Realization of three-lead remote ECG monitoring system using single chip microcomputer

1 Introduction

With the improvement of people's living standards and the acceleration of the pace of life, the incidence of cardiovascular diseases has risen rapidly and has become one of the main factors threatening human health. The electrocardiogram is the main basis for the treatment of such diseases. It has the advantages of reliable diagnosis, simple method, and no harm to patients. It has become increasingly important in modern medicine. Conventional electrocardiograms are electrocardiographs that record the patient's electrocardiograph activity while the patient is lying still. It only takes a few seconds to 1 meter and can only obtain a small amount of information about the heart state. Therefore, even if arrhythmia occurs within a limited time, the probability of being discovered is very low. Therefore, it is necessary to monitor the patient for a long time in real time through corresponding monitoring devices and record the patient's electrocardiograph data. Because the occurrence of heart disease is sudden, patients cannot lie still in the hospital for a long time, but they need to be monitored by medical staff in real time, so it is even more important to develop corresponding portable wireless electrocardiograph products.

Although there are mature wireless ECG monitoring products in China, most of them adopt the solution of "collector + transmitter (PDA or mobile phone)", which inevitably leads to high prices. In addition, the other functions of PDA or mobile phone are completely unnecessary for most patients. Therefore, the practical wireless ECG monitoring products in China are still blank. The remote ECG monitoring system described in this article is a general solution designed after a full investigation based on the hospital's proposal. It mainly realizes the following functions: three-lead ECG signal acquisition; wireless transmission of 40 seconds ECG data and diagnosis results in emergency situations; 24-hour continuous ECG recording; uploading ECG data to PC via high-speed USB; emergency call.

2 Overall system design

As a portable handheld remote mobile terminal, its design should fully consider its small size, low power consumption, large storage capacity and high processing speed. Therefore, the CPU selection is very cautious. After data collection and repeated comparison, the s3c2410 processor based on the arm920t core launched by Samsung was finally selected. The processor has rich data and high cost performance. The arm microprocessor using risc architecture generally has the following characteristics: small size, low power consumption, low cost and high performance; support thumb (16-bit)/arm (32-bit) dual instruction sets; a large number of registers are used to make the instruction execution faster; the addressing mode is flexible and simple, and the execution efficiency is high; the instruction length is fixed.

It can be seen that the embedded processor based on ARM is the best choice for portable handheld terminals, so when designing the system solution, the first focus is on this series of processors. The S3C2410 processor is based on the ARM920T processor core, a 32-bit microcontroller with a 0.18 μm manufacturing process, a five-stage pipeline and Harvard structure, and a maximum operating frequency of 203 MHz. The processor has: independent 16 KB instruction cache and 16 KB data cache, MMU, LCD controller supporting TFT, NAND flash controller, 3-way UART, 4-way timer with PWM, rich I/O ports, 8-way 10-bit ADC, touch screen interface, IICbus interface, as well as 2 USB hosts and 1 USB device and other rich peripherals.

S3C2410 provides a relatively complete set of general peripherals and minimizes the power consumption of the entire system, thus eliminating the trouble of adding and configuring additional peripheral interfaces and effectively reducing the area of ​​the circuit board. This is also the important reason why this system chooses this processor. The overall structure of the system is shown in Figure 1. With S3C2410 as the core, it expands storage chips such as 8 MB NOR flash, 64 MB NAND flash and 16 MB SDRAM. It expands human-machine interface units such as keyboard, LCD and buzzer through the GPIO port, provides communication interfaces such as USB and UART, and connects the MC35 module of Siemens to realize wireless transmission and emergency call functions. From the overall functional structure of the system, the system can be divided into five modules: power module, ECG data acquisition module, data wireless transmission module, graphical user interface module, and data storage management module.

Figure 1 System overall structure 2.1 Power module

The system is powered by a single 1700 mAh lithium-ion rechargeable battery, but as the power is released, the voltage is constantly decreasing, ranging from 4.2 to 2.75 V. In this system, a 4.3 V MC35 operating voltage, a 3.3 V I/O voltage, a 1.8 V CPU core voltage, and a 1.8 V CPU duty voltage are required. In order to meet the requirements of the system, the power supply circuit must have both a boost regulator and a low-dropout linear regulator. In order to solve this problem, the system uses a switching boost DCDC regulator, a 3.3 V extremely low-dropout linear regulator, and two 1.8 V low-dropout linear regulators with shutdown pins to form a power supply system. The power supply scheme is shown in Figure 2.

