Wristband with PIC18F85J90 implementation design (1)

Household electronic sphygmomanometer is a typical family medical testing equipment, electronic sphygmomanometer easy-to-use, can alone operation, measurement values easy to record, small and light portable, more and more ordinary family welcome.

This design to PIC18F85J90 for blood pressure control core supported by barometric sensor BP01 and used as a sensor and MCU between analog signal processing of Intersil ISL28413/ISL28196 analog circuit and LCD driver IC ISD2560 ZJM12864BSBD, voice prompts.

The working principle of electronic sphygmomanometer

System block diagram shown in Figure 1.

System from pressure sensor (BP01-type pressure sensor), constant-current source (ISL28413× 1), differential amplifier (ISL28413× 3), second-order 0.8 Hz high-pass filter (ISL28413× 1) 200 x magnification (ISL28413× 2), 38Hz second-order low-pass filter (ISL28413× 1), blood pressure and pulse triggering (ISL28196× 1), LCD display 128 x 64 dots (ZJM12864BSBD), voice reported blood pressure (ISD2560) and SCM (PIC18F85J90).

The main principle of work is MCU: PWM output control pump air/air leak in air pressure adjustment of cuffs; all ADC sampling cuff pressure DC component within a view to obtaining systolic and diastolic pressure; all ADC sampling cuff pressure alternating component within the analysis calculation analysis determines the systolic pressure and the diastolic pressure of transient time location; receive blood pressure pulse signal triggers the ADC work; to be calculated systolic and diastolic pressure results output to the LCD display and value of voice prompts.

Once the process of measuring the blood pressure is: press the activation key SCM PWM output control and air inflatable-200mmHg high, slowly decreased to about every s 5mmHg speed deflated. Pressure sensor output signal via differential amplifiers after single-ended signal all the way into single-chip ADC monitors the DC component, another second-order all the way into 0.8Hz highpass filter out the DC component; alternating component with 200 times enlarged enter 38Hz second-order low-pass filter for removing power and skin friction with the cuff and the frequency of high-frequency noise interference, and the signal is maintained at between 0 ~ 5V, filtering all the way into the alternating component of blood pressure pulse triggering and then trigger the microcontroller and the other one ADC work into a single chip ADC calculation amplitude, first identify the maximum amplitude values Amax, forward looking amplitude as transient position corresponding 0.5Amax blood DC component is the systolic pressure, back looking for amplitude as transient position corresponding 0.8Amax blood DC component is the diastolic pressure, will calculate the systolic and diastolic pressure results output to display LCD drive.

Hardware design

Master circuit (Figure 2) selects MicroChip 8-bit CMOS FLASH PIC18F85J90 64/80-pin microcontroller with LCD driver, using RISC CPU, 75 single-byte instruction, not only ensuring the necessary directives to control the efficiency of both chip costs and simplify programming complexity.

Peripheral circuits using BP01-type pressure sensor and Intersil op amp ISL28413, BP01-type pressure sensor for measuring blood pressure, specially designed for portable electronic sphygmomanometer, at full range, the accuracy of ± 1%, zero offset is not greater than ± 300mV, measuring range-300 ~ + 300mmHg. BP01 pressure sensor is in constant current power supply of Wien bridge on resistance to changes in output with double end differential voltage signal. ISL28413 is commonly used 4 amplifiers TSSOP-14 IC, ISL28196 are commonly used single comparator 6 foot SOT-23 IC with power supply voltage range, power dissipation is small, low-cost advantages. LCD display circuit adopts ZJM12864BSBD, this is a low-power LCD dot matrix graphical, display format 128 points (column) × 64 points (row) with multifunctional directive, easily attached with MPU. Integrated voice chip ISD2560 is Winbond company produced a strong feature of voice recording chip, is a permanent memory-voice recording circuit, the recording time as 60s, repeatable recording 10 million times.