3–222
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Theory of Operation
Referring to the schematic, Figure 2, the MPX5100
pressure sensor is connected to PORT D bit 5 of the
microprocessor. This port is an input to the on–chip 8 bit
analog to digital converter. The pressure sensor provides a
signal output to the microprocessor of approximately 0.5 Vdc
at 0 psi to 4.5 Vdc at 15 psi of applied pressure as shown in
Figure 4. The input range of the A to D converter is set at
approximately 0.3 Vdc to 4.85 Vdc. This compresses the
range of the A to D converter around the output range of the
sensor to maximize the A to D converter resolution; 0 to 255
counts is the range of the A to D converter. VRH and VRL are
the reference voltage inputs to the A to D converter. The
resolution is defined by the following:
Analog–to–digital converter count =
[(Vxdcr– VRL)/(VRH– VRL)]
255
The count at 0 psi = [(.5 – .302)/(4.85 – .302)]
255
≈
11
The count at 15 psi = [(4.5 – .302)/(4.85 – .302)]
255
≈
235
Therefore the resolution = count @ 15 psi – count @ 0 psi or
the resolution is (235 – 11) = 224 counts. This translates to a
system that will resolve to 0.1 psi.
MIN
TYP
MAX
O
kPa
PSI
4.5
0.5
00
25
3.62
50
7.25
75
100
14.5
10.87
TYP
SPAN
TYP OFFSET
VS=5.0Vdc
TA=25
°
C
MPX5100
Figure 4. MPX5100 Output versus Pressure Input
The voltage divider consisting of R5 through R7 is
connected to the +5 volts powering the system. The output of
the pressure sensor is ratiometric to the voltage applied to it.
The pressure sensor and the voltage divider are connected to
a common supply; this yields a system that is ratiometric. By
nature of this ratiometric system, variations in the voltage of
the power supplied to the system will have no effect on the
system accuracy.
The liquid crystal display is directly driven from I/O ports A,
B, and C on the microprocessor. The operation of a liquid
crystal display requires that the data and backplane pins must
be driven by an alternating signal. This function is provided by
a software routine that toggles the data and backplane at
approximately a 30 Hz rate.
The microprocessor section of the system requires certain
support hardware to allow it to function. The MC34064P–5
(U2) provides an under voltage sense function which is used
to reset the microprocessor at system power–up. The 4 MHz
crystal (Y1) provides the external portion of the oscillator
function for clocking the microprocessor and provides a stable
base for time based functions. Jumpers J1 and J2 are
examined by the software and are used to “rubber” the slope
constant.
OPERATION
The system must be connected to a 5 Vdc regulated power
supply. Note the polarity marked on the power terminal J3.
Jumpers J1 and J2 must either both be installed or both be
removed for the normal slope constant to be used. The
pressure port on the MPX5100 sensor must be left open to
atmosphere anytime the board is powered–up. As previously
stated, the sensor’s voltage offset with zero pressure applied
is computed at power–up.
You will need to apply power to the system. The LCD will
display CAL for approximately 5 seconds. After that time, the
LCD will then start displaying pressure.
To improve upon the accuracy of the system, you can
change the constant used by the program that constitutes the
span of the sensor. You will need an accurate test gauge to
measure the pressure applied to the sensor. Anytime after the
display has completed the zero calculation (after CAL is no
longer displayed), apply 15.0 PSI to the sensor. Make sure
that jumpers J1 and J2 are either both installed or both
removed. Referring to Table 2, you can increase the displayed
value by installing J1 and removing J2. Conversely, you can
decrease the displayed value by installing J2 and removing
J1.
J1
J2
Action
IN
OUT
OUT
IN
IN
OUT
IN
OUT
USE NORMAL SPAN CONSTANT
USE NORMAL SPAN CONSTANT
DECREASE SPAN CONSTANT
APPROXIMATELY 1.5%
INCREASE SPAN CONSTANT
APPROXIMATELY 1.5%
Table 2.
SOFTWARE
The source code, compiler listing, and S–record output for
the software used in this system are available on the Motorola
Freeware Bulletin Board Service in the MCU directory under
the filename DEVB–114.ARC. To access the bulletin board
you must have a telephone line, a 300, 1200 or 2400 baud
modem and a terminal or personal computer. The modem
must be compatible with the Bell 212A standard. Call
1–512–891–3733 to access the Bulletin Board Service.
The software for the system consists of several modules.
Their functions provide the capability for system calibration as
well as displaying the pressure input to the MPX5100
transducer.
Figure 5 is a flowchart for the program that controls the
system.
F
Freescale Semiconductor, Inc.
n
.