Detailed design of portable EEG wireless signal acquisition system

0 Preface

Electroencephalogram EEG (Electroencephalogram) is a weak low frequency physiological signal. It is a spontaneous potential activity produced by brain nerve activity, which contains a very rich information of brain activity. By recording EEG signals, it can provide data analysis and basis for the diagnosis of brain diseases. Epilepsy is caused by abnormal discharge of the brain. It is a chronic disease and a syndrome characterized by sudden appearance of transient brain neurons and transient central nervous system dysfunction [1]. The epileptiform discharge found by electroencephalography is still the main objective basis for the diagnosis of epilepsy and the location of epileptic foci.

Due to the limitations of conditions, sample collection of human epilepsy EEG data is difficult, and the data is susceptible to interference from the external environment and patient movement. At present, most of the ideas of portable EEG collecting instruments are that the collected EEG signals are pre-amplified, transmitted to the PC through the wireless module, processed and stored in the PC, and the PC is mostly used in the desktop. Run under Windows system. This system has good data processing ability, stable performance and long-term recording. However, the whole system generally needs external power supply, which is relatively large in size. It is generally used in professional institutions and hospitals. It is not really portable. Compared with some poor mountainous areas with insufficient medical resources, these portable medical devices are difficult to move. The basic medical needs cannot be met.

Therefore, how to make the EEG collecting instrument more flexible and convenient in use has begun to receive widespread attention. Designing a small, low-power, truly portable EEG signal acquisition instrument has important practical significance and application value.

1 Portable EEG wireless acquisition system overall structure

This paper proposes an EEG signal wireless acquisition system based on STC12C5A60S2 microcontroller, wireless chip nRF24L01, and true color LCD TFT6448BS-5.7. The STC12C5A60S2 MCU at the transmitting end is responsible for data acquisition and pre-processing, and the nRF24L01 module is responsible for data transmission and transmission. The receiving terminal STC12C5A60S2 MCU will receive the data through the LCD display TFT6448BS-5.7 for waveform display. The specific system composition is shown in Figure 1. According to the application characteristics of the portable EEG acquisition system, it is required to reduce its volume and weight as much as possible to achieve true portability.

Figure 1 System function block diagram

1.1 Front-end acquisition of EEG signals

Since the main frequency range of the human brain electrical signal is 0.05~100Hz, the amplitude is about 10~200μV, and the signal is very weak. Simultaneous EEG signal

Usually mixed with other bioelectrical signals, plus 50Hz power frequency interference, the measurement conditions of EEG signals are very complicated. The traditional acquisition front end usually improves the signal-to-noise ratio of the signal by simulating anti-aliasing filters, multi-stage amplifying circuits and wave circuits, which leads to disadvantages such as large system size, inconvenient operation and high power consumption. In order to accurately monitor the clinically significant EEG signals, this paper uses the method in the reference, and the front-end acquisition module uses TI's ADC1299 chip.

1.2 MCU control module

The single-chip control module of the system includes a single-chip microcomputer at the transmitting end and a single-chip microcomputer at the receiving end. The transmitting end MCU must have an on-chip integrated A/D converter, and the receiving end MCU must be connected to an external LCD. Therefore, the powerful STC12C5A60S2 microcontroller is selected. The single-chip microcomputer is a new generation of 8051 single-chip microcomputer of Hongjing Technology. The STC12C5A60S2 series single-chip microcomputer adopting Hongjing's latest sixth-generation encryption technology cannot be decrypted. It has strong anti-interference ability and integrates 8-channel 10-bit A/D converter. The A/D conversion function is used in the system, so that the system does not need to add an A/D conversion chip. At the same time, the single chip has high speed and high precision.

