FREESCALE AN1979

Freescale Semiconductor
Application Note
AN1979
Rev 3, 11/2006
Altimeter and Barometer System
by: Michelle Clifford and Fernando Gonzalez
Sensor Products Division, Tempe, AZ
INTRODUCTION
With smaller packages and lower costs, pressure sensors
can be designed into more consumer applications. This
document describes a reference design for a digital barometer
and altimeter using the MPXM2102A pressure sensor in the
low cost MPAK package, a quad op-amp, and the
MC68HC908QT4 microcontroller. This system continuously
monitors the barometric pressure and compares it to previous
pressure readings to update altitude and weather predictions.
This reference design enables the user to evaluate a
Freescale Semiconductor, Inc. pressure sensor for barometer,
personal weather station and altimeter applications. This
reference design also allows customers to evaluate barometer
pressure readings obtainable from the MPXM2102A sensor
for watches or GPS systems with this feature. In addition,
many systems require barometric pressure data to correct
system response errors. This application note describes the
reliability and accuracy that our sensors can provide in this
system.
SYSTEM DESIGN
Pressure Sensor
The barometer/altimeter system requires a pressure
sensor that has a pressure range of 64 kPa to 105 kPa.
Freescale Semiconductor, Inc. has a broad portfolio of silicon
piezo-resistive pressure sensors. They provide a very
accurate and linear voltage output directly proportional to the
applied pressure. By evaluating the application design and
cost, the right pressure sensor can be selected from our
portfolio.
measures changes in ambient pressure, we need a known
pressure reference. Therefore, an absolute pressure sensor
was selected. Freescale offers three levels of integration: uncompensated, compensated, and integrated. Since there can
be large temperature changes from one elevation to another
the sensor for this reference design needs to be offset
calibrated and temperature compensated. Therefore a
compensated sensor was selected requiring external
amplification circuitry. However, integrated solutions such as
the MPXM5100A, can also be considered, thereby eliminating
the need for the external amplification circuitry.
Knowing the range of pressure, the type of pressure
measurement, and the level of integration required for this
application, the MPXM2102A sensor was selected. The
sensor has both temperature compensation and calibration
circuitry on the silicon and is capable of producing a linear
output voltage in the range of 0 to 100 kPa, but can be pushed
further up to 105 kPa with linear results. The characteristics of
this sensor are described in greater detail in Table 2. A 5-volt
supply was used throughout the circuit to power the
components. Since the MPXM2102A is ratio metric, meaning
the output voltage changes linearly with the supply voltage,
the sensor will have a full scale span of 20 mV instead of the
specified 40 mV at a 10 V supply. The calculation of the full
scale span is shown below:
(VS actual/VS spec) x VOUT full-scale spec = VOUT full-scale
(5.0 V/ 10 V) x 40 mV = 20 mV
One of the most important decisions for a pressure
application is the packaging. Freescale has a large offering of
pressure packaging options. To minimize the space of a final
application, the MPAK package was selected. A non-ported
MPAK is the ideal pressure sensor package for hand held
GPS units or altimeter watches due to its small size. However,
a ported MPAK package can also be selected, allowing a tube
to be attached to the port for testing and demonstration
purposes.
Figure 1. Pressure Sensor
There are three types of pressure measurements: gauge,
absolute, and differential. Since this reference design
© Freescale Semiconductor, Inc., 2006. All rights reserved.
Figure 2. MPXM2102A Case 1320A-02
Table 1. MPXM2102A Operating Characteristics
Characteristic
Symbol
Min
Typ
Max
Unit
Pressure Range
POP
0
—
100
kPa
Supply Voltage
VS
—
10
15
Vdc
Supply Current
IO
—
6.0
—
mAdc
Full Scale Span
VFSS
38.5
40
41.5
mV
Voff
-1.0
-2.0
—
—
1.0
2.0
mV
—
0.4
—
MV/kPa
-0.6
-1.0
—
—
0.4
1.0
%VFSS
Offset
Sensitivity
Linearity
MPX2102D Series
MPX2102A Series
∆V/∆P
MPX2102D Series
MPX2102A Series
—
—
Amplifier Selection and Amplifier Induced Errors
The main goal of the signal conditioning circuit is to convert
the MPX2102A differential output to a single-ended, groundreferenced output. The differential output is extremely small
for the MCU to process so a conditioning circuit also needs to
provide amplification.
