A Novel Portable One Lead ECG Monitor with Low-Cost and
Long-Time Recording Based on NUC501
Lihuang She1 , Jinshuan Zhao1 , Shi Zhang1 , Guohua Wang1 , Gang Wang1
1. School of Information Science and Engineering, Northeastern University, Shenyang, 110004
E-mail: shelihuang@ise.neu.edu.cn
Abstract: In this paper, a portable low-cost long- time one lead Electrocardiogram (ECG) Monitor is present, which
is powered by one dry battery and based on NUC501. Power management module, analogous amplifier and Advanced
RISC Machines (ARM) are used in hardware design. While hardware driver, digital filter, modified baseline drift method
and difference threshold method are taken in software structure. The monitor gathers ECG signal through cable from
user’s body. A prototype instrument has been finally produced and will be put into the clinical environment. The notch
filter and median filter are used to process the data. His/her ECG tracing is displayed lively on Liquid Crystal Display
(LCD) screen. Also the heart rate is shown precisely on LCD screen and displayed with led flashing. The monitor also
can storage and transport data to computer through Universal Serial Bus (USB) cable. While being powered by one dry
battery, it works twenty-four hours. If there is any abnormally changes of heart rate, this instrument will be given alarm
through the buzzer. The testing results show that this monitor is low power consumption, light-weight, accurate and
low-cost. It can care for cardiac patients very good and provide a reliable cost-effective choice for cardio-path.
Key Words: Long-time, ECG, NUC501, Digital filter, Difference Threshold Method
1
INTRODUCTION
unusual electrocardiogram and help doctor to diagnose the
condition accurately. Along with the improvement of living standard, people pay more and more attention to their
health and the quality of life. Thus, it can be seen that developing one kind of a large storage capacity low-power
micro-Holter recorder has great significance.
In recent years, many portable ECG monitors, many of
which are based on low-power digital signal processor [1],
have been produced and improved in terms of weight, performance and power consumption [2]. However, most of
the available ECG monitors, on the market, exists certainly
insufficient in the performance: large size, high power consumption, non-patient-friendly portable and high price. So
these have a big limitation in the application. In this paper,
we present a portable ECG monitor for the actual needs.
This system takes processor NUC501 as the core processor
and gathers the heart electrical signal through single lead
line. It storages ECG signals in SD card, displays patient’s
ECG waveform and heart rate on LCD screen, and transport ECG data to computer through the USB interface. This
ECG monitor being powered by single alkaline battery can
work 24 hours. It will timely, accurately, completely collect and store ECG signals. It monitors heart activity features, which would provide effective support to diagnosis
and treatment of heart disease.
Since the 21st century, people are facing the unprecedented
life pressure and all kinds of high incidence of heart disease. The heart disease has been called one of main diseases that harm human health. Among the deaths around
the world, approximately one third dies of the cardiovascular disease. As having not been able to discover the pathological promptly change, many patients have delayed treatment. The number of people, who die of the heart disease,
accounts for 44% in the total death in our country. The prevention of heart disease has become an important question,
which the medical researchers have to face now.
By researching the ECG features and regular of the disease,
we can make the early prediction to the partial correlation
pathological change and prompt diagnosis. The medicine
practice has indicated that: if discovering the arrhythmia
omen earlier and taking the first-aid measures promptly,
most of patients may avoid dying. Therefore, long-time
recording electrocardiogram of patient has extremely important clinical value. Moreover, many heart disease patients, who are checked normal in the hospital, feel bad
while it is difficult to record electrocardiogram. Although
the existing dynamic electrocardiogram can be used for 24
hours or 48 hours recording, yet the Holter is expensive and
needs the skilled technical personnel to operate. By paying
two or three time expense of using Holter, you may purchase a household ECG machine, which is used to catch the
2
This monitor works twenty-four, while being powered by
single dry battery and under various conditions. Meanwhile the voltage of ECG signals is very weakly. So,
when designing the hardware structure, the circuit should
be low-power consumption, anti-jamming, and high preci-
This work was supported by the Open Project Program of the National Laboratory of Pattern Recognition (NLPR). This work was supported by the Informatization and Information Industry Development
Fund of Shenyang (Z200901026), Science and Technology Plan Projects
of Shenyang (1091033-4-00) and Nuvoton Technology Corp..
c
978-1-4244-5182-1/10/$26.00 2010
IEEE
HARDWARE STRUCTURE
276
Figure 2: Analog Circuit
Figure 1: Hardware Structure
sion. The hardware structure is shown in the figure 1. The
dedicated analog devices are chosen to acquire ECG signals. While a microcontroller based on ARM architecture
was used to sample, record, display and analysis the ECG
signals. Outside interfaces contain LCD, USB, Secure Digital Memory (SD)Card and so on.
