ST LPS25H Application Note

AN4450

Application note

Hardware and software guidelines for use of LPS25H pressure

sensor

By Raffaele Di Vaio

Introduction

The LPS25H is an ultra-compact absolute piezoresistive pressure sensor with enhanced digital features in a small package footprint.

Unless specifically noted, all recommendations in this document apply only to the LPS25H.

Please refer to the LPS25H datasheet (available at www.st.com) for device and feature definitions.

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www.st.com

Contents AN4450

Contents

1 Pressure sensor evaluation boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Hardware (designing PCB schematics and layout) . . . . . . . . . . . . . . . . 6

2.1 LPS25H device package, interconnect and polarization . . . . . . . . . . . . . . 6

2.1.1 Package drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.2 Pin mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.3 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Pressure sensor PCB layout and soldering recommendations . . . . . . . . . 9

2.2.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2 Stencil design and solder paste application . . . . . . . . . . . . . . . . . . . . . . 10

2.2.3 Process consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.4 Manual soldering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Power supply: consumption estimation and optimization . . . . . . . . . 12

3.1 Common rules for low power consumption . . . . . . . . . . . . . . . . . . . . . . . 14

4 Using the device step-by-step, from basic to advanced . . . . . . . . . . . 15

4.1 First time bring-up (I²C example) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2 Quick troubleshooting guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3 One-shot mode measurement sequence . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Using FIFO modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.1 Effective use of the FIFO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.1.1 Accessing the FIFO data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.1.2 Bypass mode (F_MODE[2..0]="000" in FIFO_CTRL (0x2E)) . . . . . . . . 18

5.1.3 FIFO mode (F_MODE[2..0]="001" in FIFO_CTRL (0x2E)) . . . . . . . . . . 18

5.1.4 Stream mode (F_MODE[2..0]="010" in FIFO_CTRL (0x2E)) . . . . . . . . 18

5.1.5 FIFO mean mode (F_MODE[2..0]="110" in FIFO_CTRL (0x2E)) . . . . . 18

5.1.6 Hardware digital filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.2 Extra FIFO modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.2.1 Stream to FIFO mode (F_MODE[2..0]="011" in FIFO_CTRL (0x2E)) . . 20

5.2.2 Bypass to Stream mode (F_MODE[2..0]="100" in FIFO_CTRL (0x2E)) 20

5.2.3 Bypass to FIFO mode (F_MODE[2..0]="111" in FIFO_CTRL (0x2E)) . . 21

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6 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6.1 Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6.2 One-shot mode conversion time estimation . . . . . . . . . . . . . . . . . . . . . . . 22

6.3 Reference SW to get started with LPS25H . . . . . . . . . . . . . . . . . . . . . . . 22

6.4 Pressure to altitude conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6.5 SW filtering & internal FIFO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.6 Reset the device to power-on configuration . . . . . . . . . . . . . . . . . . . . . . . 23

6.7 Absolute accuracy drift due to soldering . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.7.1 Correcting soldering drift (one-point calibration) . . . . . . . . . . . . . . . . . . 24

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

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Pressure sensor evaluation boards AN4450

1 Pressure sensor evaluation boards

UNICO/eMotion evaluation system: [STEVAL-MKI109V2] + [STEVAL-MKI142V1 or V2].

Figure 1. STEVAL-MKI142V1 LPS25H 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 DIL 24 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) from ST (Unico SW), or dedicated software routines for customized applications.

A plain terminal (such as MS windows hyper-terminal (C)) can also be used to access the sensor registers by text read/write commands.

Figure 2. STEVAL-MKI142V1 - LPS25H adapter

The STEVAL-MKI142V1 adapter board is designed to facilitate the demonstration of the LPS25H product. The board offers an effective solution for fast system prototyping and device evaluation directly within the user’s own application.

The STEVAL-MKI142V1 can be plugged into a standard DIL 24 socket. The adapter provides the complete LPS25H pin-out and comes ready-to-use with the recommended decoupling capacitors on the VDD power supply line. No pull-ups are provided.

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AN4450 Pressure sensor evaluation boards

The STEVAL-MKI109V2 MEMS (aka eMotion board, shipped with pre-loaded eMotion FW) is completed by a MS Windows™-based SW application called Unico. This tool may be used as a simple real-time demonstrator or to verify device performance.

It also allows easy monitoring of the register status and allows changes to them based on the intended scenario.

Recommended FW/SW revisions fully supporting the STEVAL-MKI142V1 are:

 eMotion V3.0.6

 Unico Rev. 3.0.1.0 beta

or newer.

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Hardware (designing PCB schematics and layout) AN4450

2 Hardware (designing PCB schematics and layout)

2.1 LPS25H device package, interconnect and polarization

2.1.1 Package drawings

Pressure sensor LPS25H is available in a holed LGA package: HCLGA-10L (1mm thickness).

Figure 3. HCLGA-10L 2.5x2.5x1.0 mm

Figure 4. Pin mapping (bottom view)

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AN4450 Hardware (designing PCB schematics and layout)

2.1.2 Pin mapping

Table 1. LPS25H pin mapping details

Pin # Name Function What to do

1 VDD_IO Power supply for I/O pins I/O supply voltage (1.7V ~ 3.6V)

3 Reserved Reserved Connect to PCB ground

6CS

7 INT1 Interrupt 1 (or data ready) Leave unconnected if unused

8 GND_IO I/O pins ground Connect to PCB ground

9 GND Ground Connect to PCB ground

10 VDD Power supply Supply voltage (1.7V ~ 3.6V)

SCL SPC

SDA SDI SDIO

SA0 SDO

I²C serial clock (SCL) SPI serial port clock (SPC)

I²C serial data (SDA) 4-wire SPI serial data input (SDI) 3-wire SPI serial data input/output (SDIO)

I²C slave address select (SA0) 4-wire SPI serial data output (SDO)

I²C/SPI mode selection SPI chip select

Optional pull-up resistor, connected to VDD_IO, could be required for I²C (see I²C standard for recommended value)

SA0=1: 0xBA/BB I²C slave address + R/W SA0=0: 0xB8/B9 I²C slave address + R/W

CS=1: I²C mode CS=ChipSelect in SPI mode

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Hardware (designing PCB schematics and layout) AN4450

2.1.3 Typical application circuit

Figure 5. LPS25H electrical connection (I²C communication example w/SA0=1)

Key notes:

The device core is supplied through the VDD line. Power supply decoupling capacitors (100 nF and at least 4.7 µF) must be placed as near as possible to the supply pads of the device.

VDD_IO should match the one requested by the host controller. In any case VDD_IO must be the same or lower than VDD. In the example shown here, VDD_IO is connected to VDD.

Please note that the LPS25H does not have an internal pull-up on the I²C lines (SCL and SDA). They must be added externally according to the I²C bus speed and load, and connected to VDD_IO.

In I²C mode, SA0 =1 (pin directly connected to VDD_IO) sets the slave address to 0xBA for write and 0xBB for read. A pull-up resistor could be a better choice if there is a need to communicate alternatively in I²C or SPI 4-wires, since pin 5 is SDO for SPI 4-wires and of course it cannot function if hard-connected to VDD_IO.

The I²C interface is compliant with fast mode (400 kHz) I²C standards as well as with normal mode.

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