This reference design has a barometric pressure range of
64 kPa to 105 kPa. The output of the sensor is ratiometric to
the supply voltage and the supply voltage is 5.0 V, the FSS,
Sensitivity, and Offset are 5.0 V/10 V, or half, of the specified
values at a 10 V supply. Using these calculated sensitivity and
offset ranges, the lowest and highest possible values were
calculated.
VOUT = (Applied Pressure * Sensitivity) ± Offset
VOUT at 64 kPa = 64 kPa * 0.2 mV/kPa — 1 mV = 11.32 mV
VOUT at 105 kPa = 105 kPa * 0.2 mV/kPa + 1 mV = 21.0 mV
each of the sensor outputs, then uses a differential
amplification as shown in Figure 2.
After the first stage of amplification, the output of op-amp A is:
VA = (1+R8/R6) x V4 – (R8/R6) x VS(1)
= (1+10/4.42k) x V4 – (10/4.42k) x 5.0 V
= (1+10/4.42k) x V4 – 11.3 mV
and the output of op-amp B is:
VB = (1+R7 / R5) x V2 – (R7 / R5) x VS(2)
= (1+10/4.42 k) x V2 – (10/4.42 k) x 0 V
= (1+10/4.42 k) x V2 – 0
The second stage of amplification connects these two
outputs to a common differential amplifier (op-amp C) also
shown in Figure 3. With some algebraic manipulation, the
output voltage (VOUT) of the entire amplification circuit is
These values were found to be 11.32 mV to 22.79 mV
differential output from the sensor.
VC = (R12/R11) x [(1+R8/R6) x (V2 - V4) – (R8/R6) x VS](3)
Two-Stage Design
= (412) x [(1.002) x (V2 - V4) – 11.3 mV]
This two-stage design level shifts the differential output
voltage of the sensor by subtracting an offset voltage from
= 412 x (V2 - V4) –11.3 mV
= (412K/1 k) x [(1+10/4.42 K) x (V2 - V4) – (10/4.42 K) * 5 V]
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R8
10 Ω
R6
4.42 K
V4 sensor
3
1
6
5
R12
412 K
–
7
VA
VCC
+
4
R11
1K
2
MPXM2102A
R5
4.42 K
V2 sensor
2
3
–
13
12
R7
10 Ω
VCC
C5
0.1 µf
R9
1K
9 –
–
+
4
14
VC
10
8
+
11
R10
412 K
1
VB
+
Figure 3. Amplification Scheme
The range of the A/D converter is 0 to 255 counts. However,
the A/D values that the system can achieve are dependent on
the maximum and minimum system output values:
Count = (VOUT – VRL) / (VRH – VRL) x 255(4)
where VXdcr = Transducer Output Voltage
VRH = Maximum A/D voltage
VLH = Minimum A/D voltage
Count (64 kPa) = (0.03 – 0.0) / (5.0 – 0.0) x 255 = 2
Count (105 kPa) = (4.85 – 0.0) / (5.0 – 0.0) x 255 = 247
Total # counts = 247 – 2 = 245 counts.
