Freescale Solutions for ECG and Heart Rate Monitor
Contents
1. INTERNAL OPAMP 1 From Left Electrode Vref1 5V INTERNAL TRIAMP 1 E 1 0K Figure 10 Instrumentation amplifier with internal OPAMPs and TRIAMPs The gain is configured with jumpers J4 and J3 of the MED EKG board and it has to be the same in both cases The equivalent gain of the amplifier is as detailed below x 100 2 3 The output of the amplifier is connected to a band pass filter 3 3 2 External instrumentation amplifier EL8172FSZ The EL8172 is an instrumentation amplifier integrated circuit IC whose gain is configurable through external resistors Figure 11 shows the use of this IC in the MED EKG board Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 11 Hardware description J 3v F ight electrod rom right electrode Ta band pass filter and DSC_ADC Pam Co hal From left electrode MM DscPwmo Output R38 fram DSC R33 402 0 C30 2 2UF Figure 11 Instrumentation amplifier with IC EL8172 The gain of the amplifier is determined with R33 and R32 The calculation is shown below G 153 17 362 17 85 57 The digital signal controller generates a PWM signal that functions as an offset generator with the intention of maintaining the EKG signal variations with a base level giving stability to the signal shown in the GUI and resulting in a more precise heart rate measurement The output of this2. Start timer Send confirmation f serial Communication Protocol Y V new data ready d i v n a Ta NT S Tr i E in Currently No measuring E Yes 1 Continue i Copy ECG buffer to OutBuffer send indication and ECG data Figure 15 Communication with host PC Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 16 Freescale Semiconductor Inc Software architecture 4 2 State machines The software for the MED EKG includes the functionality of state machines This type of execution emulates parallelism and ensures that the process follows a well defined sequence of steps or states It is this functionality that allows the device to send data packets for example while measurement is underway Figure 16 presents the concept of state machines in a graphical format State Machine A State Machine B Other tasks or state machines Figure 16 State machine outline 4 3 Timer configuration and functioning The MED EKG demo uses the timer module to generate interrupts each millisecond One sample of the electrocardiogram signal is taken every 2 milliseconds meaning that two periods of the timer have to pass between successive samples Figure 17 contains a flow chart explaining the basic functioning of the timer Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semicon
3. Table continues on the next page 1 Software related with this application note does not respond to all of these commands Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 38 Freescale Semiconductor Inc Data length and data string Table B 2 Opcodes continued Opcode CFM Opcodes Hex X SPO2 ABORT MEASURMENT X 0x22 SPO2 MEASURMENT COMPLETE OK X 0x23 SPO2_MEASURMENT_ERROR 0x24 SPO2 DIAGNOSTIC MODE START MEASURMENT 0x25 SPO2 DIAGNOSTIC MODE STOP MEASURMENT 0x26 SPO2 DIAGNOSTIC MODE NEW DATA READY 0x27 0x21 System commands SYS_CHECK_DEVICE_CONNECTION 0x29 SYS_RESTART_SYSTEM 0x2A B 3 Data length and data string The data length and data string bytes are the data quantity count and the data itself The data length byte represents the number of bytes contained into the data string The data string is the information sent just the data therefore the Data Length byte must not count the Packet Type byte the Command Opcode byte or itself B 4 Functional description Communication starts when the host sends a REQ packet indicating to the device to start a new measurement The host must send a REQ Packet Type to start transactions Figure B 3 Figure B 3 Start packet sent by host The Start Opcode can be any Opcode related with start a measurement for example 1f we wanted to start the ECG in diagnostic mode the Data Packet will look like Figure B 4 REQ S
4. Figure B 7 shows a typical MED EKG demo indication packet IND M READY Length Packet ID Packet ID i Data 2 Datan 1 HR Figure B 7 MED EKG IND packet When a Stop request is sent by the host the device stops sending data and waits for a new Start request command Figure B 8 shows the Stop Command structure Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 40 Freescale Semiconductor Inc Figure B 8 Stop command structure Immediately after this the device must acknowledge with a CFM packet shown in Figure B 9 Figure B 9 Stop CFM packet The CFM packet for stop does not require an error code it just must be received If this packet has not been received the request has been rejected or not taken and must be sent again to stop the measurements Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 41 How to Reach Us Home Page WWW freescale com Web Support http www freescale com support USA Europe or Locations Not Listed Freescale Semiconductor Technical Information Center EL516 2100 East Elliot Road Tempe Arizona 85284 3 1 800 521 6274 or 1 480 768 2130 www freescale com support Europe Middle East and Africa Freescale Halbleiter Deutschland GmbH Technical Information Center Schatzbogen 7 81829 Muenchen Germany 44 1296 380 456 English 46 8 52200080 Engli
5. MED EKG system block diagram 3 2 Signal acquisition ECG signal can be acquired either by using the on board fingertip electrodes or connecting external electrodes Figure 9 shows both the options The fingertip electrodes are marked with the number 1 and the external electrodes connector is marked with the number 2 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 9 Hardware description 1 Fingertip electrodes NN REM 2 Externalelectrodes n U4 SS 9 MA EE ld aa aa LE 3 Medical connector Figure 9 MED EKG development board connectors 3 3 Instrumentation amplifier An instrumentation amplifier consists of three operational amplifiers configured to obtain a high gain and high common mode rejection ratio CMRR Instrumentation Amplifiers are widely used in biopotentials acquisition MED EKG board provides two options for implementing the instrumentation amplifier Those options are described below 3 3 1 Instrumentation amplifier using internal OPAMPs It is possible to use the internal OPAMPs and TRIAMPs of the Kinetis and MM microcontrollers to implement an instrumentation amplifier The connection diagram is shown in Figure 10 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 10 Freescale Semiconductor Inc Hardware description From Right Electrode EN Ta the band pass filter