2.1
Power Management Module
The voltage of the system is 3.3v, but it is powered by
single alkaline battery, the voltage of which is only 1.5v.
Therefore, we need a step-up DC-DC converter to boost
voltage. The power management device of this system is
L6920 [3], which is a high efficiency and low voltage stepup DC/DC converter. The start up is 1V, while the device
is operating down to 0.6V. The circuit is simple, which requires only three external components to realize the conversion from the battery voltage to the selected output voltage.
A 120mW P-channel MetalOxide Semicoductor Field Effect Transistor (MOSFET) is used to internal synchronous
rectifier and a variable frequency control is implemented to
improve the efficiency. When the voltage of input is 1.2V
and the output is 3.3V, the current is 100mA and the efficiency is 80%. So it is very suitable for our system.
2.2
Front-End Analog Circuit
There are two ways to acquire the ECG signals: one is
using Application-Specific Integrated Circuit (ASIC), the
other is using analog amplifier Integrated Circuit (IC). In
this system, we use the analogous amplifier IC. As the
ECG signal is very weak and originates from the human
body skin surface, the Interference signal interference on
the ECG signal is very large. Therefore, when we choose
the analogous device, the accuracy of amplifier is very important. So, the first step of amplifier circuit is AD627
[4], which is a product of Analog Devices Inc. It has high
Common Mode Rejection Ratio (CMRR) for eliminating
the common mode noise. It also rejects line noise and line
harmonics. The AD627 is ideal for battery-powered applications, as its single-supply operation, low power consumption, and rail-to-rail output swing.
The second step of amplifier circuit is AD8609 [4], which
is also a product of Analog Devices Inc. It is quad micropower rail-to-rail input and output amplifier. It uses a
patented trimming technique that achieves superior preci-
sion. The AD8609 combines the features of low noise, low
offsets, very low input bias currents, and low power consumption. So it is especially useful in portable and looppowered instrumentation.
The amplification of the first amplifier step is set to 6, while
the second is 200. So the total amplification of the circuit is
about 1000. The voltage of ECG signal is right in the range
for Analog to Digital Converter (ADC). The final output of
amplifier is connected the ADC of the microcontroller. The
front-end analog circuit is shown in figure 2.
2.3
Microcontroller
Many kinds of microcontrollers are used in ECG monitors, from single chip to ARM [5], as well as Digital
Singnal Processor (DSP) and Field Programmable Gate Array (FPGA) [6]. In considering low power consumption
and less cost, we choose NUC501 [7], which is a product of Nuvoton Technology Corp. and an ARM7TDMIbased Micro Controller Unit (MCU). Unlike usual ARMbased MCU products, the NUC501 operates without the
use of Synchronous Dynamic Random Access Memory
(SDRAM), which is usually the source of complexity,
higher power consumption, and cost. It is designed to minimize the power consumption. The price of NUC501 is
equals with single chip. Meanwhile, the NUC501 is able
to operate under different power-saving modes: idle, power
down mode, and power down with Real-Time Clock (RTC)
active. The NUC501, which runs up to 108MHZ, has the
32-bits Reduced Instruction Set Computing (RISC) Central
Processing Unit (CPU) with 32KB high-speed Static Random Access Memory (SRAM), boot Read-Only Memory
(ROM), Low Dropout Regulator (LDO), Analog-to-Digital
Converter (ADC), DAC, InterIntegrated Circuit (I2C), Serial Peripheral Interface (SPI), USB 2.0 Device.
With this microcontroller, the simple rate of analogy signal
is set to 1000/sec. It can storage ECG data in SD via SPI,
display the ECG tracing on LCD screen and copy data to
PC via USB cable. It is no doubt that the NUC501 makes
the hardware low power consumption, powerful and low
cost.