The resolution of the system is determined by the
barometric pressure represented by each A/D count. As
calculated above, the system has a span of 247 counts to
represent a pressure from 64 kPa to 105 kPa. Therefore, the
resolution is:
Resolution = (System Pressure Range) /
Total # counts (5)
= (105 kPa — 64 kPa)/245 counts
= 0.17 kPa per A/D count
Microprocessor
To provide the signal processing for pressure values, a
microprocessor is needed. The MCU chosen for this
application is the MC68HC908QT4. This MCU is perfect
for appliance applications due to its low cost, small eight-pin
package, and other on-chip resources. The MC68HC908QT4
provide: a four-channel, eight-bit A/D, a 16-bit timer, a
trimmable internal timer, and in-system FLASH programming.
The central processing unit is based on the high
performance M68HC08 CPU core and it can address 64
Kbytes of memory space. The MC68HC908QT4 provides
4096 bytes of user FLASH and 128 bytes of random access
memory (RAM) for ease of software development and
maintenance. There are five bi-directional input/output lines
and one input line that are shared with other pin features.
The MCU is available in eight-pin as well as 16-pin
packages in both PDIP and SOIC. For this application, the
eight-pin PDIP was selected. The eight-pin PDIP was chosen
for a small package, eventually to be designed into
applications as the eight-pin SOIC. If added circuitry for
programming the microcontroller is added, a cyclone could be
used to program an SOIC on the PCB. If your design requires
software updates, consult the MC68HC908QT4 handbook for
adding this option.
IMPROVEMENTS
The resolution of this design is limited by the eight-bit A/D
converter on the microcontroller. Theoretically, the accuracy
achieved by this device should produce an output when
altitude change differs by about 41.54 feet (∆Z). This occurs at
approximately 1000 feet below sea level. Due to the
logarithmic relationship between pressure and elevation, the
accuracy of the results decreases as the device is elevated. At
12,000 feet above sea level, the device should recognize a
change of about 65.53 feet (∆Z) as shown in Table 3. A 10-bit,
12-bit or even a 16-bit A/D converter could be implemented in
order to increase the resolution of this reference design.
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Table 2. Microcontroller Accuracy Comparisons
Z (ft)
P (kPa)
V (mV)
Amp scheme
Vamp (mV)
Vamp – 1 bit
P0
Px
∆Z (m)
∆Z (ft)
Micro
(Vx–12.8)*650
4852.3
4832.6
20.265
20.235
12.66
41.54
8 bits
33.8
14.2
12.852
12.822
19.97
65.53
8 bits
4852.3
4847.4
20.265
20.257
3.15
10.35
10 bits
33.8
28.9
12.852
12.844
4.97
16.32
10 bits
4852.3
4851
20.265
20.263
0.79
2.59
12 bits
33.8
32.6
12.852
12.85
1.24
4.08
12 bits
4852.3
4852.2
20.265
20.265
0.05
0.16
16 bits
33.8
33.7
12.852
12.852
0.08
0.25
16 bits
-1000
105
20.265
12000
64.259
12.852
-1000
105
20.265
12000
64.259
12.852
-1000
105
20.265
12000
64.259
12.852
-1000
105
20.265
12000
64.259
12.852
(Vx–12.8)*650
(Vx–12.8)*650
(Vx–12.8)*650
Table 2 shows the theoretical maximum resolution that this
reference design can achieve. However, factors such as noise
within the circuit, sensitivity of the sensor, and voltage offsets
in the amplification scheme should be taken into
consideration. Accommodating for these factors in the
software can filter out some of these factors.
Further testing is required to determine the accuracy of the
reference design without the limiting A/D converter.
DISPLAY
The display of the barometric pressure, barometric
pressure history, current calculated altitude, and a simple
weather prediction is displayed on a 16x2 LCD.
Barometric Pressure
History
Barometric Pressure
101.3kPam
-----_----- -----_
_------steady
+1170 ft
Simple Weather
Prediction
Altitude
(must calibrate)
FREESCALE
SEL
Select Button
ENT
Enter Button
Figure 4. Barometric Display
Due to the limited number of bi-directional data pins on the
microcontroller, a system was designed to serially buffer the
display data. Using a shift register to hold display data, the
LCD is driven with only three lines of output from the
microcontroller: an enable line, a data line, and a clock signal
while the two LEDs are multiplexed with the data line and
clock signal.