6. amplifier is sent to the respective band pass filter and such output is also used by the DSC as a reference to generate the adequate PWM feedback signal 3 4 Filtering Although instrumentation amplifiers have a high CMRR it is still necessary to filter the signal Some causes for noise are the line noise muscle contractions respiration electromagnetic interference and electromagnetic emissions from electronic components A band pass filter helps in attenuating those sources of noise The low limit rejects signals caused by respiration and muscle contractions The high limit avoids the pass of electromagnetic high frequency interferences As can be seen in the system s block diagram there is one band pass filter for the internally implemented instrumentation amplifier and another for the external Each one has different cutoff frequency these cutoff frequencies are described in the following section 3 4 1 Band pass filter 0 5 Hz 250 Hz The cutoff frequencies for the first band pass filter the one located after the internal instrumentation amplifier configuration are 0 5 and 250 Hz correspondingly The values for capacitors and resistors and the respective process to obtain the cutoff frequencies are described below e High limit 153 Hz C 222nF R 47kO 1 i f arc m29 4220F 246 06Hz Low limit 0 5 Hz C20 33F R 1 MO HE S 1 f sarc ma gGzp 19 3 9Hz Freescale Solutions for Electrocardiograph and Heart Rate Monitor Ap
7. values have already been used in a previous filter from Band pass filter 0 5 Hz 250 Hz and the gain of the amplifier is simply calculated with the following equation G 1 4 1 20 56 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 13 Hardware description c Band Pass Filter 0 5 Hz 153 Hz EUM Tees Petree 250 Hz From external instrumentation R29 AK C19 n PN R30 amplifier Hu un EN n To DSC NS E g x E i Pdl TT cig 0 33UF R23 2 29 4K _ lt R25 T Erde EM ssec 10M 24 Loon E RID 100K i gt R37 dicit 5 11K Figure 13 Band pass filter from 0 5 to 153 Hz Resistors R25 and R26 help to maintain the offset of the band pass filter s input signal since negative values of the signal must not get lost The output of all these blocks is finally sent to the DSC 3 4 3 Notch filter 60 Hz The notch filter is designed to remove the 60 Hz signals The reason to include this filter is to eliminate any noise related to an electrical power source since electrical installations in America have a 60 Hz frequency Figure 14 shows the distribution of the components that form the notch filter The diagram also includes two more filters and an operational amplifier The filters are high pass and low pass but with the same cut frequencies of the band pas
8. valve V m A mL Left ventricle Chordae tendineae XN WSs aa Septum Right ventricle Inferior vena cava Figure 1 Cardiac conduction system The cardiac cycle is described next and complemented with Figure 2 which is a scheme of the blood flow in the circulatory system The cycle consists of three principal stages Atrial systole Atria contract and eject blood to the ventricles in a passive way Once the blood has been expelled from the atria atrioventricular valves located between atria and ventricles are closed to avoid blood return Ventricular systole The ventricles contract ejecting blood towards the circulatory system At this stage it is necessary to overcome the high pressure of the aortic and pulmonary valves Ventricles are never completely emptied retaining some of the blood Diastole At this point all the parts of the heart are relaxed to allow the arrival of new blood Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 2 Freescale Semiconductor Inc Heart rate and electrocardiograph fundamentals Pulmonary Vein Superior and Inferior Vena Cava Pulmonary Artery Aorta Mitral Valve Tricuspid Valve L eft Aortic Valve Right side Pulmonary Valve side Figure 2 Blood circulation scheme 2 2 Electrical activity of the heart There is an electrical activity in the heart related to depolarization and repolarization of my
9. without having to worry about USB protocol In this case the USB device acts like a communication device class CDC The graphical user interface GUI installed in the host computer controls the actions of the MED EKG demo with a series of data packet transactions Basically there are three types of packets REQ packet The host sends a REQUEST packet either to start or to stop a measurement e CFM packet After the device receives a REQ packet a CONFIRMATION packet is sent to the host in order to notify that the command is valid or that there was an error ND packet Once the buffer of the ECG with the data to be sent 1s full the microcontroller sends an INDICATION packet and the data as well Figure 15 1s a diagram showing the basic functioning of the USB protocol and a more detailed explanation can be found in Communication protocol at the end of this document Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 15 Software architecture Serial Communication Protocol 7 request received i Pi l j m Qut tm Tt ET Evaluate request gt enr Hequest start measurement Request stop measurement p E x T udi ine e ad Currently Yes No Currently Measuring 7 Measuring 7 gt ga No state changes f 0 Initialize ADCs e Sinn fim State measuring UNTEN State idle
10. Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 29 Getting started and running the MED EKG demo Figure 30 Connecting the USB cable Black electrode Left abdomen 9 Red electrode Right upper chest C White electrode Left upper chest Figure 31 Electrode connection Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 30 Freescale Semiconductor Inc n Getting started and running the MED EKG demo y Black electrode Left abdomen 9 Red electrode Right arm White electrode Left arm 4 bx Figure 32 Electrode connection 2 Press the Reset button on the microcontroller board When the demo is used for the first time it is necessary to install the driver for the USB CDC Virtual COM The default path for the driver is c Freescale Medical GUNDrivers Select X32 for 32 bit windows versions and x64 for 64 bit windows versions 3 Open the Windows Device Manager and search for the port assigned to the USB CDC virtual com This information is requested by the GUI 4 Open the graphical user interface GUI It rwill ask for a port number obtained in step 3 Select the correct port and click OK This is shown in Figure 33 E ort chooser x Figure 33 Selecting port 5 The main screen of the GUI See Figure 34 appears Make sure that Caps Lock key is not activated on your keyboard and press Shift D
