Design of ECG Simulation System Based on ARM9

With the development of society, people's awareness of medical care is getting stronger and stronger, so the training of doctors has become a very important link. As a major aspect of doctor training, ECG defibrillation technology can often save people from danger in an emergency if the operation is standardized and the movements are skillful. During training, if the scene of emergency defibrillation can be realistically simulated, it will have a good learning effect. Therefore, in emergency and invasive clinical operation training, medical simulation teaching is increasingly showing its advantages of low cost, high repeatability, high teaching efficiency and compliance with medical ethics requirements.

The defibrillation simulation system can be selected from 34 states (including adults and children) and can also be connected to a medical monitor to make the defibrillation simulation more realistic. Students can practice defibrillation with different energies, which also makes it easier for teachers to test students' learning effects.

The system is developed based on the relevant principles of electrocardiogram and the signal synthesis principle of the monitor, and strictly follows the relevant medical regulations. The waveform generated achieves the purpose of medical teaching. A more realistic effect is achieved at the key points of the relevant pathological electrocardiogram. When the system receives the high-voltage defibrillation signal, the corresponding waveform transformation is performed according to the system's preset settings. The system can be used as a training tool for medical training institutions to enable trainees to quickly master the method of ECG defibrillation. The system is used in conjunction with emergency simulators and monitors, and has broad market prospects.

The ECG defibrillation simulation generation system introduced in this paper is based on ARM9 as the control core, making full use of ARM9's rich I/O resources and powerful processing functions. It adopts an embedded development solution and comprehensively considers the system's versatility and usability. The system outputs a signal with an amplitude of 0 to 5 mV, which can continuously output ventricular and supraventricular premature beats, etc. It can also generate a square wave with a period of 1 s, a pulse width of 100 ms, and an amplitude of 1 mV. It is convenient for calibrating the monitor, and the signals are all output using three-lead synchronous signals.

1 System structure and design

The system mainly includes ARM9 central processing unit, high voltage defibrillation signal acquisition module, D/A conversion module, monitor signal matching module, ECG waveform simulation and data extraction, application design, etc. This system uses ARM9 embedded development platform. The following is the main structure and characteristics of ARM9 processor.

(1) 32-bit fixed-point RISC processor, improved ARM/Thumb code interleaving, enhanced multiplier design, and support for real-time debugging;

(2) On-chip instruction and data SRAM, and the memory capacity of instruction and data is adjustable;

(3) On-chip instruction and data cache capacity ranges from 4 KB to 1 MB:

(4) Setting up the Protoction Unit is very suitable for segmenting and protecting memory in embedded applications;

(5) Using AMBA AHB bus interface to provide a unified address and data bus for peripherals;

(6) Support external coprocessors, and the instruction and data buses have simple handshake signaling support;

(7) Support standard basic logic unit scan test method;

(8) Support BIST (Built-in-self-test);

(9) Support embedded trace macro units and real-time trace of instructions and data.

The overall design of the ECG defibrillation simulation system is shown in Figure 1.

2 System Hardware Design

This part is mainly divided into ARM9 hardware platform, D/A conversion, filtering circuit, and high-voltage defibrillation signal acquisition. The system hardware connection diagram is shown in Figure 2. Under the control of ARM9, the system converts waveform data into analog quantity for output by D/A conversion. When receiving the high-voltage acquisition signal, the processor will convert and output another ECG waveform.

2.1 D/A conversion and resistor attenuation network

This part is the core of the system. In order to ensure the stability of the system and the requirements of ECG signals, the D/A conversion chip uses an 8-bit parallel DAC0832 chip, which is powered by a single 12 V power supply. Each DAC has its own independent reference input to transform the data provided by ARM9. The output part uses a 4th-order Butterworth filter. The output waveform is attenuated to obtain the required ECG signal. The peak value of the output waveform can reach 10 V after active filtering, and the voltage output range of 0 to 5 mV is obtained through a resistor voltage divider network. Considering that three D/A channels are used, if each channel occupies 8 I/O ports, plus several control ports, the number of I/O ports provided by the processor is far from meeting the requirements, so it is planned to use a shared data port and an external I/O port chip select method to achieve it, which can save 16 I/O ports and meet the requirements of signal output synchronization.

2.2 High voltage signal acquisition circuit design

This part collects the high-voltage discharge signal from the defibrillator. Since the discharge current of the high-voltage defibrillation signal is bidirectional and the discharge time is only 4 ms, the transient voltage can reach 3000 V, so full consideration should be given to safety performance. The circuit diagram of this part is shown in Figure 3.

The circuit uses high-power resistors and transient suppression diodes to pre-process the high-voltage discharge signal, reduce the high-voltage signal to a relatively small range, convert the current into a unidirectional flow through the rectifier circuit, and then input it to the I/O port of ARM9 through optocoupler isolation to protect the processor.

3 Software Design

The system's hardware lays a solid foundation for the realization of basic functions and extended functions. The design of the software system is to make full use of the resources of the hardware platform to achieve orderly operation of software operations.

Software development involves the following two aspects: modification and improvement of interface driver program; development of application layer software. All application layer programs are developed in C++.

FIG4 is a software module structure diagram of the entire system.

Figure 4: Software module structure of the entire system

3.1 D/A driver and high voltage signal acquisition driver

The device driver is the interface between the operating system kernel and the machine hardware. The device driver shields the application from the details of the hardware, so that from the application's point of view, the hardware device is just a device file, and the application can operate the hardware device like an ordinary file. In the past, when developing applications, there was a main function as the entry point of the program, but there was no main function when developing drivers. The module is loaded when the insmod command is called. The entry point at this time is the init module function, and the device registration is usually completed in this function. Similarly, the module is unloaded when the rmmod function is called. The entry point at this time is the cleanup module function, and the device is unloaded in this function. After the device is registered and loaded, the user's application can perform certain operations on the device, such as read, write, etc., and the driver is used to implement these operations. When the user application calls the corresponding entry function, it performs related operations. The init roodule entry point function does not need to complete other functions such as read, write, etc.

The main functions of the driver are as follows:

3.2 System application design and implementation

The application of this system is designed based on Qt/Embedded. Currently, there are Microwindows, MiniGUI, and Qt/Embedded embedded GUI systems. Qt/Embedded continues the powerful functions of Qt and can run on embedded Linux operating systems deployed on a variety of different processors. Qt/Embedded provides a programming mechanism for signals and slots. The Qt used in this part is a C++ class library for creating GUI programs. The main work of writing Qt applications is to write user classes based on the existing Qt classes. This part is mainly divided into the implementation of the waveform interface and the implementation of user key control. The waveform display uses Qt's function class library Qpainter. Since the waveform interface displays two ECG waveforms, a delay effect will occur, so a multi-threaded mechanism coordination is introduced. Qt supports multi-threading, has a platform-independent thread class, thread-safe time transfer, and a global Qt library mutex that allows different threads to call Qt methods.

4 Conclusion

This system design uses Samsung 2440 embedded processor as the core to build a hardware platform, and uses embedded Linux operating system and combines the peripheral D/A conversion part, the matching network with the monitor, the high-voltage signal acquisition part, the application control part, etc. to realize the design of the ECG defibrillation simulation generation system. The system can simulate the medical defibrillation process well, and can be connected to the medical monitor to output 34 common abnormal heart rate waveforms that meet medical standards. Since the system uses an embedded real-time multi-tasking operating system, the design has high real-time, stability and reliability.