Freescale Semiconductor
Application Note
Document Number: AN4025
Rev. 1, 4/2010
Implementing a Glucometer and
Blood Pressure Monitor Medical
Devices
by:
Roxana Suarez and Carlos Casillas
RTAC Americas
Guadalajara
Mexico
Contents
1 Introduction...........................................................1
2 Glucometers and Diabetes.....................................2
3 Blood Glucose Monitor.........................................4
4 Blood Pressure Monitor and Hypertension...........6
5 Blood Pressure Monitor .......................................7
6 Technology and Medical Devices ......................11
7 LCD Driver.........................................................11
8 Bluetooth Connectivity.......................................17
9 Micro SD Card....................................................20
10 Power Management.............................................24
11 Errors System......................................................25
12 Personal Healthcare Device Class and Medical USB
Stack Applications...............................................25
13 User Guide..........................................................26
14 Conclusion...........................................................30
15 References...........................................................30
1 Introduction
This document describes a combinational medical device
designed to integrate both a low-end glucometer and a blood
pressure monitor. Nowadays, people suffering from chronic
degenerative diseases such as hypertension and diabetes can
develop a plurimetabolic syndrome.
This syndrome and both diseases in the same patient share some
risk factors such as obesity, hypercholesterolemia, and
atherosclerosis.
Medical combinational devices target this new market and not
only is power consumption a target, but bringing a better solution
for disease control.
This application note addresses medical devices implemented
with Freescale technology. By using the MC9S08LL16 this
device is energy efficient. It includes a Medical USB Stack
programmed into the MC9S08JS16 for communication and the
MPXV5050GP pressure sensor.
A glucometer is a device for measuring levels of glucose
concentration in the blood. This device is usually portable and
is used at home for monitoring diabetic-patients.
A blood pressure monitor is a device that detects systolic and
diastolic blood pressure, heart rate, and mean arterial pressure
for patients who suffer or are at risk of developing high blood
pressure.
© 2009 Freescale Semiconductor, Inc.
Glucometers and Diabetes
2 Glucometers and Diabetes
Diabetes is one of the most common diseases today. It is essential to produce glucometers whose one of many advantages is to
empower diabetics to take care of themselves without the need to visit doctors. Glucometers help to detect and confirm
hypoglycemia and infections. High blood sugar may also be a sign of infection or illness that needs to be treated.
2.1 Diabetes Fundamentals
Diabetes mellitus commonly known as “ diabetes ” is a common health problem throughout the world. It prevents the body
from producing enough insulin, does not produce insulin, produces defective insulin, or has resistance to the same. Insulin is a
hormone produced in the pancreas. According to the World Health Organization statics, the global prevalence of diabetes
mellitus is approximately 155 million people and expected to increase to 300 million in the year 2025.
Medical Management
of Diabetes and Heart Disease Book,
Marcel Dekker Inc.
Glucometry is a technique that obtains the value of glucose concentration in peripheral or central blood. These values expressed
either in mgr/dl or mmol are important clinical values for metabolic disorders such as diabetes mellitus, denutrition, and other
consequences like hyperosmolar coma, malabsorption syndrome, and most critical hypoglycemia. A glucometer and proper
pharmaceutical treatment is fundamental for glycemic control of diabetic patients. At home, some glucometers include different
kinds of strips to monitor other variables such as ketones which are produced when a patient is experiencing hyperglycemia.
Figure 1
shows a general diagram of a blood pressure monitor. It shows different peripherals for communication between the
user and the device. The most important part is the test strip, this is the sensor to collocate the blood and get a determined
measurement with the analog-to-digital converter (ADC) of the microcontroller unit (MCU). The other peripherals depend on
the designer.
Figure 1. Blood glucose monitor block diagram
2.2 Glucose Sensors
The first step to measure the glucose in the blood is to convert the glucose concentration into a voltage or current signal, this is
possible with special sensor strips for amperometry. The sensor uses a platinum and silver electrode to form part of an electric
circuit where hydrogen peroxide is electrolyzed. The hydrogen peroxide is produced as a result of the oxidation of glucose on
a glucose oxide membrane. The current through the circuit provides a measurement of the concentration of hydrogen peroxide,
giving the glucose concentration.
Implementing a Glucometer and Blood Pressure Monitor Medical Devices, Rev. 1, 4/2010
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Freescale Semiconductor, Inc.
Glucometers and Diabetes
Figure 2. Electrode reactions between glucose and gluconic acid
The sensor used as a blood-glucose meter is based on a glucose oxide electrode. The glucose oxides were immobilized in a
platinized activated carbon electrode. The enzyme electrode was used for amperometry determination by using an electrochemical
detection of enzymically produced hydrogen peroxide. The sensor is composed of various electrodes; a glucose oxide membrane
layer, a polyurethane film that is permeable by the glucose, oxygen, and hydrogen peroxide.