11. Freescale Semiconductor Application Note Document Number AN4323 Rev 1 04 2013 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications by Jorge Gonz lez 1 Introduction This application note describes how to use the MED EKG development board a highly efficient board that can be connected to the Freescale Tower System to obtain an electrocardiogram signal and measure heart rate The demo can be implemented in the TWR K53N512 TWR MCF51MM or TWR SO8MM128 This demo uses the Freescale USB stack to send data to a PC making it possible to monitor heart activity through a graphical user interface GUI The Tower System and MED EKG board allow the user to implement heart signal conditioning in three different modes e With the internal OPAMPs and TRIAMPs of the microcontrollers With the external instrumentation amplifier and operational amplifiers featured in the MED EKG board With a mixture of both The information in this application note is intended for developers of medical applications for those in the medical field such as biomedical engineers or doctors or for people who want to know more about the operation of heart rate monitors and electrocardiograph devices 2013 Freescale Semiconductor Inc Contents LETTRES ELE ai 1 Heart rate and electrocardiograph PUM Dios dandebe ep rn a ra Far cp E ad qu EAE idu 2 HOW Gean pi Oi ea 7 Software architecture eene
12. OPAMP 2 x OPAMP 2 x OPAMP 2 x TRIAMP 2 x TRIAMP 2 x TRIAMP 2 x Programmable Gain Amplifier PGA Random Number Generator RNG LCD Driver Touch Sensing Interface Appendix B Communication protocol The application communicates with the Graphic User Interface GUI in the PC using the Freescale USB Stack with PHDC with the device acting as a CDC Communication Device Class The device communicates via a serial interface similar to the RS232 communications standard but emulating a virtual COM port After the device has been connected and a proper driver has been installed the PC recognizes it as a Virtual COM Port and it is accessible for example using HyperTerminal Communication is established using the following parameters Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 35 Bits per second 115 200 e Data bits 8 Parity None e Stop Bits 1 Flow Control None Communication starts when the host PC sends a request packet indicating to the device the action to perform The device then responds with a confirmation packet indicating to host that the command has been received At this point the host must be prepared to receive data packets from the device and show the data received on the GUI Communication finishes when the host sends a request packet indicating the device to stop The following block diagram describes the data
13. TART 0x52 0x12 Figure B 4 Starting ECG in diagnostic mode 0x52 is the HEX code for a request REQ Packet Type 0x12 corresponds to ECG_DIAGNOSTIC_MODE_START_MEASUREMENT After sending the REQ packet a CFM packet must be received indicating the status of the device The received packet must look like Figure B 5 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 39 Functional description CFM Figure B 5 Confirmation packet structure The error byte indicates the device status The table below shows possible error codes Table B 3 Error codes BUSY 0x01 If the error byte received corresponds to OK the device starts sending data as soon as a new data packet is ready If BUSY error is received the host must try to communicate later If the error received is INVALID OPCODE data sent and transmission lines must be checked If a CFM packet with an OK error has been received the device starts sending Information related with the measurement requested This is performed using indication packets IND Indication packet structure is shown in Figure B 6 Length Packet ID Packet ID f T Data n M READY n Tee eese Debet v Data n 1 a Figure B 6 Indication packet structure The first byte contains the HEX code for an Indication Packet type The second byte contains the Opcode for the kind of indication for example if the device
14. This starts the GUI doctor mode Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 31 Getting started and running the MED EKG demo Medical GUI Version 1 0 Press D to enter to Doctor Mode Ac C am z freescale wedicalkiosk semiconductor Slide your ID card English Figure 34 GUI main screen 6 In the doctor mode screen shown in Figure 35 select the ECG option Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 32 Freescale Semiconductor Inc Getting started and running the MED EKG demo Fe freescale Figure 35 Doctor mode 7 Click the ECG area and the ECG signal will start being displayed Wait for the signal to stabilize You can see the electrocardiogram signal and the heart rate as well 8 The ECG parameters and the graph are shown on the GUI Figure 36 is an example of the results of a ECG test Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 33 Reference documents 9g ECG signal easurement EMT Figure 36 MED EKG GUI results 6 Reference documents In addition to this application note the following documents are available at freescale com e Reference manuals and data sheets for Kinetis K53 K53P144M 100SF2RM K53P144M 1008 F2 MCF51MM256 MCF51MM256RM MCF51MM256
15. and MC9SO8MM128 MC9808MMI28RM MC9S08MM 128 MED EKG Board User Manual MED EKGUG Schematics of the Tower Modules and MED EKG board Quick Start Guides USB Stack with PHDC User Guide MEDUSBUG Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 34 Freescale Semiconductor Inc Appendix A Comparison between Freescale microcontrollers The demo described in this document has been implemented with three microcontrollers the MCF51MM256 the MCO9S08MM 28 and the Kinetis K50 The following table is a brief comparative analysis of those MCUs Table A 1 Freescale MCU comparison Characteristic MC9S08MM128 MCF51MM256 KinetisK50 Core HCSO8 8 bit ColdFire V1 with MAC ARM Cortex M4 48 MHz Core 8 32 bit compatibility with MAC DSP SIMD 24 MHz Bus 50 33 MHz Core 72 MHz or 100 MHz 25 165 MHz Bus Flash SRAM 128 KB Flash 256 KB Flash 64 512 KB Flash 007 hemm zemu oam ADcoweter converter 16 bit bitSARADC ADC 256 KB 256KBFlash32 KBRAM 32 KB RAM 2 x 16 bit Px16 btSARADC ADC EM Am converter 12 bit pod 12 bit pu 2 x 12 bit a 2 x 4 channel TPM with PWM 2 x 4 channel TPM with PWM 3 x 12 channel Timer Communication Interfaces 2 x SCI 2 x SCI 6 x SCI 2 x SPI 2 x SPI 3 x SPI USB Device USB Device Host OTG USB OTG 12C lC Ethernet MiniFlexBus Secure Digital Host Controller SDHC FlexBus Other PRACMP PRACMP 3 x ACMP 2 x
