ECG Portable Device

1. 978-1-4244-6498-2/10/$26.00 ©2010 IEEE 1614
2010 3rd International Conference on Biomedical Engineering and Informatics (BMEI 2010)
The Research Of Portable ECG Monitoring System
With USB Host Interface
Xin Guo, Weijie Chen, Xiaoyun Xu, He Li
(Hebei University of Technology, Tianjin, 300130)
Abstract: This paper introduces a portable ECG monitor with
USB host Interface. This system uses Microprocessor C8051F021
as its controlling core, and SL811HS as USB host Interface chip.
Based on this hardware, the paper presents USB host
specification stack, specification frame about USB Mass Storage
and FAT file system so that the collected ECG signals can be
stored in U-Disk without delay. In addition, the ECG signal
collection circuit of this system embodies several characteristics
including high input impedance, free switch of lead, full filter
function and low circuit loss, all of which are quite suitable for
portable ECG monitoring system.
Key words: USB host Interface; Portable; ECG monitor; SL811HS
1. INTRODUCTION
It is well known that heart disease has become the
disease that threatens human health and life. And the mortality
rate of the heart disease is the highest in the world. So
discovering the symptom of heart disease early will make
much meaning to the prophylaxis and diagnosis of heart
disease. At the present time, doctors can get exact forecast and
diagnosis from the analyzing of electrocardiosignal. And ECG
is the most frequently used expression of the
electrocardiosignal. The routine electrocardiogram cannot get
the arrhythmia cordis that happens when the patients are in the
active state only, so the appearance of the dynamic ECG
monitor is very meaningful for the cardiopath.
2. HARDWARE DESIGN
The hardware of the portable ECG monitor is divided to
four parts. The four parts are the acquisition circuit of
electrocardiosignal, the Single Chip Microcomputer (MCU)
controlling core, the USB Host Interface and the power source
circuit.
2.1 Acquisition circuit of the ECG
The major task of the acquisition circuit is to record and
amplify the electrocardiosignal, and filter out the undesired
signal. Its structural frame is as follows:
Figure 1 Structure of acquisition system
The major task of the input terminal is overvoltage
protection, high frequency filter and increasing input
impedance. And overvoltage protection includes high voltage
protection and low voltage protection. The high voltage
protection circuit’s task is to prevent the high voltage
damaging the acquisition circuit when the monitor is used
with the defibrillator at the same time. When the input voltage
is higher then 90V, the discharge lamp will be broke through,
and high voltage will be leaded into the ground; the low
voltage protection consists of two diodes and an operational
amplifier. It will limit the input voltage between the -8.7V and
+8.7V; the main part of the high frequency filter is a 220pF
capacitance that one end of the capacitance is connected with
the ground. The impedance of the capacitance is high for the
low frequency signal and very low for the high frequency
signal, so the high frequency signal will be leaded into the
ground. On the other hand there is an operational amplifier in
the input terminal, so the input impedance is very high. It can
restrain the trashy common-mode signal. The lead selector
circuit consists of Wilson resistance network, 4052 analog
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2. 1615
switch and negative and gate. The schematic is as follows:
Figure 2 Lead switchover circuit
There are A, B, C and D four lead switchover controlling
ends. They are connected with the I/O pins of the MCU.
The amplitude of the electrocardiosignal is between 1mV
and 5mV. It is too low for the I/O pins of the MCU. So before
transporting the electrocardiosignal to the MCU it must be
enlarged. Generally, the withstand voltage of the MCU’s I/O
pins is between 3V and 5V. So the magnification times of the
electrocardiosignal is between 500 and 1000. The
magnification times is designed 1000 in this monitor. The
amplifying circuit is divided to the head amplifier and the post
amplifier. The magnification times of the head amplifier is 20
and the post amplifier’s is 50. And the filter circuit is between
the head amplifier and the post amplifier.
In order to collecting exact electrocardiosignal, we must
know about the electrocardiosignal and its undesired signal.
The main frequency of the human body’s electrocardiosignal
is between 0.05Hz and 100Hz. And the most common
undesired signal of the electrocardiosignal are the power
frequency interference (generally 50Hz) and the
myoelectricity frequency interference (its frequency is about
35Hz). So the band-pass filter consists of the RC high pass
filter (its cut-off frequency is 0.05Hz) and the RC low pass
filter (its cut-off frequency is 100Hz). And in order to
eliminating the interference between the high pass filter and
the low pass filter, an operational amplifier is put between
them. On the other hand, in order to eliminating the power
frequency interference and the myoelectricity frequency
interference, a 35Hz Double T-notch filter and a 50Hz Double
T-notch filter are designed in the circuit[1]
.
