Development of Miniature Low Power and large capacity ECG Recorder

Development of Miniature Low Power and large capacity ECG Recorder

[Date: 2008-9-3]

Source: Electronic Technology Application Author: Cao xianxiang

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Conventional ECG recorder is one of the main methods for doctors to diagnose heart disease, but it can only record transient heartbeats. In a considerable number of cases it is difficult to record changes in the ECG at immediate onset, resulting in failure to make a proper diagnosis and treatment. Therefore, the long-term ECG record has an extremely important clinical value. The micro-dynamic ECG recorder can promptly record transient abnormal ECG activities that are not easy to occur in general ECG detection, providing an important objective basis for clinical analysis of the disease [1]. The current portable ECG recorder has a low storage capacity [2] and can only record eight hours of ECG data. However, although the microcomputer-controlled monitoring analyzer has excellent performance [3], it is large in size, it is not suitable for patient family monitoring and patient carrying with transient abnormal ECG. Therefore, a miniature dynamic ECG recorder with low power consumption and large storage capacity is developed. This recorder can fully record ECG information over 200 hours, it is applicable to small and medium-sized hospitals and families.
1 main device of ECG Recorder
Figure 1 is the basic circuit diagram of the micro ECG recorder. All chips in this circuit are 3.3V low-power supply voltage and low-power chips. the power consumption of the whole machine is about 12mA × 3 V.

1.1 low-power high-capacity FLASH memory k9k2g08
Data Storage Using Micro ECG RecorderSamsungFlash data storage device K9K2G08 In the NAND structure. The storage chip is currently one of the Flash chips with a large capacity, with a single chip capacity of 256 MB + 8 Mb. Memory reads and writes by PAGE, and deletes by block. It reads and deletes commands, addresses, and data by using the I/O port. The K9K2G08 storage chip consists of 2048 blocks, each with 64 pages, 2 K + 64 bytes per page, and 2 K + 64 bytes of buffer RAM. As shown in table 1, the addressing of each page needs to send five addresses through the I/O port, from the third to fifth line address (A11 ~ A27) specify addressing to a page. The first and second addresses specify a byte in the specified area of the page.

This memory allows users to program and write buffering any page or word. The random reading time is 25 μs, and the continuous reading time is 50 ns. The chip's block erasure time is 2 ms, the programming time is 300 μs, the erasure/programming cycle is up to 0.1 million times, and the data storage time is up to 10 years. Use 2.7V ~ for chips ~ 3.6V power supply voltage. The current is 10mA during read/write operations, and the current is only 50μa in the standby mode. The power consumption is very small. Its storage capacity is 2mA mb. It is a low-power, high-density, non-loss dynamic data storage medium. It is very suitable for dynamic ECG recorder data storage with an average power consumption less than × 3. 3 V.
It is calculated based on 200 ECG signals sampled per minute. The 24-hour data storage capacity is approximately 34 MB, and the storage capacity of 180 MB can exceed hours, the storage duration can be more than 500 hours after repeated data is removed according to certain rules, which can meet the requirements. At the same time, the software design adopts a circular record. When the m record is completed, a block is erased and new ECG data is written. Therefore, this instrument records the ECG data in the last few days.
It takes about 30 minutes to read m data from a PC using a USB interface. Therefore, the Flash data memory is welded to a separate Printed Board and antistatic encapsulation is added, and the interface between the Flash data memory and the single-chip microcomputer is connected by a pluggable interface. In this way, the doctor can unplug the memory board that records the ECG data and plug it into the USB interface circuit. As shown in 2, the PC reads the data to process, display, and analyze the ECG waveform. In addition, the memory board can be changed in a very short period of time without delaying patient use. The storage board is equipped with access identification, capacity identification and other hardware identification, the microcontroller can identify whether to plug in the storage Board and its capacity. The storage capacity is 64 M, 128 M, and M bytes.