The ADC of STC12C5A60S2 is a successive comparison ADC. After power-on, the EEG signal is collected by the 8-lead electrode and then controlled by the electronic switch to enter the MCU of the transmitting end, ensuring that only one signal enters at a time. The ADC input channel is multiplexed with the P1 port. After the power-on reset, the P1 port is a weak pull-up type I/O port. The port that is not used as an ADC can continue to be used as an I/O port. The MCU converts the analog signal into a digital signal through the ADC, and simultaneously controls the wireless module to transmit the digital signal of the EEG to the wireless module of the receiving end, and enters the MCU of the receiving end to realize the real-time display and storage of the signal.

1.3 Wireless Module

The system adopts 2.4GHz wireless single-chip transceiver chip nRF24L01, adopts FSK modulation, and can realize point-to-point or one-to-six wireless communication. Wireless communication speed can reach 2Mb/s. It is small in size, low in power consumption, low in peripherals, and high in speed, making it ideal for wireless transmission applications. The nRF24L01 can be connected to the microprocessor via the SPI interface, through which the setup and transceiving of data can be done. The STC12C5A60S2 MCU integrates the SPI controller, which can be easily set by software. It only outputs data when the local address is received. Programming is very convenient. The connection diagram of nRF24L01 and single chip microcomputer is shown as in Fig. 2.

Figure 2 Connection diagram of STC12C5A60S2 and nRF24L01

1.4 Display section

The display part uses a true color LCD TFT6448BS-5.7 with a viewing angle of 5.7 inches and a resolution of 640 & TImes; 480. The display voltage is 3.3/5V and supports 256 colors. Designed specifically for microcontroller users, it provides a simple high-speed 8-bit bus to interface with the microcontroller. This display has low power consumption and is designed to be lightweight. The program design of the system includes a single-chip program and a liquid crystal display driver. The transmitting end performs A/D conversion and wireless transmission through the single-chip microcomputer, and the receiving end receives data by the single-chip microcomputer through nRF24L01, and sends it to the liquid crystal display for display.

After receiving the EEG data, the receiving end MCU transmits the data to the LCD display for display. Each point in the display maps a byte in the memory. The X and Y coordinates on the display are in one-to-one correspondence with the address of the display memory. Therefore, you only need to input the X and Y coordinates to directly read and write the corresponding point data, without calculating the address of the pixel in the display memory. After the data is written, the X coordinate is automatically incremented by 1. When the line is written, the line is automatically changed, and the Y coordinate is automatically added. The connection between the MCU and the LCD screen is shown in Figure 3.

Figure 3 MCU and TFT LCD connection diagram

2 system software design

The system consists of the STC12C5A60S2 microcontroller and the nRF24L01 wireless transceiver chip consisting of the transmitting end and the receiving end. The transmitting end performs A/D conversion and wireless transmission through the single-chip microcomputer, and the receiving end receives data through nRF24L01, and then sends it to the STC12C5A60S2 single-chip microcomputer for display and analysis. All configuration work of the wireless module nRF24L01 is done through SPI, with a total of 30B configuration words. The Enhanced Shock BurstTM transceiver mode is generally used. In this mode of operation, the system is programmed to be simpler and more stable. The Enhanced Shock BurstTM configuration word enables the nRF24L01 to handle the RF protocol. After the configuration is complete, the nRF24L01 can be switched between receiving mode and transmitting mode by changing the contents of the lowest byte. The data flow is shown in Figure 4.

Figure 4 microcontroller program flow chart

3 Conclusion

This paper designs a small-capacity, light-weight, low-power EEG signal acquisition and wireless transmission system based on single-chip microcomputer. The STC12C5A60S2 single-chip microcomputer is selected as the main controller, and its own two SPI modules are used to control nRF24L01 and TFT6448BS-5.7 respectively to realize WiFi wireless transmission and waveform display of EEG signals. This system does not need to use PC, control and display are completed by single-chip microcomputer. Due to the characteristics of easy carrying and high integration, it can provide a feasible solution for the diagnosis of EEG diseases in poor mountainous areas with insufficient medical resources.

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