16. astically For this reason several successive heart rate indicators are averaged to obtain a more approximate value The algorithm is similar to the one used to obtain the baseline but in this case the heart rate median is updated every 4 peaks although the last 18 pulses are considered The algorithm corresponds to the software routine called CalculateHeartRateMedian whose flow chart is in the Figure 21 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 21 Software architecture y Calculate heart rate median Ly ae Median counter d ae E uc Mo Obtain median HR sum HR sum median Calculate average Increment median counter current heart rate HR sum 0 Han Return J Figure 21 Heart rate flowchart 4 5 4 Gain control The gain control algorithm allows the user to modify the amplitude of the signal shown on the GUI by pressing a pair of buttons featured in the tower board for each microcontroller This action consists of changing the gain for one of the internal OPAMPs of the microcontroller With these buttons it is possible either to increase or decrease the gain of the operational amplifier so that the ECG signal is at a more convenient level Software dedicated to this function consists simply in polling the signals coming from the buttons and changing the gain of the OPAMP with the ap
17. ce may vary over time All operating parameters including Typicals must be validated for each customer application by customer s technical experts Freescale Semiconductor does not convey any license under its patent rights nor the rights of others Freescale Semiconductor products are not designed intended or authorized for use as components in systems intended for surgical implant into the body or other applications intended to support or sustain life or for any other application in which failure of the Freescale Semiconductor product could create a situation where personal injury or death may occur Should Buyer purchase or use Freescale Semiconductor products for any such unintended or unauthorized application Buyer shall indemnify Freescale Semiconductor and its officers employees subsidiaries affiliates and distributors harmless against all claims costs damages and expenses and reasonable attorney fees arising out of directly or indirectly any claim of personal injury or death associated with such unintended or unauthorized use even if such claims alleges that Freescale Semiconductor was negligent regarding the design or manufacture of the part RoHS compliant and or Pb free versions of Freescale products have the functionality and electrical characteristics as their non RoHS complaint and or non Pb free counterparts For further information see http www freescale com or contact your Freescale sales representative For inf
18. diately after the occurrence of a new pulse e The function CalculateHeartRateMedian is called to obtain a more reliable heart rate one that considers not only the last value but the previous ones as well e The heart rate calculated in the last step is stored in a buffer When the buffer is full the data is sent to the PC via the communication protocol Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 19 Software architecture r B State measuring ian A a No lt _ New sample l1 CM wes k Haturn F New sample FALSE Haad ADC feedback value Haad ADC ECG signal value Start timer 7 Time without Yes pulses gt 2 58s m pl CX l m gt A B pr an Ps i aum Wes conditions Calculate and store new raal time heart rate in HR array E ert Pu E ae T lt _ECG buffer full gt put Figure 19 Measurement flow chart 4 5 2 Baseline acquisition In order to provide a feedback signal and set a base level so the MED EKG demo signal is stable on the screen of the GUI the microcontroller first has to obtain the current approximate offset of the signal To do this the software performs an averaging algorithm that takes a certain number of samples in this case 15 orders them and obtains the median of the data Freescale Sol
19. ductor Inc 17 Software architecture Decrement count PT S c P s i E s a E T Count 0 No i ur e ve Figure 17 Timer events flow chart With each interrupt generated by the timer the interrupt routine verifies whether or not the count is over in order to indicate that a new sample of the ECG signal can be taken When the count is zero the timer is deactivated until the software starts it again 4 4 ECG initialization The following sequence is used to initialize the peripherals of the microcontroller used in the MED EKG demo The flow chart for this stage is shown in Figure 18 Fr mH Peripherals initialization s a Initialize DAC Initialize OPAMPs and TRIAMPs Initialize lO ports Initialize ECG Timer Figure 18 ECG initialization flow chart Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 18 Freescale Semiconductor Inc Software architecture 4 5 Measurement and conditioning The software described in this section handles the ECG signal data and calculates the heart rate as well All this information is sent afterwards to a host device using the Freescale USB stack This block also controls the offset voltage provided by the microcontroller s DAC which sets a base level for the ECG signal and the gain for one of the internal OPAMPs 4 5 1 State measuring The principal function performed by the system consists
