Design of electronic blood pressure meter based on PIC18F85J90

This blood pressure meter design uses PIC18F85J90 as the control core, supplemented by the pressure sensor BP01 and the Intersil ISL28413/ISL28196 analog circuit used for analog signal processing between the sensor and the MCU, as well as the LCD driver chip ZJM12864BSBD and the voice prompt ISD2560. The home electronic blood pressure meter is one of the typical home medical testing equipment. The electronic blood pressure meter is easy to use and can be operated by one person alone. The measured value is easy to record. It is light and easy to carry, and it is becoming more and more popular among ordinary families.

Working principle of electronic blood pressure monitor

The system block diagram is shown in Figure 1. The system consists of a pressure sensor (BP01 pressure sensor), a constant current source (ISL28413×1), a differential amplifier (ISL28413×3), a 0.8 Hz second-order high-pass filter (ISL28413×1), a 200-fold amplifier (ISL28413×2), a 38 Hz second-order low-pass filter (ISL28413×1), a blood pressure pulse trigger (ISL28196×1), a 128-dot × 64-dot LCD (ZJM12864BSBD), a voice blood pressure alarm (ISD2560), and a single-chip microcomputer (PIC18F85J90).

The main working principle of the single-chip microcomputer is: PWM output controls the air pump to inflate/leak to adjust the air pressure in the cuff; one ADC samples the DC component of the air pressure in the cuff to obtain the systolic and diastolic pressures; one ADC 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 to trigger the ADC to work; outputs the calculated systolic and diastolic pressure results to the LCD display and gives a voice prompt of the numerical value. The process of measuring blood pressure once is: press the activation key, the single-chip microcomputer PWM output controls the air pump to inflate to 200mmHg, and slowly deflates at a rate of about 5mmHg per second. The output signal of the pressure sensor is converted into a single-ended signal after a differential amplifier and sent to the microcontroller ADC to monitor the DC component, and the other way is sent to a 0.8Hz second-order high-pass filter to filter out the DC component; the AC component is amplified 200 times and input into a 38Hz second-order low-pass filter to remove the high-frequency noise and power frequency interference caused by the power supply and the friction between the skin and the cuff, and the signal is maintained between 0~5V. The filtered AC component is sent to the blood pressure pulse trigger and then triggers the microcontroller ADC to work, and the other way is sent to the microcontroller ADC to calculate the amplitude. First find the maximum amplitude value Amax, and then find the transient position with an amplitude of 0.5Amax in the front, which corresponds to the blood pressure DC component, which is the systolic pressure, and find the transient position with an amplitude of 0.8Amax in the back, which corresponds to the blood pressure DC component, which is the diastolic pressure. The calculated systolic and diastolic pressure results are output to the LCD driver for display.

Hardware Design

The main control circuit (Figure 2) uses MicroChip 8-bit PIC18F85J90 CMOS FLASH 64/80-pin single-chip microcomputer with LCD driver, adopts RISC CPU, and has 75 single-byte instructions, which not only ensures the necessary instruction efficiency but also has both control chip cost and simplified programming complexity. The peripheral circuit uses BP01 pressure sensor and Intersil op amp ISL28413. BP01 pressure sensor is specially designed for blood pressure detection and is mainly used for portable electronic blood pressure monitors. Within the full range, the accuracy is ±1%, the zero offset is no more than ±300mV, and the measurement range is -300~+300mmHg. The BP01 pressure sensor outputs a double-ended differential voltage signal when the resistance on the Wien bridge powered by a constant current source changes with the air pressure. ISL28413 is a commonly used 4-op-amp TSSOP-14 integrated circuit, and ISL28196 is a commonly used single-channel comparator 6-pin SOT-23 integrated circuit, which has the advantages of wide power supply voltage range, low static power consumption, and low price. The LCD display circuit uses ZJM12864BSBD, which is a low-power dot matrix LCD with a display format of 128 dots (columns) × 64 dots (rows). It has multi-function instructions and is easy to connect to the MPU. The integrated voice chip ISD2560 is a powerful voice recording and playback chip produced by Winbond. It is a permanent memory voice recording and playback circuit with a recording time of 60s and can be repeated 100,000 times.

The peripheral analog circuit is shown in Figure 3. The positive input terminal of the op amp U1A is a settable DC bias voltage Vref. The output terminal and the negative input terminal of U1A provide a loop for the constant current bias of the pressure sensor bridge to provide a constant current source for sensor bias. After differential amplification by op amps U1B, U1C, and U1D, the single-ended output amplifier circuit is directly input into ADC0 to monitor the DC component of blood pressure. U2A is used to form a second-order high-pass Butterworth filter. For the specific design, please refer to the high-pass filter design method of reference [1]. Two-stage op amps U2B and U2C are used to form a 200-fold proportional amplifier circuit, with the first stage amplifying 20 times and the second stage amplifying 10 times. A second-order Butterworth second-order low-pass filter is used, and op amp U2D is connected as a follower with an amplification factor of 1. For the specific design, please refer to reference [1] to filter out the high-frequency noise and power frequency interference caused by the friction between the skin and the cuff, and obtain a voltage signal matched by the microcontroller, which enters ADC2 to monitor the AC component of blood pressure. The comparator ISL28196 is used to set a certain voltage threshold, and the blood pressure AC signal is formed into a pulse, which enters RA1 and triggers ADC1 to work.