level pressure” is weather and location dependent and can vary by
more than 5 mbar (compare SFO and SIN airports on the web)
●Also beware that pressure altitude may be different than GPS altitude, which may also
be different than density altitude.
Case 3:
“The PCB design rule on page 4 says:
‘The area below the sensor (on the same side of the board) must be defined as
keepout area. It is strongly recommended not to place any structure in the top metal
layer underneath the sensor.’
Does this mean I can’t even place the ground plane immediately under the LPS331AP? Can
I place the ground plane in the PCB middle layer that is under the LPS331AP?”
●You can place the ground plane in the PCB middle layer under the LPS331AP, but not
in the plane immediately under the LPS331AP.
Case 4:
“We are going to mount the LPS331AP and the LIS3DH on an accessory. The accessory is
powered by coin batteries and Vdd fluctuations are possible.
For this device/accessory, we plan to use lithium coin battery (CR2032), and directly supply
the voltage to the sensors.
However, depending on the load conditions, the voltage supplied by the coin battery greatly
fluctuates (as much as 0.4 V).
Assuming that the voltage supplied to the sensor is within the “normal operating voltage
conditions” (even after this 0.4 V drop),
Do you foresee any issues with the performance and/or characteristics of LIS3DH and
LPS331AP?”
●The max peak current from pressure sensor is 1 mA. A CR2032 is usually 220 mAH,
3.3 V with around 33 Ωintrinsic resistance
●As the battery ages, the supply may drop down to 2.6 V.
●As long as the recommended decoupling capacitors are placed near the sensor, our
tests with batteries and switched resistive load shows that it should be fine even with
fast slew rates.
Case 5:
“Does LPS331AP have internal pull-up resistor on the I2C lines?”
●No