Development of Digital Wireless Telemetry System for ECG

Introduction

The market for wireless telemetry products is developing rapidly. Recently, the industry has also set off a revolution in wireless applications. Wireless telemetry technology has become an important factor in product competitiveness. From a development perspective, wireless and networked medical monitoring products are the development trend. Mobile monitoring products with wireless networking functions will become the mainstream of the future market. In addition, the development of TELEMEDICINE will also make wireless monitoring and wireless interconnection technologies very useful. The prospects for wireless applications are broad, so it is imperative to develop wireless monitoring products.

The system uses TI's single-chip wireless transmitter chip TRF4400 and receiver chip TRF6900. The chip has low power consumption, strong anti-interference ability, and uses the ISM frequency band, and the frequency does not need to be applied for. It can be widely used in wireless data acquisition systems, wireless monitoring systems, charging systems, smart cards, equipment remote control and other occasions, and has a very broad application prospect.

1. System structure and function overview

The whole system mainly consists of a transmitter, a receiving box, a PC terminal display, a remote central station, etc. (see Figure 1). The receiving box outputs TTL asynchronous serial data. The advantage of modularization is that channels can be added or subtracted very easily, and a telemetry receiving module can be easily configured for bedside monitors (Figure 2). The receiving box outputs a 10 BASE-T interface, which directly interfaces with the remote central station product. The capacity of a single receiving box is 8 beds. By using multiple receiving boxes at the same time, a capacity of n*8 can be achieved, and the upper limit of the capacity is mainly limited by the available frequency bandwidth.

First, the patient's information is collected and processed by a transmitter carried on the patient and then transmitted. The receiving box receives the data and sends it to the bedside monitor for display and printing. Finally, it is sent to the central station via a 10BASE network cable to realize remote diagnosis and treatment.

2. Hardware circuit design

2.1. Transmitter The transmitter

includes an ECG parameter circuit, an MSP430 microcontroller for baseband digital signal processing, and a TRF4400 RF transmission module circuit. The wireless RF transceiver chip TRF4400 is the core of the entire system and is directly related to the performance of the entire system. The circuit is shown in Figure 2

ECG parameter circuit

The weak ECG signal of the human body is obtained after the protection circuit, differential mode amplification, filtering and other circuits to obtain ECG signal, lead-off FAIL_RA, FAIL_LL signal, and then through the 12BIT AD conversion of MSP430, the parameter circuit can be controlled by the controller through SHUTDOWN, i.e. the switching power supply signal. The PACE pulse signal is obtained by picking up the PACE wave and captured by the I/O port interrupt function of MSP430.

Microcontroller

The microcontroller used in this system is TI's MSP430. It has the characteristics of low power consumption, small size, and rich hardware resources. The system also expands the key circuit, such as nurse call buttons, low battery voltage, sampling lead wire detachment, etc. LED status indication and alarm.

Power supply

circuit The transmitter is powered by two No. 5 nickel-metal hydride batteries with a capacity of 1300mAh, which can directly power the microcontroller and TRF6900. The power conversion chip is used to convert it to +/-5V to power the ECG parameter circuit. In order to achieve a working time of more than 20 hours, the power consumption of the transmitter is required to be within 120mW. The average power consumption of the transmitting part is estimated to be around 80mW, so the power consumption allocated to other parts is 40mW, which puts high demands on the design. The ECG circuit needs to be optimized for low power consumption. This goal can be achieved by selecting a low-power chip. [page]

2.2.RF wireless link

This part is completed by TI's wireless transceiver module TRF4400, which can realize transparent transmission of baseband digital signals. TRF6900 uses DDS and PLL technology, which directly generates two corresponding sine waves of different frequencies from baseband digital signals 0 and 1. Specifically, when 0 is transmitted, a certain frequency is generated according to the external reference frequency and the programming setting of DDS, while 1 is programmed to generate an offset frequency based on the transmission rate of the modulation coefficient. It has the advantages of wide frequency range, high frequency resolution, and convenient control of output frequency by software. However, due to the limitation of reference frequency, the output frequency of DDS is usually low. In order to meet actual needs, the intermediate frequency signal output by DDS must be multiplied by PLL. The multiplication factor N can be set to 256 or 512. There are two working modes, mode 0 and mode 1. Through the MODE line selection, both can be set to transmit or receive. The working mode or standby mode is selected by STANDBY. The frequency scheme can be flexibly set by programming four special working registers A, B, C, and D. It has a special serial interface for programming, which are the serial clock line CLOCK, data line DATA, and select line STROBE. At each rising edge of CLOCK, one bit of data on the DATA line is written into the 24-bit shift register of TRF6900 in a descending order. The first 2 and 3 bits are the address bits. When STROBE is high, they are written into the corresponding working register, where A and B are the frequency setting registers for mode 0 and mode 1 respectively, and the frequency deviation can be set by D.

