MPXM2102AS datasheet(504/670 Pages) MOTOROLA

504 / 670 page

AN1571

3–358

Motorola Sensor Device Data

www.motorola.com/semiconductors

Referring to the schematic, Figure 4, the MPX5050GP

pressure sensor is connected to PORT D bit 5 and the output

of the amplifier is connected to PORT D bit 6 of the

microcontroller. This port is an input to the on–chip 8–bit

analog–to–digital (A/D) converter. The pressure sensor

provides

signal

output

the

microprocessor

approximately 0.2 Vdc at 0 mmHg to 4.7 Vdc at 375 mmHg of

applied pressure whereas the amplifier provides a signal from

0.005 V to 3.5 V. In order to maximize the resolution, separate

voltage references should be provided for the A/D instead of

using the 5 V supply. In this example, the input range of the A/D

converter is set at approximately 0 Vdc to 3.8 Vdc. This

compresses the range of the A/D converter around 0 mmHg

to 300 mmHg to maximize the resolution; 0 to 255 counts is the

range of the A/D converter. VRH and VRL are the reference

voltage inputs to the A/D converter. The resolution is defined

by the following:

Count = [(VXdcr – VRL)/(VRH – VRL)] x 255

The count at 0 mmHg = [(0.2 – 0)/(3.8 – 0)] x 255

≈ 14

The count at 300 mmHg = [(3.8 – 0)/(3.8 – 0)] x 255

≈ 255

Therefore the resolution = 255 – 14 = 241 counts. This

translates to a system that will resolve to 1.24 mmHg.

The voltage divider consisting of R5 and R6 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 (LCD) is directly driven from I/O

ports A, B, and C on the microcontroller. The operation of a

LCD requires that the data and backplane (BP) 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.

Other than the LCD, there are two more I/O devices that are

connected to the pulse length converter (PLM) of the

microcontroller; a buzzer and a light emitting diode (LED). The

buzzer, which connected to the PLMA, can produce two

different frequencies; 122 Hz and 1.953 kHz tones. For

instance when the microcontroller encounters certain error

due to improper inflation of cuff, a low frequency tone is alarm.

In those instance when the measurement is successful, a high

frequency pulsation tone will be heard. Hence, different

musical tone can be produced to differential each condition. In

addition, the LED is used to indicate the presence of a heart

beat during the measurement.

The microcontroller section of the system requires certain

support hardware to allow it to function. The MC34064P–5

provides an undervoltage sense function which is used to

reset the microprocessor at system power–up. The 4 MHz

crystal provides the external portion of the oscillator function

for clocking the microcontroller and provides a stable base for

time based functions, for instance calculation of pulse rate.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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