Application of electronic blood pressure meter based on Sinowin SH79F164 microcontroller

1. Overview:

Electronic sphygmomanometers are currently widely used and promoted due to their advantages such as non-invasiveness, simple operation and portability. There are many methods for non-invasive blood pressure detection, such as Korotkoff sound method, vibration measurement method, ultrasound method, double cuff method, constant cuff method, beat-by-beat tracking method, tension measurement method and constant volume method. Among them, the vibration measurement method is what we often call the oscillometric method. Due to its good anti-interference ability, it can more reliably judge blood pressure and realize automatic blood pressure detection, making it the mainstream of non-invasive blood pressure. At present, most electronic sphygmomanometers at home and abroad use the oscillometric method.

The principle of the oscillometric method is the same as that of the Korotkoff sound method. It also requires an inflated cuff to block the arterial flow. However, during the deflation process, the Korotkoff sound is not detected, but the oscillation wave of the gas in the cuff is detected (hence the name of the oscillometric method). These oscillation waves are the result of the coupling between the cuff and the artery, and originate from the pressure pulsation caused by the contraction and relaxation of the blood vessel wall during the cardiovascular cycle. Both theoretical calculations and practice have proved that the amplitude of this oscillation wave has a certain regularity and has a certain functional relationship with the arterial systolic pressure, mean pressure, and diastolic pressure.

Regarding the oscillometric method, this article will introduce in detail the blood pressure monitor system solution and software and hardware implementation based on the SinoWise SH79F164 microcontroller.

2. Hardware Design

In terms of hardware circuit design, the author referred to a lot of information and finally selected SH79F164 microcontroller as the main control IC. The reason is that SH79F164 has rich built-in resources, which can not only save a lot of peripheral devices, but also facilitate system debugging. The built-in resources of SH79F164 mainly include: programmable instrument amplifier (PGA), bandpass filter, fixed gain amplifier, constant current source amplifier, 10-bit A/D converter, and real-time timer (RTC).

The hardware part consists of: pressure sensor, SH79F164 microcontroller, LCD, cuff, air pump, deflation valve, buttons, etc. (see Figure 3).

2.1. Chip Introduction

SH79F164 is an enhanced MCU developed by Sinowin Electronics specifically for blood pressure monitor applications. It integrates a high-speed 8051 compatible MCU with a low-power clock and memory cycle. At the same oscillation frequency, it has the superior characteristic of running faster than the traditional 8051 chip.

SH79F164 retains most of the features of the standard 8051 chip, including built-in 256 bytes of RAM and two 16-bit timers/counters, one UART and external interrupts INT0, INT1 and INT2.

SH79F164 has a built-in 16K-byte Flash. In addition to storing program codes, the Flash area can also store data. Therefore, the blood pressure history data collected by the user can be saved in the Flash area through the program, without the need for an external EEPROM to save history records and calibration initial values, saving hardware costs.

SH79F164 integrates not only standard communication modules such as EUART, but also LCD driver, ADC with built-in comparison function, programmable gain amplifier, operational amplifier, PWM timer and time base timer (RTC) and other modules.

In order to achieve high reliability and low power consumption, SH79F164 has built-in PLL clock, watchdog timer, low voltage reset function. Considering the application of blood pressure meter, SH79F164 also provides 2 low power saving modes.

The following figure is the SH79F164 system block diagram:

2.2. Circuit Principle

The pressure sensor converts the blood pressure signal of the user's cuff into a voltage signal. After being amplified by the instrumentation amplifier (PGA), one path is directly sent to the A/D converter to obtain the absolute pressure value inside the cuff (see Figure 1), and the other path is sent to the bandpass filter and fixed gain amplifier composed of OP2 and OP3 to extract the pulse signal (see Figure 2).

The purpose of the bandpass filter composed of built-in OP2 and OP3 is to filter out the DC component, as well as the high-frequency noise caused by the friction between the human body and the cuff, 50Hz power frequency noise, circuit noise, PUMP and valve noise, etc., so that the software can accurately extract the pulse vibration wave.

The built-in OP1 constitutes a constant current source to provide a constant current for the pressure sensor. According to the sensor specifications and actual applications, the constant output of OP1 is selected to be 0.3mA ~ 1mA.

Built-in programmable instrumentation amplifier (PGA), the main feature is that it has high input impedance without affecting the constant current circuit of the sensor, while ensuring high gain for small signal amplification. The instrumentation amplifier contains two parts of programmable gain, with a total of 64 programmable gain levels, of which the programming gain range of PGA1 is: 16 times, 32 times, 64 times, 128 times, and the programming gain range of PGA2 is: 1.33 times to 2.58 times. If you need to adjust the output range, you can use the two bias voltages: VREF1 and VREF2 to achieve it.

