Application of Single Chip Microcomputer in Automatic Blood Pressure Monitoring System

introduction

At present, most of the automatic blood pressure monitoring systems used in hospitals are imported equipment, which are expensive and increase the cost of medical treatment. The system discussed in this article is low-priced, accurate, unique, and has high practical value.

1 System Function and Composition

The system is implemented by a PC to receive and manage data, and is combined with an automatic blood pressure monitor used in each ward (with a single-chip microcomputer installed inside the automatic blood pressure monitor)
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MCU is the abbreviation of Single-Chip Microcomputer, which is a microcomputer system that integrates the central processing unit CPU, random access memory RAM, read-only memory ROM, multiple I/O ports and interrupt systems, timers/counters and other functions (may also include display drive circuits, pulse width modulation circuits, analog multiplexers, A/D converters and other circuits) on a silicon chip using very large-scale integrated circuit technology. [Full text]

The measuring and control board is modified to form a complete automatic blood pressure monitoring system, as shown in Figure 1.

The management PC is set up at the nurse station. The startup and measurement time of the blood pressure monitor used in each ward can be set on the PC, and the blood pressure monitor can be controlled to work according to the set time, and then the measurement data sent back by the blood pressure monitor can be received; finally, the data sent back by each ward 24 hours a day can be plotted into graphs or tables for doctors' reference.

There are many types of blood pressure monitors on the market that can perform automatic measurement, but they can only perform stand-alone measurement and store a small amount of data, and cannot complete the timed start and automatic measurement when no one is operating, as well as the communication and transmission of measurement data. The design of the automatic blood pressure monitor is to install a single-chip microcomputer measurement and control board inside the automatic blood pressure monitor, which can receive instructions from the PC, automatically start and measure according to the instructions, and sample the measurement signal from the LCD screen of the automatic blood pressure monitor. After analysis and decoding, the measurement data is finally obtained and transmitted to the PC for storage, statistics, and analysis.

2 LCD display of automatic blood pressure monitor
LCD display

A device that uses the electro-optical effect of liquid crystal to modulate external light for display. [Full text]

How it works

LCD screen of Japan Seiko automatic blood pressure monitor
LCD screen

Liquid crystal screens are displays made of LED liquid crystal as the main material. Liquid crystal is a polymer material. Because of its special physical, chemical and optical properties, it has been widely used in thin and light display technology since the mid-20th century.

Composed of 7 digital tubes
Digital tubes
Digital tubes are a type of semiconductor light-emitting device with light-emitting diodes as the basic unit. In our daily lives, for example, the display of electronic scales and electronic digital clocks is realized by digital tubes, and digital tubes have been widely used today. [Full text]

And 1 sign bit, of which 6 eight-segment digital tubes display high voltage, low voltage and pulse number respectively, and 1 small digital tube is used to display the number of measurements; 1 sign bit can display 4 symbols, reflecting 4 different working states. Through experimental observation and logic analysis, it is found that each eight-segment digital tube of the LCD screen is controlled by 2 signal pins, the sign bit has 1 signal pin, and there is 1 synchronization signal pin as the reference for signal synchronization. See Figure 2. The timing relationship of the LCD screen signal pins is shown in Figure 3.

The pulse signal on the pin of the LCD screen has a signal cycle for every 4 synchronization signals. Within 1 signal cycle, the 4 fields of the digital tube can be controlled. The control of these 4 fields is realized in a time-division manner in terms of timing; the symbol bit only displays 4 different symbols, so only 1 signal pin is needed. In the periodic signal of each pin, there are 4 pulses corresponding to the pulses of the synchronization signal. Each pulse controls 1 field. When there is a positive pulse at this position, the corresponding field is lit. In this way, the pulse waveforms at 4 different positions correspond to the 4 different fields of the digital tube (such as a, b, c, d). Therefore, an eight-segment digital tube only needs 2 signal pins. The waveforms shown on pins 1 and 2 in Figure 3 are the pulse signal waveforms of the number 3 (see Figure 2).