PTA3
PTA4
EN
RS
HC908QT4
PTA5
R2
1K
R3
1K
A
B
RW
CLK
D80
D81
D82
D83
D84
D85
D86
D87
LCD
HC164
VEE
Figure 5. Multiplexed LCD Circuit
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Multiplexing of the microcontroller output pins allows
communication of the LCD to be accomplished with three pins
instead of eight or 11 I/O pins usually required. With an eightbit shift register, we are able to manually clock in eight bits of
data. The enable line, EN, is manually enabled when eight
bytes have been shifted in, telling the LCD the data on the data
bus is available to execute. The LCD will only be written to and
the contrast can be held at a constant brightness, allowing the
read/write and the VEE bits to be held low, also minimizing
additional I/O lines.
Table 3. Parts List
Ref
Qty.
Description
Value
Vendor
Part No.
U3
1
Pressure Sensor
1.0
Freescale
MPXM2102A
C1
1
VCC Cap
0.1 µF
Generic
C2
1
Op-Amp Cap
0.1 µF
Generic
C3
1
Shift Register Cap
0.1 µF
Generic
D1
1
Red LED
—
Generic
D2
1
Green LED
—
Generic
S2, S3
1
Push buttons
—
Generic
U1
1
Microcontroller
8-Pin
Freescale
MC68HC908QT4
U2
1
16x2 B&W LCD
16x2
Seiko
L168200J000
U4
1
Shift Register
—
Texas
74HC164
U5
1
Voltage Regulator
5.0 V
Fairchild
LM78L05ACH
AD8544
U6
1
Quad Op-Amp
—
ADI
R1, R4
1
1/4 W Resistor
10 K
Generic
R2, R3
2
1/4 W Resistor
1.0 K
Generic
R5, R6
2
1/4 W Resistor
3.65 K
Generic
R7, R8
2
1/4 W Resistor
10 K
Generic
R9, R11
2
1/4 W Resistor
1.0 K
Generic
R10, R12
2
1/4 W Resistor
200 K
Generic
OTHER
This system is designed to run on a 9.0 V battery. It
contains a 5.0 V Regulator to provide a 5.0 V supply to the
pressure sensor, microcontroller, and LCD. The battery is
mounted on the back of the board using a space saving spring
battery clip.
ALTIMETER/BAROMETER SOFTWARE
This application note describes the software version that
was available during publication. However updated software
versions may be available with further functionality and menu
selections. Check our website update for updates to Sensor
Products Reference designs.
Software User Instructions
When the system is turned on or reset, the microcontroller
will flash the select LED and display the program title on the
LCD for five seconds or until the select (SEL) button is
pushed. Then the menu screen is displayed. Using the select
(SEL) push button, the user can scroll through the menu
options for a software program. To run the altimeter program,
use the (SEL) select button to high-light the “Alti/Barometer”
option, then press the enter (ENT) push button. The Altimeter
program will display current barometric pressure reading, the
calculated altitude in feet, a message displaying a simple
weather prediction such as “sunny”, “rainy”, “steady” without a
pressure change, and “history” before enough history is
collected to make a prediction. In the top right corner of the
display, a scrolling graphical history displays data points
representing the past forty pressure readings.
Calibration and Calibration Software
There are two forms of calibration for this system. The first
calibration is used for the barometer part of the system. This
calibration was already done before you received the
reference design and only needs to be done once per system.
To calibrate the barometer module, a two-point calibration is
performed using a highly accurate pressure generator. The
system takes a calibration point at 64 kPa and another at 105
kPa. Holding down both the SEL and ENT buttons on system
power-up will put the system into calibration mode. At this
point, the calibration menu will be displayed with the
previously sampled offset voltage. To recalibrate the system,
apply a pressure of 64 kPa and press the SEL button (PB1).