Since TRF6900 does not require many other external circuits, we can flexibly set it according to the requirements due to its flexible programming characteristics. The transmission and reception of this system both adopt FSK modulation and demodulation. The system works in the 430MHz UHF band and adopts frequency division multiple access. The transmitter and receiver are one-to-one. The bandwidth occupied by each transmitter is 50KHz, and the transmission rate is 9600kbps. If the power of the transmitter complies with the wireless regulations and the sensitivity of the receiver is large enough, the transmission distance can reach more than ten kilometers in theory. Due to various losses, it can actually reach four or five hundred meters. The circuit of the RF part of TRF4400 is shown in Figure 3 below:

Figure 3 Transmitter RF circuit

In order to realize wireless data communication, a set of transmission protocols must be designed according to the general requirements of wireless transmission and the special requirements of the wireless transmission transceiver module chip used. Due to the low efficiency of asynchronous transmission, one frame can only transmit one byte. In addition, TRF6900 does not support asynchronous data format, so a synchronous frame format is used. First, a bit synchronization code with 1 and 0 intervals of two bytes must be sent. On the one hand, it is used to identify a frame of valid data and invalid data, and on the other hand, it is required for the comparison reference level establishment time required for the final output of TRF6900. Next is a frame synchronization code, which usually starts with a falling edge. Next is the status information and data information that the user wants to upload. Finally, in order to ensure the reliability and efficiency of wireless transmission, there must be an error detection and correction code. The CRC cyclic redundancy code widely used in communication is used to achieve the purpose of forward error correction. The format is shown in Figure 3:

2.3. Receiving box circuit

Each receiver module in the receiving part can switch between the two antennas of spatial diversity through software control to ensure the signal strength, which is equivalent to the transmitter module. First, let TRF6900 complete the demodulation, send the received data to MSP430 for baseband signal processing to restore the ECG signal, and finally send it to the bedside monitor through the serial port according to the agreed communication protocol to display the ECG waveform of each channel. The circuit of the wireless part of the receiver is very similar to the peripheral circuit of the transmitter, so it will not be described in detail here.

Figure 5 Receiver circuit

2.4. Remote part

This part can be transmitted to the remote central station through Ethernet via 10BASE-T line, thus achieving the purpose of remote monitoring or diagnosis and treatment.

Figure 6 Remote part

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3. Software Implementation

The system software design of telemetry includes two parts. One part is the telemetry acquisition, compression, verification, packaging, and sending program running on the transmitter. The characteristics of this part of the software are large amount of calculation, many interrupts and timing, strict time requirements, and relatively complex. The AD interrupt of MSP430 is used to collect biomedical signals such as electrocardiogram. Since the collected data must be sent in the format of the wireless protocol, it undoubtedly increases the code overhead and increases the code rate of the data stream. If the data is not compressed, not only the speed of the data stream cannot be too high due to bandwidth limitations, but also the real-time transmission of the data cannot be achieved. Therefore, the collected data must be compressed to reduce the code rate, and then the CRC cyclic redundancy check code is calculated to complete the framing. Finally, the interrupt is used to send the data bit by bit through the I/O port to TRF4400. As shown in Figure 5 below:

Figure 7 Transmitter system software block diagram Figure 8 Receiving system software block diagram

The other part is the program running on the receiver, which includes receiving, error detection and correction, decompression, unpacking and finally sending the program to the main control board for display. The characteristics of this part of the program are that the communication is very complex, especially the reception, and the amount of calculation is also very large. As for the program and the transmission, they are equal. First, the data is received through the interruption of the I/O port, and then the data is corrected and decompressed. Then, according to the requirements of the bedside monitor, the data is packaged and sent using the serial port interrupt. As shown in Figure 6 above:

The development tool chosen is the IAR Embebbed Workbench integrated software development system that is compatible with TI's MSP430 series microcontrollers. It can realize online programming of the target system through the emulator. In the integrated environment, the program written in C language can be simulated in two ways. One is software simulation. The development system comes with a very complete processor driver library. You can choose to perform single-step, breakpoint and other debugging as needed. The other is hardware simulation. The program is written to the microcontroller through the emulator and runs in real time. Breakpoints can be set for debugging, and the running results can be observed and analyzed through the powerful debugging window function.

4. Conclusion

Through development tests and hospital clinical applications, it has been proven that the system has achieved distortion-free transmission of ECG signals, thus achieving the expected purpose and having a good diagnostic effect, which has been well received by hospitals and doctors. The system has a certain degree of versatility. It is not only used to collect biomedical signals, but also provides a way of thinking and a set of solutions for low-cost remote communication, which can be widely used in other industries.

References

[1] Hu Dake, MSP430 Series FLASH Ultra-Low Power 16-Bit Microcontroller, Beijing University of Aeronautics and Astronautics Press, first edition, November 2001.
[2] Joe Campbell, translated by Xu Guoding et al., Serial Communication C Programmer's Guide, second edition, Tsinghua University Press, first edition, June 1995.
[3] Zhang Weizhen, Biomedical Electronics, Tsinghua University Press, first edition, October 1990.
[4] John Schilling. Designing With the TRF6900 Single-Chip RF Transmitter[Z]. Texas Instrument Application Report, SWRA033A-January 2001
[5] TRF6900 Single-Chip RF Transceiver, Texas Instruments data sheet SLAA1213, September 2000.