20. embedded in the MED EKG board this way users can program their own applications The use of the DSC is optional The ARM Cortex M4 Kinetis K50 and ColdFire V1 MCF51MM cores can execute multiply accumulate MAC DSP instructions to perform the digital filtering internally The HCS08 core is capable of filtering the signal even though it doesn t have the MAC feature but with a loss in performance Besides the peripherals in the microcontroller modules can obtain the different signals and achieve both the conversion to digital and the computation of heart rate as well as the offset control 4 Software architecture This section describes the software functions most relevant for obtaining a valid heart rate and maintaining the signal in a stable operation baseline It also explains some of the routines related to the ECG demo NOTE The respective software for TWR K53N512 TWR MCF51MM and TWR SO8MM128 is available on freescale com along with this application note 4 1 Freescale USB stack The microcontroller communicates with the host PC through a USB interface Freescale offers a solution to implement medical applications and easily connect medical devices to the PC Freescale s USB stack with personal healthcare device class PHDC which is a portable and easy to use source code designed for many applications that require interconnection with a host computer The USB stack makes it possible for developers to focus only on their applications
21. ev 1 04 2013 Freescale Semiconductor Inc Hardware description Ultra low power operation e 2 operational amplifiers OPAMP e 2 trans impedance amplifiers TRIAMP 16 bit SAR analog to digital converter ADC 4 differential channels and up to 12 external single ended channels e 12 bit digital to analog converter DAC e nter integrated circuit IC Universal serial bus USB connectivity e Multiply accumulate unit MAC only in MCF51MM e ColdFire V1 and HCSOS cores respectively In addition the MED EKG board includes the MCS56F8006 a 16 bit digital signal controller DSC which performs the digital filtering of the signal The main features of the MC56F8006 DSC are e 3 analog comparators ACMP e 2x 12 bit ADC e 6 PWM outputs e PC JTAG ONCE for programming The DSC communicates with the microcontroller through the I7C bus 3 1 EKG system implementation Figure 8 is a general block diagram of the MED EKG system Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 8 Freescale Semiconductor Inc Hardware description r 3 E D O 5 ADC BASELINE Internal amplifiers In Out Blocks featured on the MED EKG board Internal blocks of MM or Kinetis microcontroller User selectable These stages are connected to several blocks See SCH 2652 for reference Optional block digital filtering can be performed internally zo p Figure 8
22. flow Host sends start request Device sends start confirmation Device sends measurement data Hostsend WW Device sends stop stop request confirmation Figure B 1 Communication protocol data flow Packets sent between host and device have a specific structure The Packet is divided in four main parts Packet Type e Command Opcode Data length Data The image below shows the packet structure Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 36 Freescale Semiconductor Inc Packet type Packet Command Data DataO Datan Type Opcode Length LC oOo Figure B 2 Packet structure B 1 Packet type The Packet Type byte defines the kind of packet to be sent There are three kinds of packets that can be sent between host and device B 1 1 REQ packet This is a request packet this kind of packet is used by the host to request to the device to perform some action like a start or stop measurement A REQ packet is usually composed of 2 bytes Packet Type and Command Opcode Data Length and Data Packet bytes are not required B 1 2 CFM packet This is a confirmation packet this kind of packet is used by the device to confirm to the host that a command has been received and sends a response indicating if the command is accepted or if the device is busy B 1 3 IND packet This is an indication packet This kind of packet is used to indicate to the host t
23. folder 6 Verify that the selected debugger is PE micro in the project options panel Project Options Debugger as shown in Figure 25 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 26 Freescale Semiconductor Inc Getting started and running the MED EKG demo Options for node USB CDC Category Output Converter Linker Setup macros Angel Use macro file s levice description file TO i DL E IT_ DIRS CI 1h Figure 25 Selecting appropriate debugger 7 To load the microcontroller with the project click on the Download and debug icon The IAR screen of Figure 26 shows the location of the icon 4 IAR Embedded Workbench IDE File Edit View Project Tools Window Help naugiereRoo u g y yv sed wn sop i Morkspace x Files amp BR OMA CDC Flash Debug K40 EE Download and debug app Ep mm a 3 driver a Oo MED EKG pr Ha APP Re A a G Drivers enn a G prm a 1 startup x I B readme t La 73 Output Figure 26 IAR screen Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 27 SOSA Getting started and running the MED EKG demo 8 After programming the Kinetis K53 refer to Graphical user interface for the MED EKG for instructions on how to connect the electrodes and use the graphical interface 5 1 2 MED EKG demo wit
24. h Flexis MM The steps for the TWR MCF51MM and the TWR SO8MM128 are listed below 1 Download the MEDICAL GUI from freescale com Follow the instructions included in the installer s ZIP file for GUI installation 2 Ensure that the host computer has CodeWarrior v6 3 and the latest service pack for MCF51MM2506 if necessary Otherwise download and install them 3 Connect the modules of the tower system elevators TWR SER TWR MCF51MM256 or TWR SO8MM128 and the MED EKG board The primary and secondary elevators need to match the corresponding sides of the microcontroller board Figure 27 Tower system assembled for the MED EKG demo 4 Using the USB ports connect the serial and microcontroller modules from the tower system to the host computer The first time you connect the tower to the PC it will ask for the driver of the Open Source BDM You can either choose the option Install the software automatically or specify the path to the driver which is by default C Program Files Freescale Code Warrior for Microcontrollers V6 3 Drivers Osbdm jm60 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 28 Freescale Semiconductor Inc Getting started and running the MED EKG demo Tower System Figure 28 USB connections 5 Open CodeWarrior v6 3 and program the microcontroller with the ECG for MCF51MM or ECG for SO8SMM project included in the compressed folder AN4323 zip Figu
