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Motorola Sensor Device Data
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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 a signal output to the microprocessor of
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.
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Freescale Semiconductor, Inc.
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