Design of an intelligent electronic blood pressure meter based on LPC3250

In recent years, due to the rapid development of social economy and changes in people's lifestyles, the incidence of cardiovascular diseases and related risk factors in my country have shown an increasing trend, and people are more and more concerned about their health. Electronic sphygmomanometers are medical devices that use modern electronic technology and the principle of indirect blood pressure measurement to measure blood pressure. They are the embodiment of the intelligence of home medical devices. In the past, people had to go to the hospital to measure their blood pressure, but now, as long as they have a home electronic sphygmomanometer, they can monitor blood pressure changes at any time while sitting at home.

1 System working principle
Blood pressure refers to the lateral pressure of blood in blood vessels on the vascular wall per unit area, that is, pressure. A normal heart is a strong muscle organ that expands and contracts rhythmically day and night to make blood flow in the circulatory organs. When blood flows in the blood vessels, whether the heart contracts or relaxes, it exerts a certain pressure on the vascular wall. There are two types of blood pressure. One is systolic pressure, which refers to the sharp increase in aortic pressure when the ventricle contracts, and reaches the highest value in the middle of the systolic period. The arterial blood pressure value at this time is called systolic pressure, also known as "high pressure"; the other is diastolic pressure, which refers to the decrease in aortic pressure when the ventricle relaxes, and the lowest value of arterial blood pressure at the end of diastole is called diastolic pressure, also known as "low pressure".
This multifunctional electronic sphygmomanometer uses the oscillometric method for measurement. Its principle is to measure the vibration of the blood vessel wall when the blood flows. During the cuff deflation process, as long as the pressure in the cuff is the same as the blood vessel pressure, the vibration is the strongest. Its advantages are: easy to use, can be operated by one person alone, easy to record the measured value, and light and easy to carry.

2 Hardware Design
2.1 System Overall Structure
The overall structure of the multifunctional electronic blood pressure monitor system is shown in Figure 1, which mainly includes six modules: LPC3250 main control module, power supply and reset module, detection module, LCD touch screen module, WiFi module, voice module and USB module.

2.2 Main Controller LPC3250
The main controller adopts the newly launched highly integrated LPC3250 microprocessor of NXP, which has the characteristics of high performance, high integration, low power consumption, etc., and is very suitable for the design requirements of this solution. It adopts 90 nm process and powerful ARM926EJ-S core, with a main frequency of up to 208 MHz, and has a full range of standard peripherals. These include a 24-bit LCD controller with a dedicated DMA controller that can support STN and TFT panels; a three-channel 10-bit 400 kHz A/D converter with a touch screen interface; internal integration of up to 11 PWM channels; a USB OTG interface that can connect the host and the device at full speed; an external storage controller that supports DDR and SDR SDRAM, SRAM, FLASH and static devices. It fully meets the needs of this design. Only a few chips are needed to realize the system functions, and
the entire system can be reduced in size, power consumption, stability and cost. [page]

2.3 Pressure sensor XFGN-6025KPGSR
This design uses the new generation pressure sensor XFGN-6025KPGSR produced by Fujikura Company of Japan. It weighs only 0.35 g and is mainly used for portable electronic blood pressure monitors. It uses precision thick film ceramic chips and nylon plastic packaging. It contains amplification, temperature compensation and pre-correction of offset voltage and range, which improves the measurement accuracy and stability and eliminates the need for an amplifier circuit. It directly converts blood pressure into an electrical signal of 0 to 4.5 V, and the corresponding blood pressure value is 0 to 25 kPa, that is, 0 to 187.5 mmHg, which is very consistent with the design requirements of the blood pressure monitor.
2.4 Filter The analog signal measured by MAX267
must also be filtered before A/D conversion. The filtering is used to filter out the DC component in the signal, the high-frequency noise of the power supply and the friction between the skin and the cuff, and the power frequency interference. This design uses Maxim's MAX267, which is one of the simpler of Maxim's many switched capacitor filter (SCF) chips. It contains two second-order SCFs and an op amp that have been fixed to a bandpass type and use the same Q parameter and frequency conversion ratio. By selecting appropriate feedback resistors and Q parameters, Butterworth or Chebyshev filters with different ripple rates can be formed. This greatly reduces the number of peripheral circuits and is flexible to use. The performance is far superior to the filter circuit composed of an integrated op amp, which is very suitable for this design.