2.3 Amperometry
Amperometry measures electric current between a pair of electrodes that are driving the electrolysis reaction. Oxygen diffuses
through the membrane and a voltage is applied to the Pt electrode reducing O
2
to H
2
Figure 3. Test strip basic block diagram
These reactive electrodes are amperometric type sensors that use a three electrode design. This approach is useful when using
amperometric sensors due to the reliability of measuring voltage and current in the same chemical reaction. Three electrode
models use a working electrode (WE), reference electrode (RE), and a counter electrode (CE). After this current is produced
this must be changed to voltage for processing by the MCU. This action is performed by the transimpedance amplifier. Finally,
the MCU detects and processes this signal with the ADC module. For a transimpedance amplifier and sensor designs for medical
applications go to Medical Application User Guide at
www.freescale.com.
Implementing a Glucometer and Blood Pressure Monitor Medical Devices, Rev. 1, 4/2010
Freescale Semiconductor, Inc.
3
Blood Glucose Monitor
Figure 4. Chip schematic
Use an amperometric determination method with a constant potential of 0.3V used in the portable meter. The current response
of the sensor is linear with a glucose concentration in the range of 5 to 30 mmol/ L and a fast response time of about 20 seconds.
3 Blood Glucose Monitor
This section explains how to develop a medical device for measuring the blood glucose level. This device operates placing a
relatively small drop of blood on a disposable test strip that interfaces with a digital meter. Within seconds the level of blood
glucose is shown on the liquid crystal display (LCD).
3.1 Transimpedance Amplifier
The transimpedance amplifier consists of an operational amplifier and a feedback resistor between the output and the negative
input. The positive input can be connected to either GND or used for offset calibration.
3.2 Glucose Software Overview
The voltage source is always at 3.3 V. To start taking ADC samples, the source voltage must go to 0.3 V. The connection
between the source and the application contains a voltage regulator of 3.3 V. You can provide voltage to the system by using
a 9 V battery.
TakeSample function
—The function configures the ADC module and starts conversion. It compares the ADC conversion
obtained with
Value1
which is 60. If the ADC conversion is lower than this value, the MCU goes into stop mode, and after 20
seconds sends an error message to the LCD.
void TakeSample (void)
{
.
.
.
ADC_Start();
Strip_CTRL =StripVoltage300mV;
CountSec=0;
ADC_Start_conversion (2);
//0.3V Supply
Drop=1;
while(ADC_Get_Newconversion(2)<=Value1)
{
ADC_Start_conversion (2);
_Stop;
if(CountSec==20)
{
Error(1);
Option=6;
Drop=0;
Implementing a Glucometer and Blood Pressure Monitor Medical Devices, Rev. 1, 4/2010
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Freescale Semiconductor, Inc.
Blood Glucose Monitor
break;
}
}
If no errors occur. The ADC conversion continues and sets the ranges as shown in the code below. The samples obtained must
be the following:
•
•
•
•
•
•
Range 0— ADC Conversion < 128,
Range 1— 141 < ADC Conversion <= 292
Range 2— 239 < Range 2 ADC Conversion<=407
Range 3— 408 < Range 3 ADC Conversion<= 537
Range 4— 539 < Range 4 ADC Conversion<= 752
Range 5— ADC Conversion >752. Indicates a high level of glucose
NOTE
The ADC module resolution is 0.8058 mV/count.
while(CountSec<6)
{
bLCD_CharPosition = 10;
vfnLCD_Write_Char (0x30+(5-CountSec));
if(CountSec==1)
{
ADC_Start_conversion (2);
Sample=ADC_Get_Newconversion(2);
if(Sample<128)
{
bLCD_CharPosition = 0;
vfnLCD_Write_Char ('0');
Range=0;
}
.
.
.
}
The code below sets the glucose levels for each range. You have to change the information in range 2, range 3, and range 4 in
the lines as commented below. Finally, determine the glucose level with the equation:
Glucose = x + midpoint
The x variable for:
• Range 1 = 35
• Range 2 = 86
• Range 3 = 166
• Range 4 = 201
if(Range==1) // changes for 2, 3 or 4
{
low=0;
high=51;
midpoint=0;
while (low<high)
{
midpoint =(low+high)/2;
if (Sample<Range1[midpoint]) // changes for Range2, Range3 or Range4
{
high=midpoint-1;
}
else
{
low=midpoint+1;
}
}
Glucose=35+midpoint;
Implementing a Glucometer and Blood Pressure Monitor Medical Devices, Rev. 1, 4/2010
Freescale Semiconductor, Inc.
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Microchip MCU
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