AN1571, Digital Blood Pressure Meter - Freescale Semiconductor

Freescale Semiconductor 

Application Note 

Digital Blood Pressure Meter 

by: C.S. Chua and Siew Mun Hin, Sensor Application Engineering 

Singapore, A/P 

INTRODUCTION 

This application note describes a Digital Blood Pressure 

Meter concept which uses an integrated pressure sensor, 

analog signal-conditioning circuitry, microcontroller 

hardware/software and a liquid crystal display. The sensing 

system reads the cuff pressure (CP) and extracts the pulses 

for analysis and determination of systolic and diastolic 

pressure. This design uses a 50 kPa integrated pressure 

sensor (Freescale Semiconductor, Inc.P/N: MPXV5050GP) 

yielding a pressure range of 0 mm Hg to 300 mm Hg. 

CONCEPT OF OSCILLOMETRIC METHOD 

This method is employed by the majority of automated noninvasive 

devices. A limb and its vasculature are compressed 

by an encircling, inflatable compression cuff. The blood 

pressure reading for systolic and diastolic blood pressure 

values are read at the parameter identification point. 

The simplified measurement principle of the oscillometric 

method is a measurement of the amplitude of pressure 

change in the cuff as the cuff is inflated from above the systolic 

pressure. The amplitude suddenly grows larger as the pulse 

breaks through the occlusion. This is very close to systolic 

pressure. As the cuff pressure is further reduced, the pulsation 

increase in amplitude, reaches a maximum and then 

diminishes rapidly. The index of diastolic pressure is taken 

where this rapid transition begins. Therefore, the systolic 

blood pressure (SBP) and diastolic blood pressure (DBP) are 

© Freescale Semiconductor, Inc., 2005. All rights reserved. 

Vi 

C2 

0.33 µ 

+DC Offset 

R3 

1M 

R1 

C1 

1k 

33u 

3 

+ 

2 

- 

Figure 1. Oscillation Signal Amplifier 

AN1571 

Rev 1, 05/2005 

obtained by identifying the region where there is a rapid 

increase then decrease in the amplitude of the pulses 

respectively. Mean arterial pressure (MAP) is located at the 

point of maximum oscillation. 

HARDWARE DESCRIPTION AND OPERATION 

The cuff pressure is sensed by Freescale's integrated 

pressure X-ducer‰. The output of the sensor is split into two 

paths for two different purposes. One is used as the cuff 

pressure while the other is further processed by a circuit. 

Since MPXV5050GP is signal-conditioned by its internal opamp, 

the cuff pressure can be directly interfaced with an 

analog-to-digital (A/D) converter for digitization. The other 

path will filter and amplify the raw CP signal to extract an 

amplified version of the CP oscillations, which are caused by 

the expansion of the subject's arm each time pressure in the 

arm increases during cardiac systole. 

The output of the sensor consists of two signals; the 

oscillation signal ( ≈ 1 Hz) riding on the CP signal ( ≤ 0.04 Hz). 

Hence, a 2-pole high pass filter is designed to block the CP 

signal before the amplification of the oscillation signal. If the 

CP signal is not properly attenuated, the baseline of the 

oscillation will not be constant and the amplitude of each 

oscillation will not have the same reference for comparison. 

Figure 1 shows the oscillation signal amplifier together with 

the filter. 

11 

4 

+5.0V 

R2 

150k 

U1a 

1 

LM324N 

Vo

The filter consists of two RC networks which determine two 

cut-off frequencies. These two poles are carefully chosen to 

ensure that the oscillation signal is not distorted or lost. The 

Attenuation (dB) 

10 

0 

-10 

-20 

-30 

-40 

-50 

-60 

-70 

-80 

The oscillation signal varies from person to person. In 

general, it varies from less than 1 mm Hg to 3 mm Hg. From 

the transfer function of MPXV5050GP, this will translate to a 

voltage output of 12 mV to 36 mV signal. Since the filter gives 

an attenuation of 10 dB to the 1 Hz signal, the oscillation signal 

becomes 3.8 mV to 11.4 mV respectively. Experiments 


CP Signal (0.04 Hz) 

0.01 

0.1 

f P1 = 

f P2 = 

1 

2πR1C1 1 

2πR3C2 Figure 2. Filter Frequency 

two cut-off frequencies can be approximated by the following 

equations. Figure 2describes the frequency response of the 

filter. This plot does not include the gain of the amplifier. 

10 100 

indicate that, the amplification factor of the amplifier is chosen 

to be 150 so that the amplified oscillation signal is within the 

output limit of the amplifier (5.0 mV to 3.5 V). Figure 3 shows 

the output from the pressure sensor and Figure 4 illustrates 

the extracted oscillation signal at the output of the amplifier. 

Sensors 

2 Freescale Semiconductor 

1 

Frequency (Hz) 

Oscillation Signal (1 Hz)

Vi (Volts) 

Vo (Volts) 

3 

2.5 

2 

1.5 

1 

0.5 

0 

0 5 10 15 20 25 30 35 40 

3.5 

3 

2.5 

2 

1.5 

1 

0.5 

SBP 

Figure 3. CP Signal at the Output of the Pressure Sensor 

Figure 4. Extracted Oscillation Signal at the Output of Amplifier 

Referring to the schematic, Figure 5, 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 mm Hg to 4.7 Vdc at 375 mm Hg 

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 mm Hg to 300 mm Hg to maximize the resolution; 0 to 255 

Time (seconds) 

Oscillation signal is extracted here 

0 

10 15 20 25 30 35 

Time (seconds) 

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 mm Hg = [(0.2 - 0)/(3.8 - 0)] x 255 ≈ 14 

The count at 300 mm Hg = [(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 mm Hg. 

