User Guide
www.kitronik.co.uk
Barometric Sensor
Barometric Sensor Breakout -
5102
Circuit Overview
This board uses a MPL115A2 digital barometer which can provide a temperature compensated
pressure reading. The sensor can measure pressures between 50 kPa to 115 kPa Absolute Pressure
with an accuracy of ±1 kPa. The board supports a wide range of input voltages (2.4V – 5.5V) and uses
I2C. When selecting an input voltage to use, keep in mind that pulling the SHDN and RST pins above
the input voltage will cause malfunctions. On board IIC bus pull-ups are included and can be enabled
by attaching a jumper across PL2.
Block Diagram
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User Guide
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Barometric Sensor
Electrical Characteristics
Supply Voltage
Current Consumption
(sampling)
Current Consumption
(SHDN)
Range
Resolution
Accuracy
Conversion time
Wakeup time
Min
2.4V
Typical
3.3V
5uA
Max
5.5V
6uA
1uA
50kPa
0.15kPa
1.6ms
3ms
±1 kPa
3ms
5ms
115kPa
Pinout
SDA
SCL
Vcc
GND
SHDN
I2C data
I2C clock
Supply voltage
Ground pin
Shutdown pin
2.4-5.5V
0V
Low = shutdown
High = active (must not
exceed Vcc)
Low = reset
High = normal (must not
exceed Vcc)
Connect pads with solder
bridge to enable pull-ups
RST
Reset Pin
PL2
Pull-up jumper
Mechanical Information
All dimensions in mm. Tolerance 0.1mm. All mounting holes are on 10mm spacings.
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User Guide
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Barometric Sensor
Arduino Uno Connections
Barometric Sensor
Arduino Uno
SDA
A4*
SCL
A5*
Vcc
5V or 3.3V (to match IOREF)
GND
GND
SHDN
Pin 11
RST
Pin 10
*SDA and SCL pins vary between Arduino models. If you wish to check which pins your Arduino uses
for SDA/SCL you can visit
http://arduino.cc/en/reference/wire
Arduino Sketch
The sketch for this board is quite long so it is provided in a .zip.
In simple terms the program retrieves stored calibration information, and then uses it along with a
pair of readings taken by the device to calculate pressure. The time taken for a reading to be taken
and calculated is around 7.5ms at 5V
If ran “as-is” the program will simply output the pressure every 5 seconds in kPa to the serial
monitor. The refresh rate can be altered by changing the value of the variable
‘refreshtime’,
which is
measured in milliseconds and is set to 50,000 by default.
If you are interested in how this is calculated or are looking to debug a problem you might have, you
can increase the amount of data displayed on the serial monitor. To do this simply look at the 2
nd
line
of code “//#define DEBUG” and delete the “//”. Putting them back in again will hide the information.
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User Guide
www.kitronik.co.uk
Barometric Sensor
Function Overview
The MPL115A2 works in several steps:
The initial power-up causes all circuit elements to become
active. I2C port pins are high impedance and associated
registers are cleared. The device then enters standby mode.
The user then accesses the ROM in the device and reads the
coefficients, these are then usually stored in the
microcontroller, but can be re-read at any time.
The user sends a CONVERT command to the device to begin
data conversion, when this is complete the raw data is stored
in the device’s pressure and temperature ADC registers.
The microcontroller then uses the coefficients and raw
pressure and temperature data to give a compensated
pressure reading by using a compensation formula shown
below.
Padc is the 10-bit pressure ADC output of the MPL115A
Tadc is the 10-bit temperature ADC output of the MPL115A
a0 is the pressure offset coefficient
b1 is the pressure sensitivity coefficient
b2 is the temperature coefficient of offset (TCO)
c12 is the temperature coefficient of sensitivity (TCS)
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User Guide
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Barometric Sensor
I2C Commands (simplified for communication)
Device Address + write bit “To Write”
0xC0
Device Address + read bit “To Read”
0xC1
Command to Write “Convert Pressure and Temperature”
0x12*
Command to Read “Pressure ADC High byte”
0x00
Command to Read “Pressure ADC Low byte”
0x01
Command to Read “Temperature ADC High byte”
0x02
Command to Read “Temperature ADC Low byte”
0x03
Command to Read “Coefficient data byte a0_MSB”
0x04
Command to Read “Coefficient data byte a0_LSB”
0x05
Command to Read “Coefficient data byte b1_MSB”
0x06
Command to Read “Coefficient data byte b1_LSB”
0x07
Command to Read “Coefficient data byte b2_MSB”
0x08
Command to Read “Coefficient data byte b2_LSB”
0x09
Command to Read “Coefficient data byte c12_MSB”
0x0A
Command to Read “Coefficient data byte c12_LSB”
0x0B
*The
command byte needs to be paired with a 0x00 as part of the I2C exchange to complete the
passing of Start Conversions.
Glossary of terms
kPa
– kilopascal, standard unit for pressure equal to 1000 pascals
I2C/IIC
– inter-integrated circuit, a computer bus invented by Philips used for attaching low-speed
peripherals to a motherboard, embedded system or other electronic device
SDA
– Serial Data
SCL
– Serial Clock
SHDN
– Shutdown
RST
– Reset
MUX
– A multiplexer (or mux) is a device that selects one of several analogue or digital input signals
and forwards the selected input into a single line.
ADC
– Analogue Digital Converter
Vcc
– Supply Voltage
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