
3–360
Motorola Sensor Device Data
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SOFTWARE DESCRIPTION
Upon system power–up, the user needs to manually pump
the cuff pressure to approximately 160 mmHg or 30 mmHg
above the previous SBP. During the pumping of the inflation
bulb, the microcontroller ignores the signal at the output of the
amplifier. When the subroutine TAKE senses a decrease in
CP for a continuous duration of more than 0.75 seconds, the
microcontroller will then assume that the user is no longer
pumping the bulb and starts to analyze the oscillation signal.
Figure 5 shows zoom–in view of a pulse.
–7.1
–7.3
–7.5
–7.7
–7.9
–8.1
–8.3
–8.5
Time (second)
Figure 5. Zoom–in view of a pulse
V
450 ms
Premature pulse
1.75
First of all, the threshold level of a valid pulse is set to be 1.75
V to eliminate noise or spike. As soon as the amplitude of a
pulse is identified, the microcontroller will ignore the signal for
450 ms to prevent any false identification due to the presence
of premature pulse ”overshoot” due to oscillation. Hence, this
algorithm can only detect pulse rate which is less than 133
beats per minute. Next, the amplitudes of all the pulses
detected are stored in the RAM for further analysis. If the
microcontroller senses a non–typical oscillation envelope
shape, an error message (“Err”) is output to the LCD. The user
will have to exhaust all the pressure in the cuff before
re–pumping the CP to the next higher value. The algorithm
ensures that the user exhausts all the air present in the cuff
before allowing any re–pumping. Otherwise, the venous blood
trapped in the distal arm may affect the next measurement.
Therefore, the user has to reduce the pressure in the cuff as
soon as possible in order for the arm to recover. Figure 6 is a
flowchart for the program that controls the system.
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Freescale Semiconductor, Inc.
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