25. hat an event has occurred in the device and data needs to be sent For example this is used when the device has a new data to be sent to the GUI The following table shows the HEX codes for every Packet Type Table B 1 HEX codes CFM OM os wo o B 2 Command opcode The Command Opcode byte indicates the action performed for a REQ packet and the kind of confirmation or indication in case of CFM and IND packet types There are different Opcodes for every Packet Type The following table shows the different Opcodes See Note Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 37 Command opcode Table B 2 Opcodes Do mme P m Pm Im Hex En X X X X BPM START LEAK TEST OxOA BPM ABORT LEAK TEST OxOB BPM LEAK TEST COMPLETE X OxOC X BPM SEND PRESSURE VALUE TO PC 0x28 Electro Cardiograph Opcode X ECG DIAGNOSTIC MODE NEW DATA READY 0x14 e CO CR ue wae E E E E meee Thermometer m x X X WGT READ WEIGHT 0x17 o Spirometer SPRH DIAGNOSTIC MODE START MEASURMENT X X OxOC SPR DIAGNOSTIC MODE STOP MEASURMENT X X OxOD A E X a X SPR_DIAGNOSTIC_MODE_NEW_DATA_READY OxOE s J H H p J dn a see Pulse oximetry SPO2 START MEASURMENT X X 0x21
26. is sending an Indication Packet for ECG_DIAGNOSTIC_MODE_NEW_DATA_READY the HEX code read in this position is 0x14 because this is the Indication Opcode for a new set of data from the ECG diagnostic mode The next byte is the Length which indicates the quantity of data sent The first couple of bytes after the Length byte are the Packet ID bytes The Packet ID is a 16 bit data divided in 2 bytes to be sent and contains the number of packets sent The Packet ID number of a data packet is the Packet ID of the previous packet 1 For example if the Packet ID of the previous packet sent was 0x0009 the Packet ID of the next packet must be 0x000A This allows the GUI to determine if a packet is missing The following data bytes are the Data String and contain the information of the measurement requested The Data quantity is determined by the Data Length byte and data is interpreted depending on the kind of measurement For example for the MED EKG Demo from Data 2 to Data n 1 contains the data graphed Every point in the graph is represented by a 16 bit signed number this means that every 2 data bytes in the packet means it is one point in the graph The first byte 1s the most significant part of the long 16 bits and the second byte is the less significant part The long is signed using 16 bit complement The last byte contains the Heart Rate measurement This byte must be taken as it is an unsigned char data that contains the number of beats per minute
27. ndow we look for the lowest and highest sample of the ECG signal by constantly updating two registers that contain the current minimum and maximum values Once the time window is over the difference between those values determines the maximum peak to peak amplitude of the signal during that period The software checks if the amplitude value is between the proper ranges if it is not the gain of the internal amplifier is changed either to increase or decrease it Figure 23 is a graphical representation of this algorithm Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 23 hee Getting started and running the MED EKG demo Amplitude Max Min Initialize Max and Min Figure 23 Automatic gain control flow chart 5 Getting started and running the MED EKG demo This section explains how to use the demo and the graphical user interface GUI Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 24 Freescale Semiconductor Inc a Getting started and running the MED EKG demo The GUI communicates with the microcontroller via USB port using the Freescale USB stack which performs an algorithm that creates a virtual serial port to send and receive data For further reference and information about Freescale USB stack with PHDC please visit freescale com 5 1 Hardware settings and connections The contents of thi
28. nnn 15 Getting started and running the MED Comparison between Freescale merotan o BER arent ed ede eei pe e e otro dex e opis 34 Communication protocol sess 33 e oS gt freescale Heart rate and electrocardiograph fundamentals 2 Heart rate and electrocardiograph fundamentals This section provides an introduction to the basic concepts of heart activity in order to explain the development of the electrocardiograph demo 2 1 Heart functional description The heart is located in the middle of the thorax positioned slightly on the left and surrounded by the lungs Its function is to pump blood to all parts of the body so that the organs receive oxygen The heart can be divided into four chambers two upper atria right and left and two lower ventricles right and left Atria receive blood coming respectively from the tissues or lungs through the venous system and ventricles eject blood towards the tissues or lungs Figure 1 shows the cardiac conduction system with the structure just mentioned Left common carotid artery Left subclavian artery _ Aorta Brachiocephalic artery superior vena cava Left pulmonary Right pulmonary arteries arteries Right nul _ Left pulmonary i ulmonary veins E i om p ry 8 M a veins Left atrium Right atrium Po 4 semilunar valves i _ Atrioventricular s mitral valve Atrioventricular E e tricuspid
29. ocardial cells that accompanies the blood flow The electrical impulse starts in the sinoatrial node and flows first through the atria reaching the atrioventricular node and generating the atrium contraction After that there 1s a brief pause and the current flows through the His bundle towards the ventricles generating their contraction Finally current arrives to the Purkinje fibers and repolarization of the heart tissue occurs The following sequence represents electrical activity during a heartbeat It is depicted in Figure 3 L P E P Eu Freescale Semiconductor Inc Atrium begins to depolarize Atrium depolarizes Ventricles begin to depolarize at apex Atrium repolarizes Ventricles depolarize Ventricles begin to repolarize at apex Ventricles repolarize Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Heart rate and electrocardiograph fundamentals w a Figure 3 Electrical activity in myocardium 2 3 Heart signals and the QRS complex Figure 3 represent the voltage signals generated during heartbeat The sequence of pulses comprise the typical heart signal shown in Figure 4 Figure 4 Electrocardiogram signal Heart muscles generate different voltages in the order of hundreds of microvolts This signal has very low amplitude and an important amount of electrical noise The QRS complex is usually the central and largest part of the electrocardi