This A/D value is then saved to a location in the
microcontroller memory. To obtain the second calibration
point, using the accurate pressure generator apply a pressure
of 105 kPa directly to the sensor. Then press the ENT button
(PB2). This signal is similarly sampled, averaged and saved to
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OUTPUT (mVDC)
a location in FLASH. To exit the calibration mode, press the
SEL (PB1) button.
The second calibration is done for the altimeter. The
Altimeter requires a one-point calibration where a known
altitude is entered with a known pressure. This ensures that
changes in atmospheric pressure are due to increases or
decreases in altitude and not changes in barometric pressure.
By returning to the main menu, and selecting the “Set
Elevation”, the user can select an elevation by pressing the
SEL button to cycle through the Elevation options from 0 to
12000 feet in 100-foot increments. Once the selection has
been made the elevation is flashed into the microcontroller
and the user is brought to the Altimeter/Barometer function.
Calibration is required for each use of the altimeter module.
kPa
PSI
40
35
30
25
20
15
10
5
0
–5
VS = 10 VDC
TA = 25°C
P1 > P2
MAX
these known pressures are saved in the flash memory of the
microcontroller.
ATD = (Po – P64kPa)/(P105kPa - P64kPa) x 255
By algebraic manipulation, the following equation is
reached to find the barometric pressure:
Po = (ATD/255) x (P105kPa - P64kPa) + P64kPa
Converting Pressure to Altitude
The method of determining altitude for this reference
design is measuring the changes in barometric pressure. The
relationship of pressure vs. altitude is not linear. As pressure
decreases, altitude increases, but the higher the altitude gets
the less pressure changes. The equation that was used for
this reference design is:
P = (P0) e^[-(g/(RT)) x (Z — Z0),
TYP
where P = pressure at an unknown altitude,
SPAN
RANGE
(TYP)
P0 = pressure at a known altitude,
e = a constant,
MIN
g = gravitational constant 9.8 (m/s^2),
0
25
3.62
50
7.25
75
10.87
100
14.5
OFFSET
(TYP)
Figure 6. Analog Output to Pressure
CONVERTING ANALOG OUTPUT TO PRESSURE
Freescale pressure sensors have an extremely linear
analog voltage output that is proportional to the pressure
input. Since the sensor output is linear, the pressure can be
calculated by using the equation of a line, y = mx + b, where y
is the output voltage, the slope, m, is the Sensitivity, and the y
intercept, b, is the Offset:
VOUT = Sensitivity x Pressure + Offset
With algebraic manipulation, pressure can be determined by:
Pressure = (VOUT – Offset)/Sensitivity
Below is an example of determining the pressure from the
analog output of 9.5 mV using the Sensitivity and Offset of the
MPX2102a sensor specified in the datasheet:
Pressure = (VOUT – Offset)/Sensitivity
= (9.5 mV – 0.5 mV) / 0.1 mV/kPa
= (9.0)/0.1 mV/kPa
= 90 kPa
where 0.5 mV is the typical offset for the MPX2102 and
0.1 mV/kPa is the sensitivity with a 5.0 V supply
This system uses additional amplifiers and an A/D
converter that all add additional offset and gain errors;
however, the translation function was corrected with the twopoint calibration. The known pressure values that are used for
calibration are the maximum and minimum pressures for the
system, 105 kPa and 64 kPa respectively. The A/D values for
R = dry air constant 287 J/(kg x K),
T = temperature at unknown elevation in Kelvin,
Z = unknown altitude in meters,
and Z0 = known altitude also in meters.
This equation originates from the hydrostatic equation:
dP = -ρgdZ
in conjunction with the ideal gas law:
P = ρRT
After some algebraic manipulation, plugging in constant
values and converting meters to feet, the following equation
was generated:
Z = Z0 — 27,887 in (P/P0),
where Z = unknown altitude in feet,
Z0 = known altitude also in feet,
P = known pressure at unknown altitude,
and P0 = known pressure at known altitude.