Figure 2 Power module solution 2.2 ECG data acquisition

Since the detection of ECG signals belongs to the detection of ultra-low frequency (0.5-100 Hz) weak (0.1-5 mV) signals under strong noise background, it has the characteristics of weakness, stability, low frequency characteristics and randomness, so the pre-stage should meet the requirements of high input impedance, high common mode rejection ratio (CMRR), low noise, low drift and high safety. The weak ECG signal is subject to various interferences from inside and outside the human body, and its characteristics are submerged in complex signals. In order to make its characteristics stand out, it is necessary to pre-process it. The ECG signal acquisition principle adopted by the system is shown in Figure 3. The pre-stage adopts a differential amplifier circuit with an amplification factor of 22.4 times; the amplification factor of the post-stage amplifier circuit is 37 times, and the total amplification factor is 828.8 times.

Figure 3 ECG signal acquisition principle Since ECG signals are low-frequency signals, a first-order low-pass filter with a cutoff frequency of 100 Hz is designed in the analog circuit to filter out high-frequency interference, and a second-order VCVS band-stop filter is used to filter out 50 Hz power frequency interference. In digital processing, in order to suppress the power frequency interference and baseline drift that have a greater impact on ECG signals, a 2,048-point FFT is used to transform the input ECG data from the time domain to the frequency domain, and then the low frequency below 0.5 Hz and the power frequency of 50 Hz are removed; at the same time, in order to suppress the interference caused by high-frequency noise and 50 Hz frequency doubling, the frequency above 100 Hz is filtered out, and then the ifft is performed to transform this group of data back to the time domain.

2.3 Data Wireless Transmission Module

This system is a remote mobile terminal, involving wireless data transmission. To achieve this function, the MC35 module of Siemens is used, and the TCPIP protocol stack and PPP protocol are transplanted to complete the transmission of ECG data and the reception of diagnostic results. MC35 is the first GSM/GPRS module launched by Siemens that supports GPRS. It is small in size and easy to integrate into portable handheld terminals. It supports voice, data, fax and SMS services. The processor S3C2410 is connected to MC35 through an asynchronous serial communication interface, and the module is controlled and data is transmitted through AT commands.

When sending data, first, the application layer submits the collected ECG data to the TCPIP protocol stack; then, the TCPIP protocol stack encapsulates the ECG data into a complete IP datagram according to the destination address and port, and then submits it to the PPP layer; finally, after the IP datagram is encapsulated by the PPP layer, it is submitted to the MC35 byte by byte through the serial port and sent. When receiving data, the MC35 first submits the received data byte by byte to the PPP layer; after the PPP layer reassembles the scattered bytes into a complete IP datagram frame, it is submitted to the TCPIP layer for detailed processing. The specific process is shown in Figure 4.

During the initialization of the power-on, the MC35 should be started and logged into the Mobile Dream Network gateway to establish a connection with the service provider. Generally, before sending a command, a test command should be sent to detect the current status of the MC35. The format of the command is "ATR". After the parameters such as the access gateway and flow control are set through the AT command, the service code 99 can be used to start calling and establishing a connection with the service provider. The command format is ATDT*99***1#RN. If the connect message is returned within a given time after the execution of the command, it indicates that the connection with the service provider is successfully established; otherwise, it indicates that the dial-up fails and the wireless transmission function cannot be started normally. After the MC35 successfully logs into the Mobile Dream Network gateway, it will automatically switch from the command mode to the data communication mode, and the serial port communication mode will be changed from the original query mode to the interrupt mode. At this time, the system actively sends a frame of PPP request information, and the service provider actively sends an inquiry frame after receiving the request information to negotiate the setting of relevant parameters. After the service parameters and user identity authentication are successful, the service provider allocates an independent IP to the system, and it can be considered that the GPRS is successfully online.