3 MCU Hardware Design

The work of the single-chip microcomputer control board needs to be implemented according to the work flow of the automatic blood pressure monitor. The hardware circuit must meet the following functions:

a) The two I/O ports of the microcontroller send control signals to start and measure the blood pressure monitor.

b) Shaping and level conversion. The signal level sampled by the microcontroller from the signal pin of the blood pressure monitor LCD screen is inconsistent with the level of the microcontroller I/O interface, and the pin signal obtained has burr interference on the rising and falling edges, which requires an analog voltage comparator
.

The comparator is a widely used circuit element. In fact, it is also an operational amplifier with very high gain, which can amplify a very small differential signal at the input end and drive the output end to switch to one of the two output states. So that it cannot stabilize in the middle amplification area, and no longer jumps to a low level or a high level. [Full text]

Realize shaping and level conversion functions.

c) Signal sampling. In order to obtain the numbers displayed on the LCD screen, 16 pin signals need to be sampled, that is, 16 ports of the microcontroller need to be occupied. In order to place the circuit board inside the blood pressure monitor, the circuit board should be as small as possible to reduce the occupation of the input channel of the microcontroller. First, the 16 pin signals are converted into 2 output signals through 2 eight-to-one analog data selectors. In this way, only two I/O ports of the microcontroller need to be occupied, and then 3 I/O ports are used as the decoding control signals of the eight-to-one analog data selector. The sampling of the pin signals is completed by the software scanning method, which greatly reduces the occupation of the I/O ports.

d) Communication with PC. To construct an RS-232 serial communication port, three I/O ports of the microcontroller and a MAX232 integrated circuit are used
.

Integrated circuits are made by using semiconductor manufacturing technology to manufacture many transistors, resistors, capacitors and other components on a small single-crystal silicon wafer, and then combine the components into a complete electronic circuit by using multi-layer wiring or tunnel wiring. It is represented by the letter "IC" (also with the letter symbol "N", etc.) in the circuit. [Full text]

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e) MCU selection. Based on the above analysis, a MCU with 10 I/O ports can meet the system requirements. In order to shape the signal waveform to a level suitable for the MCU, multiple analog voltage comparators are required. Since the PIC16C622 MCU has 13 I/O ports and a comparator module is also installed inside, making full use of this feature of the MCU can not only save multiple comparator integrated chips, but also its I/O ports can meet the system requirements. The schematic diagram of the hardware circuit composition is shown in Figure 4.

4 MCU Software Design

The task that the microcontroller needs to complete is to receive instructions from the management computer and turn on the power
supply at the set time.

A power supply is a device that provides power to electronic devices. It is also called a power supply. It provides the electrical energy required by all components in a computer. [Full text]

First, start the blood pressure monitor to start measuring; second, monitor the entire measurement process. This requires not only the corresponding hardware circuit, but also the cooperation of software to filter out interference signals, and take out the pin signal and sign bit signal of each digital tube from the corresponding pin according to the correct timing, and compare the corresponding relationship between the synchronization signal and the different pin signals. Use the software scanning method to combine the signals on the two pins of the same digital tube, and obtain the final measurement result after analysis and decoding. Finally, the communication between the microcontroller and the PC must be completed, and the measurement results must be sent to the management machine. The main program flowchart is shown in Figure 5.

5. Program Debugging

The difficulty of program debugging is how to accurately extract the display data according to the correct timing of the LCD screen pin signal. This not only requires the coordination of the hardware circuit and the observation of the signal timing of the LCD screen pin, but also requires repeated experiments and measurements, adjustment of software parameters, finding accurate timing data, and completely extracting the waveform data of the displayed digital, and then summarizing and decoding the waveform data to obtain the correct measurement data. Finally, the communication between the microcontroller and the PC also requires accurate coordination to complete the data transmission.

6 Conclusion

Since most of the automatic blood pressure monitoring systems currently used in hospitals are imported equipment, they are expensive and the cost of medical treatment is high, so compared with automatic ECG monitors, automatic blood pressure monitoring is far from being popularized. The automatic blood pressure monitoring system discussed in this article uses a PC to realize data reception and data management. It has a unique design concept, simple system structure, easy operation and use. Compared with similar equipment, it has high cost performance, accurate measurement, and high practical value.