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					         eZ DESIGN CONTEST




BLOOD PRESSURE HOLTER MONITORING
            SYSTYEM


         Technical description




            Yachnyy Maxim, 2006.
                                                                                 eZ DESIGN CONTEST
                                 Device composition and functioning

The system is designed for 24- to 48-hour measurement of systolic, diastolic pressure and pulse rate in
medical institutions and at home. It does not require any special skills for it's user. The measured
during the period of up to 48 hours values are stored in internal memory. After the end of measurement
period all obtained data from device are transferred to PC trough Bluetooth interface. A PC program
takes those data and allows a specialist to view it in graphical or table form, to eliminate erroneous
samples, to calculate statistical characteristics and to store the data and the patient personal data in a
data base. Before the start of measuring period the device must be programmed, that is the doctor who
treats the patient must enter active and passive periods limits (7:00-22:00 and 22:00-7:00 by default)
and time step between measurements for both periods. Programming can be performed as from PC as
through a built-in menu. A patient may start an out-of-order measurement at any time when he feels
that something goes wrong with him.
The device is extremely compact and light. Due to this it is placed directly on the plastic cuff which
surrounds the patient’s arm above the elbow thus avoiding pipes and wires winding around patient’s
body as in other devices.

The main goals of this project:
- fully automated measurement with high precision;
- minimal time of measurement, pressure force on patient’s arm and level of acoustic noise by using
   measurement on inflation. It means that air pump is controlled by PWM to achieve constant
   speed of inflation (2-5 mm hg per second), when pressure in cuff reaches Psystolic air valve opens
   and lets the air out of cuff – the measurement is done.
- minimal size and weight;
- minimal measurement time and power consumption.

The device is built on TI's MSP430F449 Flash MCU which gives the following advantages.
    Powerful processor core and large enough RAM and FLASH allow implementing rather
       complex data processing algorithms in real time. Thus precision may be achieved with noisy
       data and the CPU core may control the motor to provide a constant pressure increasing rate.
    Two programmable 16-bit timers allow using a cheap capacitive pressure sensor with
       frequency output and getting pressure resolution of 1/16 mm hg. Any analog circuitry with it's
       interference susceptibility, temperature and time drifts becomes unnecessary. Individual sensor
       calibration provides required accuracy. Timers also allow using measurement on inflation.
    FLASH memory self programming provides internal measurement and calibration data storage
       without external memory.
    Internal ADC allows continuous monitoring of battery voltage without using core resources.
    Hardware UART requires no extra chips to communicate with Bluetooth controller.
    Extremely low power consumption with versatile power saving modes allows avoiding
       mechanical power switch.
    Between measurements the device stays in Low Power Mode 3 (consumed current is less than
       10 μA).
    LCD module allows using large 160-segment LCD.
    Second TI's MSP430F1132 MCU (Patient Protection Unit – PPU) is used for protecting patient
       of overpressure (>300 mm hg) or overtiming (>2 minutes of single measurement) in case of
       main MCU malfunction. Both of the CPU can be independently be reprogrammed using a
       bootloader trough single JTAG/Bootloader connector.




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                                                                              eZ DESIGN CONTEST




                                                                                       cuff




                                                                                     MENU
                                                                                     button


                                                                                      START
                                                                                      button
   2-channel
   tube




               Figure 1. The automatic blood pressure Holter monitor. Overall view.


The cuff must be placed tightly around a patient's hand above his elbow and connected to the
electronic unit via the attached 2-channel air tube.

The cover of the electronic unit contains two buttons for device control and a transparent window
through which the LCD screen is visible. The buttons are labeled "MENU" and "START". LCD
displays numeric measurement results, time, date, current battery voltage and some icons indicating
the meter state.




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                                                                eZ DESIGN CONTEST




                                                                      DataFlash 2MB
                                                                      (not mounted)




                                                                       150 segment
    MOSFET                                                                    LCD
   transistors



  JTAG/BSL                                                            Power/
   connector                                                          peripheral
                                                                      connector

      Buzzer
                                                                      TI charge
                                                                      pump


   Gold cap
   0.22F




                    Figure 2. Electronic block without cover.