3 Software Design
3.1 Program Flow
The software part is the core of the effective operation of the system, and its program flow is shown in Figure 2. After the system is started, the embedded Linux is started by U-boot, the relevant hardware and programs are initialized, and the main menu is entered. Among them, data query can view previous measurement results on the LCD, and the machine can perform a simple health analysis; network service can manually transmit the measured results to the hospital via WiFi, and have a simple conversation with the doctor; start the test, the blood pressure meter enters the test mode; system settings can set the system time, network parameters, etc., and can upgrade the system firmware; personal information can be entered into your height, weight, gender, age and other information, so that the system can manage each person's measurement data separately and conduct targeted health analysis.

[page]

3.2 Electronic blood pressure meter detection mode flow
When measuring blood pressure, the microprocessor PWM output controls the air pump to inflate, first inflate to the maximum rated value of the pressure sensor 25 kPa, that is, 187.5 mmHg, and then slowly deflate at a uniform rate of about 3 mmHg per second to adjust the air pressure in the cuff to achieve automatic blood pressure measurement. One A/D samples the DC component of the air pressure in the cuff to obtain the systolic and diastolic pressures, that is, high pressure and low pressure, and averages the heartbeat cycle to calculate the heart rate; the other A/D samples the AC component of the air pressure in the cuff, and determines the transient time position of the systolic and diastolic pressures after analysis and calculation, receives the blood pressure pulse signal, triggers the ADC to work, and calculates the results of the systolic and diastolic pressures. The heart rate and blood pressure calculation flow chart is shown in Figure 3.

3.3 Design of database system
The data management of this design adopts SQLite embedded database, which is a small and medium-sized embedded database developed by D. Richard Hipp in 2000. It can be easily used in embedded systems. Its source code is completely open and can be used for any purpose, including commercial purposes. It provides most of the support for SQL92, such as supporting multiple tables, indexes, transactions, views, triggers and a series of user interfaces and drivers. It is simple and easy to use, fast, and provides rich database interfaces. After analyzing and optimizing the requirements, the ER diagram of the database system can be designed as shown in Figure 4. It fully and accurately reflects the functional requirements of users, and can reduce the number of entity types and the number of attributes contained, and there is no redundancy between entity types.
3.4 System interface design
In the system interface, various commands are issued through the touch screen, and then displayed by the graphical interface through the database connection. The application window interface of this design is implemented in Qt, which is a cross-platform C++ graphical user interface application framework. It is fully object-oriented, easy to expand, and allows real component programming. It is widely used in various embedded products. Figure 5 shows the interface designed using the lightweight cross-platform integrated development environment Qt creator. After booting up, first enter the main menu as shown in Figure 5(a), tap Start Detection, and the system enters the detection menu as shown in Figure 5(b), displaying the measured data. Considering the poor eyesight of the elderly, the fonts in this interface have been enlarged.

4 Conclusion
With the continuous improvement of living standards and the proportion of urban aging, the home-based and intelligent medical electronic equipment has gradually become a trend. This article gives a complete design scheme for an intelligent electronic blood pressure monitor, which is different from traditional products with only a single detection function. It has three major innovations: large-screen display and control; using embedded database SQLite for data management; and connecting to the hospital through the network. Practical applications have shown that the system is small in size, low in power consumption, intelligent, and has a fast detection speed. It realizes a health detection network that integrates individuals, instruments, and medical institutions, which is very suitable for home users, especially the elderly. If this solution can be widely promoted and applied, it will produce immeasurable economic and social benefits.