30. of the process depicted in Figure 19 It is one of the possible states in the state machine and its name is StateMeasuring This function is called after a request from the host device to start ECG measurement As the flow chart in Figure 19 shows execution departs from the function through the blocks labeled Return The purpose of this is to perform some tasks related to USB communication while waiting for another lapse to sample the signal It is a periodic routine therefore the system always comes back to it while the host device does not send a request for abortion The basic algorithm consists of the following e While the current state is State Measuring the software waits for the timer interrupt routine to indicate when a new sample of the signal must be taken Once the time for a new sample has passed the microcontroller performs two analog to digital conversions one of them is merely to adjust the baseline and the other is the ECG signal itself The software calls a routine that handles the baseline adjustment That routine is PerformControlAlgorithm The successive samples are compared in order to detect when a big variation occurs by checking the difference between samples against a predefined threshold Heart rate is calculated with each new pulse detected in the base of a counter incremented with each sample taken between successive peaks This value is called Real Time Heart Rate because it is obtained imme
31. ogram signal The P wave represents the atrium contraction while the QRS complex and T wave represent the actions of the ventricles The higher amplitude of the QRS complex compared with the P wave is due to the fact that the ventricles contain more muscle mass than atria Actually there is one atrium repolarization wave that resembles an inverse P wave but it is overlapped by the QRS wave The electrocardiogram signal is useful in diagnosing cardiac arrhythmias conduction abnormalities ventricular hypertrophy myocardial infarction electrolyte derangements and other heart diseases Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 4 Freescale Semiconductor Inc Heart rate and electrocardiograph fundamentals 2 4 Biological electrical potentials As mentioned previously the heart generates potentials that can be expressed as vector quantities meaning that it is possible to assign a numeric value with its respective sign to those quantities The heart can be represented as a dipole located in the thorax with a specific polarity at a certain moment and an inverted polarity the next moment The potential in a specific moment depends on the amount of charge and the separation between charges When we refer to potentials we are actually talking about potential differences therefore heart signals must be acquired with a reference point which is used to determine significant variations from the p
32. oints whose potentials we want to record An example of how we can acquire signals is shown in Figure 5 This example uses an operational amplifier since its principal function is to obtain the potential difference between two signals and amplify it m m Heference electrode Vo Electrically unrelated tissue Muscle L Mr 34 n V Son Vo V4 V 4 n S2 n 7 S4 S2 Figure 5 Signal acquisition A lead is one pair of electrodes or a combination of electrodes In cardiology there are three basic leads at 0 Lead I 60 Lead II and 120 Lead III Figure 6 is a representation of those leads that together form an imaginary figure known as Einthoven s Triangle Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 5 AEE EEE Heart rate and electrocardiograph fundamentals LL Left Leg 7 l B Figure 6 Einthoven s Triangle The three electrodes for measuring the electrical signals are placed on the limbs one on the left arm LA one on the right arm RA and the last on the left leg LL The reason for connecting electrodes that way is to obtain the bio potentials coming from the chest more specifically from the heart 2 5 Heart rate ranges Heart rate varies depending on several factors such as age gender metabolism genetics physical activities temperature and so on One of the most common situati
33. ons in which heart rate is measured is during exercise Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 6 Freescale Semiconductor Inc Hardware description Figure 7 correlates average heart rate with physical condition and age Figure 7 200 HEART RATE BEATS PER MINUTE 5 80 20 25 30 35 40 45 50 55 60 65 AGE S Recovery Zone 5 Target Heart Rate Zone Fat Burning Zone Bi Anaerobic Threshold Zone Exercises zones 3 Hardware description This section describes how to implement an EKG system acquire signals use instrumentation amplifiers filtering and the digital signal controller When it comes to measuring heart rate or displaying an electrocardiogram the first consideration is the acquisition of an accurate noiseless signal with enough amplitude to be quantified The Kinetis K53 ColdFire MCF51MM and S08MM128 offer many advantages in this regard The features of Kinetis K53 are listed below Ultra low power operation e 2 operational amplifiers OPAMP e 2 transimpedance amplifiers TRIAMP e 2x 12 bit DAC 2 x 16 bit SAR ADC up to 31 channels with programmable gain amplifiers PGA Inter integrated circuit I7C USB connectivity e ARM Cortex M4 core with digital signal processor DSP instructions The MCF51MM and SO8MM128 have the following features Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications R
34. ormation on Freescale s Environmental Products program go to http www freescale com epp Freescale and the Freescale logo are trademarks of Freescale Semiconductor Inc All other product or service names are the property of their respective owners 2013 Freescale Semiconductor Inc a LLI on LU c a a gt J K 4 P freescale v
