At present, there are mercury sphygmomanometer and electronic sphygmomanometer on the market. Every time a mercury sphygmomanometer needs someone to wear a stethoscope to help with the measurement. The measurement process is complex. The electronic sphygmomanometer makes up for the shortcomings of the mercury sphygmomanometer. The sphygmomanometer scheme introduces a sphygmomanometer chip to collect human blood pressure signals, transmits, stores and uploads these data through low-power Bluetooth, and analyzes the changes of blood pressure in a day through software.

1、 Hardware design

1. Working principle

In the scheme of sphygmomanometer, the sensor converts the user's blood pressure into a voltage signal, which is sent to the MCU through a filter and displayed after being processed by the MCU. These data can be uploaded through Bluetooth, and can be conveniently observed through software. During the diagnosis and treatment process, changes in blood pressure can also be displayed to doctors.

2. Problems encountered in hardware design

In hybrid voltage systems, there are three main issues when logic devices with different power supply voltages interface with each other: firstly, the allowable voltage limit applied to the input and output pins; The second is the current crosstalk between two power sources; The third is to meet the threshold level of input conversion. Devices typically limit the voltage applied to input and output pins, which have diodes or independent components flowing to the power supply. For example, if the input terminal of a 3V device is connected to a 5V signal, the 5V power supply will charge the 3V power supply, and the continuous current will damage the diode and circuit components.

2、 Software Design

1. Serial data format issues

Because it is a serial interface, the serial data format should be consistent. Frame format of Asynchronous communication: 1 start bit, 7 or 8 data bits, check bits (odd/even/optional), 1 address bit (in address mode) and 1 or 2 stop bits. Asynchronous mode is divided into idle multi machine mode and address multi machine mode. In synchronous mode, allows 7-bit or 8-bit data streams to enter and exit the MCU. In asynchronous mode, this character contains an additional bit as the address flag. The character in the data block has a set address bit, indicating that the character is an address.

2. Problems with software filtering programs

Although the filter is used on hardware, the filtering program needs to be written in software. There are 11 common software filtering methods, including finite amplitude filtering, median filtering, arithmetic mean filtering, recursive average filtering, etc. For example, the median filtering method is equivalent to the "median filtering method" and "arithmetic mean filtering method", which is to continuously sample N data, remove one value and one value, and then calculate the arithmetic mean value of N-2 data. Selection of N value: 3-14. Its advantage is that it combines the advantages of two filtering methods, which can eliminate the sampling value deviation caused by pulse interference. The disadvantage is that the measurement speed is slow, which is the same as the arithmetic mean filtering method. Because the sphygmomanometer scheme does not require high measurement speed, this filtering method can be selected.