2.2 The Single Chip Microcomputer controlling core
In this monitor, the Microprocessor C8051F021 is chose
to be the controlling core of the system. The C8051F021 has a
lot of functional module, such as: the GPIO, the timer, the
external interrupt the 12-bit analog-to-digital converter and so
on. In addition, the 32 I/O pins of the C8051F021 are
adequate to the control switch and the control output.
2.3 The USB Host Interface
In this part, SL811HS is chose to be the USB host
interface chip. The SL811HS is an embedded USB Host/Slave
controller capable of communicating in either full-speed or
low-speed. SL811HS has 8-bit bidirectional data port I/O,
on-chip SIE, on-chip USB transceivers, and 256-byte internal
SRAM buffer. It supports suspend/resume, wake up and
low-power modes. So the SL811HS is quite suitable for being
the USB Host Interface chip. The surrounding connection of
the LS811HS is as follows:
Figure 3 Surrounding connection of the SL811HS
The SL811HS connects the MCU by D0-D7, nCS, nWR,
nRD, A0, INTRQ, nRST. The D0-D7 is the 8-bit bidirectional
data port I/O. They are in charge of transporting data between
the SL811HS and the MCU. The nCS can enable the
SL811HS when a low electric level on it. The nWR and nRD
are the Write and Read enabling end. The INTRQ can send the
Interrupt Request to the MCU. And the nRST is the restart
signal receiving end.
3. SOFTWARE DESIGN
The software system of the portable ECG monitor
includes: the program of analog-to-digital conversion, the
implementation of USB Host Specification framework, the
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3. 1616
implementation of USB Mass Storage framework, the
implementation of FAT file system.
The USB Host Specification framework consists of four
layers. The first layer is the achievement of the
communication between the C8051F021 and the SL811HS.
The second layer is the achievement of the data packet
between the USB Host and the USB Slave. The third layer is
the achievement of the USB Specification standard request.
The fourth layer is the achievement of the USB Host Flow.
The higher the layer is, the more advanced it is. The lowest
layer and the hardware are interrelated closely. The higher the
layer is, the less relation between the layer and the hardware is.
And the highest layer has nothing with the hardware. So it is
quite suitable for transplanting.
The USB Mass Storage framework is used to achieving
the disk management and the transmission of mass data. This
framework bases on 19 UFI commands. The commands
include Format Unit, Inquiry, Mode Select, Verify and so on.
If the 19 commands are achieved, the USB Mass Storage
framework is achieved.
The FAT file system connects the U-Disk by the
achievement of 19 UFI commands[2]
. And the FAT file system
will establish a struct of the U-Disk in the USB Host. The
struct is used to create, search, write, read and delete the files.
The relationship of these modules is as follows:
Figure 4 Structure of USB Host software system
3.1 The configuration of the U-Disk
After the initialization of the MCU and the restoration of
the USB serial bus, the main circle program will enable the
interrupt of the device detection. Then the monitor will wait
the U-Disk insert.
When the U-Disk connects with the monitor, the
SL811HS will send the interrupt request to the MCU[3]
. Then
the MCU will get the type of the interrupt by reading the
interrupt state register. When the type of the interrupt indicates
that there is a U-Disk connects with the monitor, the MCU
will configure the U-Disk. Before the U-Disk is configured,
its default addresses and default communications pipe are
both 0. The steps that configure the U-Disk are as follows[4]
:
(1) The USB Host sends the Get-Descriptor command to the
U-Disk. Then a few Device Descriptor of the USB Slave will
be sent back.
(2) The USB Host sends the Set-Address command to the
U-Disk. This command will set the new address of the
U-Disk.
(3) The USB Host sends the Get-Descriptor command again.
At this time, all of the Device Descriptor of the USB Slave
will be sent back.
(4) The USB Host sends the Get-Configuration command to
the U-Disk. The USB Slave will send the Configuration
Descriptor, the Interface Descriptors and the endpoint
Descriptors back.
(5) The USB Host sends the Set-Configuration command to
the U-Disk for configuring it.
After the U-Disk is configured, the U-Disk will use the
new address for the communication between the USB Host
and the U-Disk.