For this system to calculate an altitude, Z, at a known
pressure P, the user must enter a known pressure, P0, and its
corresponding altitude, Z0. To accommodate for changes in
barometric conditions, the known pressure and altitude data
must be re-entered during each use to ensure accuracy.
Simple Weather Prediction
Atmospheric pressure at the Earth’s surface is one of the
measurements used to make weather predictions. Air in a
high-pressure area compresses and warms as it descends.
The warming air inhibits the formation of clouds. Therefore,
the sky is normally sunny in high-pressure areas with a small
chance of haze or fog. However, in an area of low atmospheric
pressure, the air rises and cools. With enough humidity in the
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CONCLUSION
air, the rising air will cool, the air will condense forming clouds
and precipitation in the form of rain or snow.
This reference design saves the current pressure reading
and compares it to past pressure measurements. It
determines if there was a pressure drop or a pressure
increase. Using this information, it makes a simple weather
prediction by sending a message of ‘sunny’ for a pressure
increase, ‘rainy for a pressure drop, and ‘steady’ for no
significant change in pressure.1
.
The Altimeter is one of many applications for the
MPXM2102AS pressure sensor. This reference design can be
used as a reference for developing more integrated
barometer applications such as hand-held weather stations,
altimeter features for camera or GPS systems, as well as
barometric pressure monitoring systems for industrial
systems. The MPXM2102AS is an excellent pressure sensor
for this application since it is calibrated and temperature
compensated. By having these features available on-chip,
there is a large savings in PCB real estate in addition to
savings in cost for external components
Table 4. Elevation Pressure and Temperature Changes
Altitude Above Sea Level
Temperature
Barometer
Atmospheric Pressure
Feet
Meters
F
C
mm * Hg
psi
kPa
-1000
-305
63
17
787.9
15.23
105.0
-500
-153
61
16
773.9
14.96
103.1
0
0
59
15
760.0
14.69
101.33
500
153
57
14
746.3
14.43
99.49
1000
305
55
13
733.0
14.16
97.63
1500
458
54
12
719.6
13.91
95.91
2000
610
52
11
706.6
13.66
94.19
2500
763
50
10
693.9
13.41
92.46
3000
915
48
9
681.2
13.17
90.81
3500
1068
47
8
668.8
12.93
89.15
4000
1220
45
7
656.3
12.69
87.49
4500
1373
43
6
644.4
12.46
85.91
5000
1526
41
5
632.5
12.23
84.33
6000
1831
38
3
609.3
11.78
81.22
7000
2136
34
1
586.7
11.34
78.19
9000
2441
31
-1
564.6
10.91
75.22
9000
2746
27
-3
543.3
10.5
72.40
10,000
3050
23
-5
522.7
10.1
69.64
15,000
4577
6
-14
429.0
8.29
57.16
1. This information was found from the USA Today Weather Book from USAToday.com.
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Start
A/D converter is set up and enabled
and variables initialized
PB1 and PB2
pressed
Start in Calibration
Mode?
PB1 and PB2 not
pressed
Display Calibration
Message
Display Welcome
Message
Apply 64 kPa Pressure
Average 256 Readings
PB1 pressed
Display Altitude or
Set Elevation?
Apply 105 kPa Pressure
Average 256 Readings
“Select Elevation”
Menu Displayed
Cycle through 0 to
12000ft selections
PB2 pressed
Read 256 A/D values
Average 256
Convert digital reading
to history graph
Convert A/D Values
to Pressure (kPa)
Calculate Altitude
in Feet
Display Barometric
Pressure, Altitude and
History
Figure 7. Altimeter/Barometer Software Flow Diagram
REFERENCES
Williams, Jack. (2001). Understanding Air Pressure. The
Weather Book, 5, 117–123. Retrieved April 4, 2003, from
http://www.usatoday.com/weather/wfront.htm
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NOTES
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AN1979
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11/2006
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