LPS25H Pressure/Altitude Sensor Carrier with Voltage Regulator

LPS25H Pressure/Altitude Sensor Carrier with Voltage Regulator
LPS25H Pressure/Altitude Sensor Carrier with Voltage Regulator
Overview
This board is a compact (0.4″ × 0.8″) carrier for ST’s LPS25H MEMS absolute pressure sensor, or
barometer; we therefore recommend careful reading of the LPS25H datasheet (1MB pdf) before
using this product. The LPS25H is a great IC, but its small, leadless, LGA package makes it difficult
for the typical student or hoyist to use. It also operates at voltages below 3.6 V, which can make
interfacing difficult for microcontrollers operating at 5 V. This carrier board addresses these issues
by incorporating additional electronics, including a 3.3 V voltage regulator and level-shifting
circuits, while keeping the overall size as compact as possible. The board ships fully populated
with its SMD components, including the LPS25H, as shown in the product picture.
Compared to the earlier LPS331AP, the LPS25H features improved accuracy and reduced noise in
the output. The addition of a built-in FIFO (First In, First Out) buffer allows the sensor to store
pressure readings for burst transmission, reducing overall power consumption by allowing the host
processor to sleep longer between data requests. Alternatively, the FIFO can be configured to
perform a running average of pressure readings to further decrease the output noise.
This LPS25H carrier board is pin-compatible with our LPS331AP carrier, but due to the removal of
one of the interrupt pins on the LPS25H, the position of the mounting hole has changed relative to
the remaining pins. The two sensors use the same I²C addresses, but some of their configuration
register fields are different, so code written to interface with an LPS331 might need to be modified
to work with an LPS25H.
The LPS25H features embedded temperature compensation and has many configurable options,
including selectable resolutions, a choice of output data rates, several FIFO operating modes, and
a programmable external interrupt signal. Its pressure output has an absolute accuracy as low as
±0.2 mbar (0.02 kPa), with RMS noise of 0.01 mbar (0.001 kPa) in the highest-resolution mode
with embedded filtering enabled. Pressure and temperature sensor data are available through a
digital interface, which can be configured to operate in either I²C or SPI mode, and can be used for
altimetry. (See the Sample Code section below for an Arduino library that can be used to turn this
sensor into an altimeter.)
The carrier board includes a low-dropout linear voltage regulator that provides the 3.3 V required
by the LPS25H, which allows the sensor to be powered from a 2.5 V to 5.5 V supply. The regulator
output is available on the VDD pin and can supply almost 150 mA to external devices. The
breakout board also includes a circuit that shifts the I²C/SPI clock and data in lines to the same
logic voltage level as the supplied VIN, making it simple to interface the board with 5 V systems,
and the board’s 0.1″ pin spacing makes it easy to use with standard solderless breadboards and
0.1″ perfboards.
Specifications
Dimensions: 0.4″ × 0.8″ × 0.1″ (10 mm × 20 mm × 3 mm)
Weight without header pins: 0.5 g (0.02 oz)
Operating voltage: 2.5 V to 5.5 V
Supply current: 2 mA
Output format (I²C/SPI): 24-bit pressure reading (4096 LSb/mbar)
Sensitivity range: 260 mbar to 1260 mbar (26 kPa to 126 kPa)
Included components
A 1×8 strip of 0.1″ header pins and a 1×8 strip of 0.1″ right-angle header pins are included, as
shown in the picture below. You can solder the header strip of your choice to the board for use with
custom cables or solderless breadboards, or you can solder wires directly to the board itself for
more compact installations.
The board has one mounting hole that works with #2 and M2 screws (not included).
Using the LPS25H Connections
Regardless of the interface being used to communicate with the LPS25H, its VIN pin should be
connected to a 2.5 V to 5.5 V source, and GND should be connected to 0 volts. (Alternatively, if
you are using the sensor with a 3.3 V system, you can leave VIN disconnected and bypass the
built-in regulator by connecting 3.3 V directly to VDD.)
A minimum of two logic connections are necessary to use the LPS25H in I²C mode (this is the
default mode): SCL and SDA. These pins are connected to built-in level-shifters that make them
safe to use at voltages over 3.3 V; they should be connected to an I²C bus operating at the same
logic level as VIN. The remaining pins are not connected to level-shifters on the board and are not
5V-tolerant, but our 4-channel bidirectional logic level shifter can be used externally with those pins
to achieve the same effect.
To use the LPS25H in the default SPI mode, four logic connections are required: SPC, SDI, SDO,
and CS. These should be connected to an SPI bus operating at the same logic level as VIN. The
SPI interface operates in 4-wire mode by default, with SDI and SDO on separate pins, but it can be
configured to use 3-wire mode so that SDO shares a pin with SDI.
