Design of portable cerebral blood oxygen monitor based on ARM9 and its Ethernet interface

Oxygen is an important substance for human metabolism, and brain tissue consumes the most oxygen. If there is a lack of monitoring means for brain tissue oxygen supply during the clinical rescue and treatment of patients with cardiovascular and cerebrovascular diseases and brain trauma, it may cause loss or damage of brain tissue neurological function. However, there are no reports abroad on the use of dual channels to simultaneously monitor the blood oxygen of bilateral brain tissues and reflect the changes in the blood oxygen content of the affected side of the brain tissue by comparison [1-2].
The brain blood oxygen monitoring device introduced in this article can not only collect blood oxygen parameters at symmetrical positions on the left and right sides of the brain at the same time, providing a new method for clinical evaluation of the changes in the blood oxygen content of the affected side of the brain by comparison, but also uses an embedded system to share data using Ethernet, achieving the effect of both real-time monitoring of data on both sides of the brain and data sharing.
1 Principle of cerebral blood oxygen detection
This device is based on the different absorption characteristics of oxygenated hemoglobin and reduced hemoglobin to infrared and near-infrared light. Using the modified Lambert-Beer law and the different absorption characteristics of hemoglobin in near-infrared light (wavelength 760~850 nm), the blood volume and oxygen capacity expressions can be derived as follows [1]:

Since the process that light undergoes in tissue is related to the wavelength, the changes in the concentrations of oxygenated hemoglobin and reduced hemoglobin for two different physiological states before and after can be expressed as [2]:

In actual measurements, the differences in skin color and tissue properties of different patients make it difficult to measure the absolute concentrations of Hb and HbO2. Therefore, in actual monitoring, the change in optical density is generally used to characterize the changes in blood volume and blood oxygen.
2 Hardware design of portable cerebral blood oxygen detection device
2.1 Overall hardware design
This device consists of a probe part and a host part. The two parts use a wireless transceiver mode for data transmission. The specific system block diagram and workflow are shown in Figure 1.

As shown in Figure 1, the device uses a 760 nm and 850 nm dual-wavelength light source controlled by a clock pulse sequence generated by a Samsung S3C2410A microprocessor. The dual light sources alternately illuminate the cranial tissue, and the transmitted light is collected in real time by a photoelectric converter at a certain distance from the probe, and is divided into two paths for amplification. The amplification process is controlled by an analog switch controlled by a timing pulse. At the same time, the monitoring device integrates the filtering and noise reduction, sum and difference operations, A/D conversion and other circuits of the amplified transmitted light in the probe part. This design can reduce the attenuation and noise interference of the signal transmission to a certain extent, and also saves the volume of the instrument and realizes a portable design. The host part uses a new S3C2410A microprocessor, which can better process information; voice prompts and Ethernet data sharing interfaces are added to quickly and intuitively prompt the patient's health status and data transmission and sharing; the previous power spectrum, correlation function, and blood oxygen saturation display are retained, which can provide doctors with a certain scientific basis when formulating prescriptions. Generally, wired transmission is used between the probe part and the host part, which has many shortcomings such as space occupation, high cost, and limited transmission distance. In order to overcome these shortcomings, this device adopts wireless transmission, and realizes wireless transmission between ARM7 and ARM9 through software, which can be easily moved and monitored in real time within the transmission range, eliminating a large amount of area and storage space occupied by wired transmission, and truly achieves portable monitoring.
Samsung S3C2410A 16/32 bit RISC microprocessor uses ARM920T core. The main features of this processor are low power consumption, low cost, small size and high performance. Its components include: 16 KB instruction cache and 16 KB data cache, NAND flash boot loader, MMU virtual memory manager, 3-channel UART, 4-channel DMA, 4-channel PWM timer, LCD controller (super twisted LCD screen), 8-channel 10 bit ADC and touch screen interface, 2-channel SPI and PLL clock generator.
In this cerebral blood oxygen monitoring device, the S3C2410A microprocessor control circuit generates a pulse signal, so that the 760 nm and 850 nm light-emitting diodes emit light alternately; when the data is input from the probe part to the host part, the S3C2410A can process, store, and calculate the data, and control the LCD display output and the voice circuit to output audio. Controlling the voice circuit to output audio is the core of this device.
2.2 Design of Ethernet interface based on ARM9
In previous cerebral blood oxygen monitoring devices, the Ethernet circuit selected the CS8900A chip, which has 100 pins and occupies a large amount of PCB layout area. The DM9000A has only 48 pins, which can save a lot of PCB layout area. Considering the requirements of portable device size, this paper adopts the Ethernet controller DM9000A chip. The DM9000A Ethernet controller is an integrated, low-cost, low-pin single-chip fast Ethernet controller developed by DAVICOM Semiconductor. Its main components include: 1 10/100 M PHY and 4 KB double-byte SRAM. It has low power consumption and high performance when the input voltage is 3.3 V~5 V. DM9000A supports 8-bit and 16-bit data internal memory access for various interface processors, complies with IEEE 802.3u standard specifications, and its auto-negotiation function automatically configures DM9000A and makes the best use of it; it supports IEEE 802.3x full-duplex flow control, 48-pin LQFP, and integrates 10/100 M adaptive transceiver and automatic MDIX.