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 


Sensors 

Freescale Semiconductor 3 

MAP 

DBP

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. 

Sensors 

4 Freescale Semiconductor

R10 

+5.0 V 

10M 

+5.0 V 

5.0 V Regulator 

MC78L05ACP 

+5.0 V 

X1 

4MHz 

1 

Input Output 

3 

2 

R9 

4.7k 

C4 

C3 

GND 

22p 

22p 

100u 

C5 

MC34064 

Reset Input 

GND 

1 

C6 

C8 

2 

0.33u 

9.0 V Battery 

100n 

3 

+5.0 V 

LCD5657 

16 

OSC1 

OSC2 

17 

10 

37 

36 

35 

34 

7 

6 

5 

18 

19 

G4 

F4 

A4 

B4 

C4 

D4 

E4 

/RESET 

/IRQ 

VDD 

+5.0 V 

+5.0 V 

Buzzer 

4 

22 

23 

50 

TCAP1 

TCAP2 

RD 

TCMP1 

TCMP2 

2 

1 

100R 

R8 

Pressure Sensor 

MPXV5050GP 

52 

51 

TDO 

SCLK 

20 PLMA VRH 

21 

PLMB VRL 

49 PC0 PA0 

48 PC1 PA1 

47 PC2/ECLK PA2 

46 PC3 PA3 

45 PC4 PA4 

44 PC5 PA5 

43 PC6 PA6 

42 PC7 PA7 

14 PD0/AN0 PB0 




9543 PD4/AN4 PB4 

PD5/AN5 PB5 

PD6/AN6 PB6 33 

PD7/AN7 PB7 32 

MC68HC05B16CFN 

34 

35 

36 

37 

24 

39 

38 

25 

26 

27 

28 

29 

30 

31 

8 

7 

Vs 

1 

Vout 

3 

DP 

16 

23 

22 

21 

20 

19 

18 

17 

LED 

3 

DP1 

G1 

F1 

A1 

B1 

C1 

D1 

E1 

R5 R6 

4.7k + 36R 

15k 

+5.0 V 

GND 

28 

L 

40 

1 

2 

BP 

BP 

DP L L 

24k 

R4 

8 

32 

31 

30 

29 

11 

10 

9 

12 

27 

26 

25 

24 

15 

14 

13 

Figure 5. Blood Pressure Meter Schematic Drawing 


Sensors 

Freescale Semiconductor 5 

DP3 

G3 

F3 

+5.0 V 

10k 

R0 

A3 

A 

B3 

C3 

D3 

E3 

2 

DP2 

G2 

F2 DP E F 

A2 

D 1 G 

B2 

C2 

C B 

D2 

E2 

1M 

R3 

11 

C2 

3 

1 

LM324N 

0.33u 

330u 

2 

C7 

+5.0 V 

4 

R2 

150k 

1k 

R1 

33u 

C1


SOFTWARE DESCRIPTION 

Upon system power-up, the user needs to manually pump 

the cuff pressure to approximately 160 mm Hg or 30 mm Hg 

above the previous SBP. During the pumping of the inflation 

bulb, the microcontroller ignores the signal at the output of the 

V O (volt) 

-8.5 

-8.3 

450 ms 

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 repumping 

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 7 on 

the following page is a flowchart for the program that controls 

the system. 

SELECTION OF MICROCONTROLLER 

Although the microcontroller used in this project is 

MC68HC05B16, a smaller ROM version microcontroller can 

also be used. The list below shows the requirement of 

-8.1 

Premature Pulse 

-7.9 

-7.7 

Time (second) 

Figure 6. Zoom-In View of a Pulse 

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 6 shows zoom-in view of a pulse. 

microcontroller for this blood pressure meter design in this 

project. 

• On-chip ROM space: 2 kilobytes 

• On-chip RAM space: 150 bytes 

• 2-channel A/D converter (min.) 

• 16-bit free running counter timer 

• LCD driver 

• On-chip EEPROM space: 32 bytes 

• Power saving Stop and Wait modes 

CONCLUSION 

This circuit design concept may be used to evaluate 

Freescale pressure sensors used in the digital blood pressure 

meter. This basic circuit may be easily modified to provide 

suitable output signal level. The software may also be easily 

modified to provide better analysis of the SBP and DBP of a 

person. 

REFERENCES 

Lucas, Bill (1991). “An Evaluation System for Direct 

Interface of the MPX5100 Pressure Sensor with a 

Microprocessor,” Freescale Application Note AN1305. 

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6 Freescale Semiconductor 

-7.5 

-7.3 

1.75 

-7.1

Output a high 

frequency 

musical tone 

Display pulse rate. 

Display "SYS" follow by SBP. 

Display "dlA" follow by DBP. 

Y 

Main Program 

Initialization 

Clear I/O ports 

Display "CAL" and 

output a musical tone 

Clear all the variables 

Take in the amplitude of all the 

oscillation signal when the 

user has stop pumping 

Repump? 

N 

Calculate the SBP and DBP 

and also the pulse rate 

N 

Is there any error 

in the calculation or the 

amplitude envelope 

detected? 

Y 

N N 

Exhaust cuff 

Exhaust cuff 

Y Y 

before repump 

before repump 

Figure 7. Main Program Flowchart 

Display "Err" 

Output a low 

frequency alarm 


Sensors 

Freescale Semiconductor 7

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Rev. 1 

05/2005 

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AN1571, Digital Blood Pressure Meter - Freescale Semiconductor

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