LPS25H pressure/altitude sensor carrier with voltage
regulator, labeled top view.
LPS25H pressure/altitude sensor carrier with
voltage regulator in a breadboard.
Pinout
PIN
Description
VDD
Regulated 3.3 V output. Almost 150 mA is available to power external
components. (If you want to bypass the internal regulator, you can instead
use this pin as a 3.3 V input with VIN disconnected.)
VIN
This is the main 2.5 V to 5.5 V power supply connection. The SCL/SPC
and SDA/SDI level shifters pull the I²C and SPI bus high bits up to this
level.
GND
The ground (0 V) connection for your power supply. Your I²C or SPI control
source must also share a common ground with this board.
SDA/SDI/SDO
Level-shifted I²C data line and SPI data in line (also doubles as SDO in 3wire mode): HIGH is VIN, LOW is 0 V
SCL/SPC
Level-shifted I²C/SPI clock line: HIGH is VIN, LOW is 0 V
SDO/SA0
SPI data out line in 4-wire mode: HIGH is VDD, LOW is 0 V. This output is
not level-shifted. Also used as an input to determine I²C slave address
(see below).
CS
SPI enable (chip select). Pulled up to VDD to enable I²C communication by
default; drive low to begin SPI communication.
INT1
Programmable interrupt, a 3.3-V-logic-level output. This output is not levelshifted.
Schematic diagram
The above schematic shows the additional components the carrier board incorporates to make the
LPS25H easier to use, including the voltage regulator that allows the board to be powered from a
2.5 V to 5.5 V supply and the level-shifter circuit that allows for I²C and SPI communication at the
same logic voltage level as VIN. This schematic is also available as a downloadable PDF (162k
pdf).
I²C communication
With the CS pin in its default state (pulled up to VDD), the LPS25H can be configured and its
pressure reading can be queried through the I²C bus. Level shifters on the I²C clock (SCL) and
data (SDA) lines enable I²C communication with microcontrollers operating at the same voltage as
VIN (2.5 V to 5.5 V). A detailed explanation of the I²C interface on the LPS25H can be found in its
datasheet (1MB pdf), and more detailed information about I²C in general can be found in NXP’s
I²C-bus specification (371k pdf).
In I²C mode, the sensor’s 7-bit slave address has its least significant bit (LSb) determined by the
voltage on the SA0 pin. The carrier board pulls SA0 to VDD through a 10 kΩ resistor, making the
LSb 1 and setting the slave address to 1011101b by default. If the pressure sensor’s selected
slave address happens to conflict with some other device on your I²C bus, you can drive SA0 low
to set the LSb to 0.
The I²C interface on the LPS25H is compliant with the I²C fast mode (400 kHz) standard. In our
tests of the board, we were able to communicate with the chip at clock frequencies up to 400 kHz;
higher frequencies might work but were not tested.
SPI communication
To communicate with the LPS25H in SPI mode, the CS pin (which the board pulls to VDD through
a 10 kΩ resistor) must be driven low before the start of an SPI command and allowed to return
high after the end of the command. Level shifters on the SPI clock (SPC) and data in (SDI) lines
enable SPI communication with microcontrollers operating at the same voltage as VIN (2.5 V to
5.5 V).
In the default 4-wire mode, the pressure sensor transmits data to the SPI master on a dedicated
data out (SDO) line that is not level-shifted. If the SPI interface is configured to use 3-wire mode
instead, the SDI line doubles as SDO and is driven by the LPS25H when it transmits data to the
master. A detailed explanation of the SPI interface on the LPS25H can be found in its datasheet
(1MB pdf).
Sample Code
We have written a basic Arduino library for the LPS25H that makes it easy to interface this sensor
with an Arduino. The library makes it simple to configure the LPS25H and read the raw pressure
data through I²C, and it provides functions for calculating altitude based on the measured pressure
for those looking to use this sensor as an altimeter.
Protocol hints
The datasheet provides all the information you need to use this sensor, but picking out the
important details can take some time. Here are some pointers for communicating with and
configuring the LPS25H that we hope will get you up and running a little bit faster:
The pressure sensor is in power down mode by default. You have to turn it on by
writing the appropriate value to the CTRL_REG1 register to choose an output data rate.
You can read or write multiple registers in a single I²C command by asserting the
most significant bit of the register address to enable address auto-increment.
You can enable the same auto-increment feature in SPI mode by asserting the second
bit (bit 1, called the MS bit in the datasheet) of an SPI command.
Documentation on producer website.
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