MPXM2102AS datasheet(368/670 Pages) MOTOROLA

368 / 670 page

AN1305

3–222

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

kPa

PSI

4.5

0.5

100

3.62

7.25

10.87

14.5

TYP

SPAN

TYP OFFSET

VS = 5.0 Vdc

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.

Action

OUT

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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