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                                                                                      eZ DESIGN CONTEST




                                                                                               Power connector
                                                                                               (for testing)

                                                                                               MSP430F449

 NiCd AAA
 storage battery                                                                               PPU
                                                                                               (MSP430F1132)




Air channel                                                                                    Pressure sensor
union (rubber)                                                                                 (under shield)




                        Air compressor               Air valve           Bluetooth PCB         Bluetooth
                                                                         antenna               module KC-22




                                     Figure 3. Internal view of electronic unit.



       Specifications


         Systolic/diastolic pressure measurement range:          20 - 280 mm hg.
         Method:                                                 Oscillometric on inflation
         Maximum pressure:                                       300 mm hg.
         Pulse rate range:                                       30 - 180 pulses per minute.
         Pressure error bound:                                   ± 3 mm hg.
         Communication with PC:                                  Bluetooth v1.2
         Temperature range:                                      10 - 35 Celsius degrees.
         Power supply voltage:                                   2 AAA batteries
         Current consumed from battery:
         - during cuff inflation:                                350 milliamps.
         - in low power mode:                                    less than 10 micro Amps.
         Size (without cuff):                                    91x51x25 mm.
         Weight (with batteries):                                less than 120 g.


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                                                                                 eZ DESIGN CONTEST
Hardware
The monitor hardware is based on MSP430F449 microcontroller and uses very few additional parts.
Due to the fact that these MCUs contain almost all the peripheral blocks necessary for the device it's
schematic (file Shematic.pdf) is rather simple.
The VT1 transistor protects the electronic circuit against inverse battery polarity. The air compressor
and controlled air vent currents flow through this transistor so the voltage drop across it must be small
enough to allow the circuit functioning down to the deep battery discharge.
The charge pump regulator DA1 provides a stable power voltage for MSP430 and PPU (DD1)
independently of battery discharge level. It has 5 mV ripple level to ensure that Bluetooth module DD4
work properly. The gold cap C5 allows user to change discharged batteries without losing current date
and time. Device will work more than ten minutes without batteries.
JTAG and Bootstrap Loader are implemented as single X4 connector.
The MOSFET switches with extremely low on-resistance VT2, VT3 turn the air compressor on/off (by
PWM) and make the controlled air valve open/closed under the MSP430 control. VT4 makes
additional current, reduced by resistors R1 and R2, to flow trough pump (to avoid it’s stop at low
PWM values) and trough valve (to reduce power consumption VT3 opens just for 1 second – to
change valve state, VT4 then holds it in the same state up to the end of measurement). VT5 is driven
by PPU, stops the pump and opens valve in case of main MCU malfunction. As all MCU's ports after
reset are inputs the switch's gates are tied to ground by resistors R3, R4, R5, R7 to ensure their off
state.
The pressure sensitive element is the capacitive pressure sensor BP1 (electrically it is just condenser
with large plates). The distance between its plates is inversely proportional to the applied pressure. The
capacitor is included in the loopback circuit of the RC generator built on DD3 gates. Oscillating
frequency changes in the range from 2.0-4.0 MHz (at zero pressure) down to 1.0-2.0 MHz (at 300 mm
hg) depending on the sensor sample. Additional LDO (DA2) is used to protect sensor circuit of voltage
drops since it is very sensitive to supply voltage.
R16, C14, R18 and R17, C15, R19 circuits are RC filters suppressing button's bouncing at pressing
and releasing. MSP430 I/O ports allow not only reading the current button state but can fix the fact of
state change between the moments where the program analyses its state.
R22, R23 form a low current voltage divider the output voltage of which VCC/2. This allows using
internal 2.5 V reference source of ADC to measure current battery voltage.
DD4 is a Bluetooth module. It uses UART1 channel plus additional signals to communicate with
MCU. VT6 switches DD4 off to reduce total power consumption since DD4 even in reset state
consumes up to 20 mA.
DD5 is 2 MB Atmel DataFlash module. It uses USART0 channel of MCU. DD5 is used to record real-
time signals for algorithm debugging purposes. DD5 uses more than 30 mA of current in the moments
of programming, so DA3 is used to protect all circuits of voltage drops in such moments.
Piezoelectric buzzer is used to indicate "prepare to measurement" state and pulsation detection during
measurement by sound. As the supply voltage of the circuit is low to make the sounds louder the
buzzer is connected in differential mode. The signals on its plates produced by MSP output port
change in opposite phase so that the whole voltage change across the buzzer is twice the supply
voltage.




eZ DESIGN CONTEST                                                                                       6

				
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posted:11/7/2012
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