3
Software Structure
Software is the soul of ECG monitor system. The structure
of software can be seen in Figure 3. There are two parts of
the software structure: device driver layer and application
layer. The device driver layer is used to driver hardware
device. The application layer is presented to users. There
are many parts in software structure. Space lacks for a detailed description of all parts. We present two important
parts: front data processing and fast algorithm for R wave
2010 Chinese Control and Decision Conference
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Figure 3: Software Structure
detection.
3.1
Front Data Processing
In the design of front-end analogous circuit, there are radio frequency interference suppression and low-pass filter
which is used to filter the signal above 100HZ. But there
are other kinds of noises, which aren’t well removed in
the circuit, such as 50HZ power-line interference, baseline drift. These noises are seriously interference normal
signals. So we should take some methods to dispose of
the noises in software. Considering the ability of microcontroller, a second-step Infinite Impulse Response (IIR)
notch digital filter is chosen to remove the power line interference noise (50 Hz AC interference at China). The center
trap frequency is set to 50HZ. Due to the processor cannot
handle floating-point data very good. So the floating-point
data are changed into the fixed point data to accelerate its
processing speed.
Sorts of methods are used to remove baseline drift: curvefitting, zero-phase IIR filtering, wave-let transform, median
filtering and so on. Also considering the ability of the microcontroller, a modified median filter is used to remove
baseline drift. The median filtering is the fast and easy way
to dispose of the noise. But it is very difficult to determine the parameters of the filter. The improper parameter
will increase the complexity of the algorithm and the quality of signal. In this design, we modify the median filter
by making the parameter easily to change and removing
baseline drift effectively. By using the filter, the ECG data
processing program can provide clean and accurate data for
R wave detecting. And display original signals clearly on
LCD screen.
3.2
Fast Algorithm for R Wave Detection
Many kinds of R wave detecting methods are used in ECG
monitor. But most algorithms are very complex and timeconsuming. By considering the characteristic of embedded system, a simple and practical difference threshold
method is chosen in this paper. The differential threshold
value method bases on detecting ECG signal amplitude and
slope. Normally, the amplitude of R-wave is the largest in
ECG signals, and the rate of slope of QRS wave is also the
largest. While the other waveform amplitude and the rate
of slope is very low. So, if taking the appropriate differential calculation of ECG signal, we can easily pick out the
QRS wave. At the same time, the R-R period and heart
rate can be derived by the time interval between adjacent
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2010 Chinese Control and Decision Conference
Figure 4: Difference Threshold Method
R-wave peaks. The process is shown in Figure 4. First,
the positions of possible R wave are found out by differentiation technique; second, weather it is R wave or noise
is distinguished by threshold method; then, the peak of R
wave is extracted; finally, the heart rate is calculated.
To meet the request of real-time and the feasibility of analysis, this monitor takes average heart rate of 5 seconds as
real-time to display. The heart rate is shown by led flashing. As the peak of R wave is detected exactly, while the
abnormality of heart rate is found in the process of realtime ECG analysis, the monitor would send an alarm signal
by an alarm buzzer. It will remind patient paying attention
to their health and going to hospital timely.
4
Experiment and Results
A prototype instrument has been designed in this paper, as
shown in Figure 5. By testing the ECG of different people,
this instrument can test out the R wave accurately. And
calculative accuracy rate of heart rate is 95%. The R wave
also is shown by led flashing. Being powered by one 1.5V
battery, it works twenty-four hours. The monitor can store
ECG data accurately in SD memory card and upload the
data to the computer via USB. The stored data is able to
display on the PC screen, as shown in Figure 6. If there are
any abnormally changes of heart rate, this instrument will
be given alarm through the buzzer alarm. The test results
of the monitor have obtained consent of cardiologists.
5
Conclusions
In this paper, a low-cost portable ECG monitor has been
presented and designed. And,It will be put into the clinical
environment. The monitor, which is powered by a dry battery, can work for nearly 24 hours. It can display the ECG
trace clean and accurate on the LCD screen, store the data
on SD memory card, and transport data to a PC via USB.
The monitor quickly and accurately captures the abnormal
change of heart rate, and warns cardiac patients. With the
help of this monitor, patients will freely to move without
lines connecting to stationary instrument. It is portable and
light-weight. Users can wear the monitor during normal
activities. It can avoid frequent or long-term hospitalization and reduce overall healthcare cost of patients. That is
an excellent choice for cardiac patients taking this monitor
to ward their health.
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Figure 5: Prototype Instrument
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Figure 6: Displaying on Computer
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