Through the built-in resources of IC, this system can realize automatic calibration and initialization of the program, without manual adjustment of peripheral circuits, which is convenient for mass production.

The following figure shows the small signal amplifier circuit at the front end of the system:

2.3. System design features

Adopting SH79F164 enhanced single-chip computer as the core, the execution speed is about 10 times faster than the traditional 8051. The main control chip Flash program memory space is large: 16Kbyte, built-in EEPROM function system low power design: idle mode 12uA, power-off mode 1uA The main control chip has a built-in RTC module to record the real-time clock. The main control chip has a built-in instrument amplifier (PGA), bandpass filter, fixed gain amplifier, and constant current source. The factory calibration procedure of the OP blood pressure meter is automatically realized without manual adjustment of the peripheral circuit. The built-in wake-up circuit can be used to turn off the DC/DC device in the power saving mode to reduce the peripheral devices. The device composition: SH79F164+Sensor+resistor and capacitor.

2.4. System Block Diagram

The signal output by the pressure sensor is directly connected to the SH79F164 microcontroller, where the chip’s built-in resources complete amplification, filtering, and acquisition processing, with very few peripheral devices (Figure 6).

Figure 7 is a block diagram of a traditional electronic blood pressure monitor system compiled by the author. As can be seen from the figure, there are many peripheral devices, which makes production and debugging inconvenient, and the system cost is high.

3. Software Design

3.1. Software Algorithm

As for the oscillometric method, all methods determine systolic and diastolic pressures based on the mean pressure. There are two typical methods: the proportional method and the S discrimination method. Currently, most electronic blood pressure monitors on the market use these two algorithms to calculate blood pressure values. This article uses the S discrimination method to calculate systolic and diastolic pressures.

3.2. Software filtering

There are many commonly used software filtering methods, such as: limiting filter method, median filter method, arithmetic average filter method, recursive average filter method, first-order low-pass filter method, etc. This paper uses the limiting filter method to filter out abnormal jitters on the pressure signal, uses the second-order low-pass filter algorithm to filter out 50Hz power frequency interference on the pulse signal, and uses the median average filter method to calculate the heart rate signal.

3.3. System calibration

Since SH79F164 has built-in PGA and filter circuit, the gain and bias voltage can be set through internal registers. Therefore, sensor offset correction, pressure curve range adjustment, zero point and full scale point initial value determination can be automatically realized by software. The main features of system calibration are:

(1) The program automatically calibrates, no manual adjustment of the potentiometer is required, and the calibration is completed by pressing the button three times.

(2) The total calibration time of the program is less than 10ms (excluding the time for inflation and worker key pressing)

(3) The program automatically calibrates the sensor offset

(4). Maximum calibration zero offset: ±30mV, commonly used sensors are less than ±20mV

(5). Maximum input range: 120mV, 300mmHg pressure corresponds to about 30mV

(6) Automatically confirm the zero point value and full scale value

System calibration initialization is divided into three steps:

Step 1: Zero point calibration (see Figure 8). The purpose of zero point calibration is to ensure that the input signal is within the effective range of the PGA and that the offset values ​​of different sensors can be effectively calibrated.

Step 2: Full-scale calibration (see Figure 9). The purpose of full-scale calibration is to adjust the output slope of the absolute pressure signal to ensure the accuracy of system pressure value detection.

Step 3: Zero point confirmation (see Figure 10). The purpose of this step is to correct the slope of the pressure curve and confirm the previously adjusted parameter settings.

3.4.Procedure Flow

Press key to detect inflation Start ON / OFF Power switch ONOFF Pressure threshold detection YesNo Stop inflation Cuff starts deflation Detect pressure Detect pulse peak Save peak value and cuff pressure value End detection Complete According to the calculation results, display: systolic pressure, diastolic pressure, heart rate Enter automatic calibration program System calibration key YesNo Calibration completed System reset Judgment flag bit Determine whether calibration has been done NoYes Step 1: Zero point calibration Step 2: Full scale calibration Step 3: Zero point confirmation

4. Conclusion

This article presents a design of an electronic blood pressure meter based on the SinoWise SH79F164 single-chip microcomputer. It makes full use of the built-in functional modules of the chip and has the characteristics of simple circuit, low power consumption, high precision and strong practicality. It has a high design reference value and can be used as a reference for electronics enthusiasts or industry designers.