3.2 The identification of the file system of the U-Disk
At the present time, a majority of the U-Disk’s file
system is the FAT file system. And the FAT file system
includes the FAT16 file system and the FAT32 file system. So
it is necessary to distinguish which file system of the U-Disk
uses. The result of the identification will be stored in a
variable. When the value of the variable is ‘1’, the file system
of the U-Disk is the FAT32 file system. When the value of the
variable is’0’, the file system of the U-Disk is the FAT16 file
system. The theory of the FAT32 file system and the FAT16
file system are nearly. Here we take the FAT32 file system for
example.
Program of the main circle
Interrupt
service
routine
FAT file system
USB Mass Storage
framework
Firmware of the SL811HS
Writ and read program of the SL811HS
Hardware of the SL811HS
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4. 1617
The FAT32 file system is divided to five parts. They are
the MBR, the DBR, the FAT, the FDT and the DATA. The
relative position of the five parts is as follows:
Figure 5 Organizational structure of the FAT32
3.3 The operation of the files in the U-Disk
The operation of the files in the U-Disk is as follows:
(1) Create Directory, the function prototype is: unsigned
char CreateDir (unsigned long len, unsigned char *pBuffer,
unsigned char *pName). The input parameters are the length
of the long file name, the pointer points to the directory entry
of the short file name and the pointer points to the directory
entry of the long file name.
(2) Create a file under the present directory, the function
prototype is: unsigned char CreateFile (unsigned long len,
unsigned char *pBuffer, unsigned char *pName). The input
parameters are same with the Create Directory.
(3) Set the pointer of the files, the function prototype is;
unsigned char SetFilePointer (unsigned long pointer). The
input parameter is the pointer points to the file.
(4) Open a file, the function prototype is: unsigned char
OpenFile (unsigned char *pBuffer). The input parameter is the
file name.
(5) Write data into a file, the function prototype is:
unsigned char WriteFile (unsigned long writeLength, unsigned
char *pBuffer). The input parameters are the length of the data
and the pointer points to the data.
3.4 The data transmission between the monitor and the
U-Disk
The data transmission between the monitor and the
U-Disk is based on the Bulk endpoints. The Bulk transmission
is divided to Bulk-In and Bulk-Out[5]
. The process that the
monitor writes data into the U-Disk is the Bulk-Out.
Generally a sector consists of 512 bytes in the U-Disk. But the
maximum bytes in a Bulk-Out packet are 64. So it needs 8
circles for writing a sector in the U-Disk. The transmission
process of the Bulk-Out is illustrated in Figure 6 [6]
.
Figure 6 Process of Bulk-Out transmission
MBR
62 reserved sectors
DBR
31 reserved sectors
FAT1
FAT2
Root Folder
DATA
Remain sectors
Start
Send CBW to Bulk-Out
ACK？
Send PID-OUT to Bulk-Out
ACK？
Send data to Bulk-Out
Transmission
over？
Send PID-IN to Bulk-In
ACK？
Receive CSW
Transmission
over？
End
Error
Processing
N
Y
Y
Y
Y
Y
N
N
N
N
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5. 1618
4. CONCLUSION
The portable ECG monitor introduced in this paper is
designed for that the cardiopath can monitor their heart
condition in their daily life. This monitor is easy to use and
carry, small, low energy consumption and running stably. So it
is very suitable for the 24-hour monitoring. Moreover a USB
Host Interface is designed for the portable ECG monitor. This
is the main innovation in this paper. So we can use the U-Disk
in stead of the internal storage of the monitor. It makes the
circuit of the monitor simpler, and cut down the development
cycle. In addition, the main purpose of designing the USB
Host Interface in a portable ECG monitor is as below. With
the rapid development and spread of the USB interface
technology, there will more functional module with USB
Interface come about. These modules can display, analyze,
transfer the electrocardiosignal date, control and drive the
treatment equipment automatically. So the monitor can
connect with the relevant module in times of need.
REFERENCES
[1] Lindberg G ， Jwarzon M. Halmmarlund B.24-hour
ambulatory electrocardiosignal in healthy volunteers Scan J
Gastroenterol, 1996, 31:658-663
[2] USB Implementer, Forum USB Mass Storage Class UFI
Command Specification Revl.0 [EB/OL]. http://www.usb.org.
2006-11-22
[3] SL811HS Embedded USB Host/Slave Controller
Datasheet Cypress Semiconductor Corporation，2002
[4] Mark S.USB embedded host controller for removable
mass storage devices. Elector Electronics，2004，30(335):
58-63.
[5] USB Mass Storage Class Specification Overview Rev，
http://www. usb.org，2000
[6] USB Mass Storage Class Bulk-only Transport Revl.0，
http://www.usb.org，1999
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