The hardware interface circuit of each pin of the DM9000A chip is shown in Figure 2. In order to protect the data and address pins of the chip and protect the pins from damage by instantaneous high voltage, a zero resistor is connected behind each pin (these zero resistors are not marked in Figure 2). In the figure, resistors R7~R10 can be used to remove interference such as signal noise and coupling in the Ethernet input and output circuits; the chip RClamp2504N is used as high-voltage protection for Ethernet to prevent instantaneous overvoltage and pulse current from damaging DM9000A; X1 is a crystal oscillator circuit with an operating frequency of 25.000 MHz, which provides a stable clock signal for the normal operation of the microcontroller, and is grounded after filtering out the DC signal through capacitors C5 and C6, achieving the safety of the design; pins 19, 20, 21, and 46 are not used and are suspended.

FIG3 is a protection circuit for the ground terminals of chip pins 15, 33, 45, and 48. The circuit achieves the effect of filtering out voltage fluctuations and protecting the chip pins from burning out in the event of overcurrent by connecting semiconductor capacitors and polarized capacitors in pairs in parallel.

3 Software Design
The DM9000A driver module program in this design includes:
(1) Setting the Ethernet physical address, which can be modified before initializing DM9000A.
2) Defining the receive frame type, Ethernet data, and address port.
(3) Setting the working mode, 8-bit or 16-bit mode. This design uses the 16-bit mode to set the registers used in the data packet sending and receiving process and the interrupt mode.
(4) Sending a frame request, initializing DM9000A, and the data packet sending and receiving process.
The software flow chart of DM9000A is shown in Figure 4.

The innovation of this article is: Ethernet controllers were used in the past, but this design uses DM9000A, which has excellent performance, low power consumption and low price. The biggest advantage is that DM9000A occupies a large proportion in 10 Mb/s embedded network applications. With the same transmission speed, the chip occupies a small area of ​​PCB board, and it can be used for 8/16 bit, which is convenient for software designers to design and use.
Due to the widespread use of ARM-based embedded medical devices and the need for data sharing, the hardware and software design of ARM-based embedded Ethernet interface is imminent. Therefore, it is of great significance to carry out the design of embedded Ethernet interface. At the same time, this interface also has a good application prospect.
References
[1] Wang Qiang, Lin Shujuan, Luo Zhicheng. Non-invasive infrared spectroscopy brain blood oxygen monitor [J]. Foreign Medical Biomedical Engineering, 1998(21): 19-26.
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[3] Li Liangchen, Li Kaiyang, Qin Zhao. Development of a new near-infrared brain blood oxygen monitoring device [J]. Laser and Infrared, 2006, 36(8): 661-664.
[4] Wang Guiyun, Hou Sizu. Design and implementation of Ethernet interface based on ARM7 [J]. Microcomputer Information, 2009, 25(2): 124-125, 204.
[5] Bu Chenyuan. Implementation of Ethernet communication of ARM7 embedded processor [J]. China Science and Technology Information, 2009, 23: 87-88.