MPX2202DP datasheet(506/670 Pages) MOTOROLA

506 / 670 page

AN1571

3–360

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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