1.2 low-power microcontroller msp430f149
MSP430F149 [5] YesTiA low power supply voltage range (1.8V ~ 3.6 V) Low Power 16-bit microcontroller. The chip contains 60 Kbyte Flash EPROM and 2KByte RAM. There is a basic clock module with complete performance, including a numerical control oscillator (DCO) and two crystal oscillator. It also includes the hardware watchdog, the 16-bit timer Timer_A3 for three capture/compare registers, the 16-bit timer Timer_B7 for seven capture/compare registers, the 8-channel 12-bit A/D converter ADC12, and two serial communication interfaces.
It is particularly worth mentioning that its low-power applications. Use an external low-speed crystal oscillator 32768hz to generate accurate timing signals and set the asynchronous Handshake Communication baud rate, while A/D conversion and operation use the internal Numerical Control Oscillator to generate a faster kHz clock signal, the power consumption is less than 3mA × 3. 3 V. In addition, an 8 MHz high-speed crystal oscillator is added, which usually shuts down and does not work. When the USB interface circuit is connected, the USB interface circuit provides power. At this time, the single chip microcomputer starts the high-speed crystal oscillator according to the handshake signal command, setting an appropriate baud rate for high-speed data transmission according to instructions is undoubtedly a way to reduce power consumption. Therefore, using msp430f149 as the CPU chip is the best solution based on low power consumption.
2 Structure of micro-dynamic ECG Recorder
As shown in figure 1, the design of the recorder is divided into analog ECG signal detection, amplification and filtering, and digital ECG signal storage and processing. That is, the ECG signal is input through the lead, amplified and filtered, and the digital ECG signal is obtained after A/D conversion. The digital ECG signal is sent to the single chip microcomputer system and processed and stored by the software, finally, the waveform data is sent to the microcomputer through UART interface or memory board for analysis and diagnosis. The following sections describe them respectively.
2.1 analog ECG signal detection amplification and filtering
The ECG signal is amplified by a high-common-mode input instrument amplifier consisting of three high-precision low-power op-ops max4240, and filtered out by a low-pass filter, and then further restrained by a 50Hz trap filter, input to msp430f149 for A/D conversion to obtain digital ECG signals.
2.2 digital ECG signal storage and processing
The biggest characteristic of the ECG Recorder designed in this paper is its large storage capacity, which consists of a M Flash storage chip k9k2g08. To record ECG data with high fidelity, doctors generally do not want to compress the data, so they do not use any data compression method to directly record the original 12-bit binary ECG data. K9k2g08 reads, writes, and deletes commands, addresses, and data through the I/O port. Therefore, the pluggable interface is directly connected to the microcontroller msp430f149 for control.
2.3 other hardware and interfaces
The power supply module uses a high-efficiency, low-power boost DC/DC converter max1674 to output a voltage of 3.3v for the system. The static current is only 16 μA, and the efficiency is above 90%. The input voltage is as low as v.
To accurately analyze the information, add the time parameter to the PCF8583 clock chip using the I2C interface, and add the backup power to save the current completed flash page write address in the PCF8583 backup Ram, flash data is stored by date and time sequence each time the system is powered on or off.
The LCD uses an explicit serial SPI interface with an operating current of only 25μa. It mainly displays the number of records, battery capacity, date, and time. If you need to view the ECG in real time, you need to use a large lattice LCD screen, but the power consumption has increased to several times to dozens of times, which cannot reach a long continuous working time. Therefore, real-time ECG viewing is not used.
The average power consumption of two 20mA MH-Ni batteries is less than 100. The battery can be continuously operated for hours after the battery is fully powered. The recorder will not be affected in 15 seconds. There is a sound prompt when the battery energy is only 30 minutes.
3 additional USB Interface Circuit
As shown in figure 2, the USB interface circuit provides two functions. One is to provide a pc usb interface and a high-speed serial communication interface of the recorder, which can read Flash data or initialize the recorder. Second, the USB interface of the PC is provided as the interface for Directly Reading Flash data memory board. Replacing the memory board can further provide better home services for patients with transient ECG abnormalities. USB interface circuit is mainly composed of cygnal usb to UART Single Chip bridge adapter CP2101, MSP430F149 microcontroller and power chip SE117-3.3V. One UART port of the single-chip microcomputer is associated with CP2101, And the other UART port is associated with the recorder. The maximum communication rate is 900 kbps.
4. Software Design
The main tasks of the ECG recorder software are ECG data collection, storage control of flash memory, display of battery voltage and time, alarm when the electrodes fall off or are faulty, and serial communication with the microcomputer.
The software controls A/D sampling at A sampling rate of 5 SPS to detect and calculate the peak value of the collected ECG signal, dynamically adjust the reference voltage of A/D Conversion in pages to obtain the optimal dynamic range of A/D conversion, the 12-bit A/D conversion number is added with four binary numbers encoded into two bytes, and repeated data is removed according to certain rules to increase the storage space, time tag, dynamic adjustment coefficient, and other parameters are added in units of pages. Store all the data in the flash memory and input it to the microcomputer through the serial port or memory board. With a specially designed program, continuous and repetitive data recovery will be removed according to certain rules in the microcomputer, various parameters will be extracted based on the doctor's needs, and graphics and parameters will be displayed, it allows doctors to process, display, analyze and diagnose ECG Waveforms through a microcomputer.
The hardware of the micro ECG recorder described in this article is all designed using low-power devices, and no compression algorithm is used for ECG data. The ECG signal has high fidelity and long recording time. This recorder is an economical and practical small ECG recorder with low power consumption and convenient patient carrying.