35. plications Rev 1 04 2013 12 Freescale Semiconductor Inc Hardware description Filter implementation is shown in Figure 12 Vrefi amp V Nd 12 2 2K n Vrefl 56V BandPass Filter 5 250 Hz C8 To notch filter From internal amplifier R15 29 4K 0 33UF INTERNAL OPAMP 2 C10 v 22nF 10M Vrefl 6V Figure 12 Band pass filter from 0 5 Hz to 250 Hz After band pass filtering an extra internal operational amplifier adjusts the signal to better levels to be sent to the microcontroller s ADC Developers may configure the MED EKG board s J2 and J5 jumpers to select one of the following options EMEN Negative input of the amplifier is connected directly to Vref 1 6 V and gain is configured by software Amplifies the signal with a gain of 69 Gain of the signal 1 Amplifier acts like a buffer 3 4 2 Band pass filter 0 5 Hz 153 Hz The limit frequencies for the first band pass filter the one located after the internal instrumentation amplifier configuration are 0 5 and 250 Hz correspondingly The values for capacitors and resistors and the correspondent process to obtain the cutoff frequencies are described below High limit 250 Hz C 22 nF R 29 4 KQ 1 1 f aR m4m 123 0Hz As can be seen in Figure 13 there is another external OPAMP and a low pass filter that condition the signal with more appropriate levels to connect to the DSC The low pass filter
36. propriate register The following flowchart Figure 22 depicts the gain control Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 22 Freescale Semiconductor Inc Software architecture 5W2 pressed gt Turn on D10 Increase gain C SW2 and SW4 Ye released 7 in Turn LEDs off NE pe as End j Figure 22 Gain control flowchart The microcontroller is continuously checking for the state of the buttons since it is part of an infinite loop so the gain is updated at any time when the user presses one of the buttons 4 5 5 Automatic gain algorithm Automatic gain control AGC is an alternative to the buttons mentioned in Gain control The algorithm consists of checking the amplitude of the ECG signal constantly to change the gain of the internal amplifier in real time if necessary This offers a convenient alternative to pushing a button to maintain the signal between appropriate levels The basic algorithm consists of the following There is a counter in the software that is incremented each time a sample of the ECG signal is taken This counter serves to maintain a register of the time elapsed If we consider a minimum heart rate of 40 bpm a pulse should occur at least every 1 5 seconds A window time of 1 2 seconds 50 bpm is acceptable to detect a heartbeat since heart rate is generally higher than 40 bpm Within the time wi
37. re 29 shows an example of how to do this if you are using the TWR MCF51MM For the TWR SO8MM128 the target must be HCS08 FSL Open Source BDM Those options are placed on the left side of the CodeWarrior screen Projects title Ecg for MCF51MM mcp AJD CF71 FSL Open Source BDM db o g g Files Link Order Targets 1 Make sure that the selected target is CFYT FSL Open source BOM v File Code Data We Ela Sources 15412 3344 m 2 Click an the Debug icon BE tpm c 410 45 mj Libraries E main c 602 13 mi ES app 5676 1185 m E a Med ERG 5244 556 mi Figure 29 CodeWarrior programming interface 6 When the microcontroller has been programmed refer to Graphical user interface for the MED EKG for instructions about the connection of the electrodes and the graphical user interface 5 2 Graphical user interface for the MED EKG The graphical user interface shows the EKG results and the corresponding graph generated when performing a test To run a test using the GUI follow the steps explained below Follow the steps below to perform ECG measurement 1 After the microcontroller is programmed and the TWR SER module is connected to the computer via USB as shown in Figure 30 the next step is to connect the electrodes Figure 31 and Figure 32 are an example of the correct way to do this Freescale Solutions for
38. s filter The amplifier provides a gain of 3 3 Notch Filter 60a WreflGV VA hes Tow Pass Filter RT R18 1 0K 3 3K PM I y Duc Gain 3 3 us EOS HR Te ECG ADC Fram internal C23 o E Figure 14 Notch filter The microcontroller uses the signal from the notch filter as a reference upon which to set a baseline level The microcontroller does this by sending a feedback signal from the output of the internal DAC to the high pass filter Finally the signal resulting from all these blocks is sent to the microcontroller for sampling and measurement Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 14 Freescale Semiconductor Inc Software architecture 3 5 Digital signal controller DSC Once the DSC receives the signal through one of its ADCs it takes samples of the signal and makes conversions from the analog values to digital values These values are used to perform a digital filtering that sends an improved signal to the microcontroller through the I2C bus The DSC also calculates heart rate by itself and sends the value to the MCU As mentioned before the DSC also gives back a PWM signal to the circuit to establish a baseline on the electrocardiogram An advantage of this DSC is that despite the fact that it has been factory programmed for digital filtering it is possible to reprogram it with the external JTAG ONCE interface using the connector
39. s section are intended as a guide in the implementation and operation of the MED EKG demo Instructions are provided for both the Kinetis K50 and Flexis MM tower modules 5 1 1 MED EKG demo with Kinetis TWR K53N512 The following steps have to be completed in order to run the MED EKG demo in the TWR K53N512 1 Download the MEDICAL GUI from freescale com Follow the instructions included in the installer s ZIP file for GUI installation 2 Download the IAR Embedded Workbench from the IAR Systems website iar com The Kinetis demo software was developed using this platform Install it on your computer 3 Assemble the tower system with the elevators TWR SER TWR K53 and the MED EKG analog front end Be sure to match the primary and secondary sides of the microcontroller board with the respective elevators Refer to the jumper setting file to configure the necessary jumpers Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 Freescale Semiconductor Inc 25 Ld Getting started and running the MED EKG demo TWR SER TWR K53N512 MED EKG AFE Figure 24 Parts of the tower to be assembled 4 Download and unzip AN4323 zip file Open the AR system and search for the project in the software folder The name is USB_CDC eww 5 Connect the microcontroller board to the computer using a USB cable The computer must recognize the Open Source BDM debug port Install the driver for the OSBDM in the IAR
40. sh 49 89 92103 559 German 33 1 69 35 48 48 French www freescale com support Japan Freescale Semiconductor Japan Ltd Headquarters ARCO Tower 15F 1 8 1 Shimo Meguro Meguro ku Tokyo 153 0064 Japan 0120 191014 or 81 3 5437 9125 support japan freescale com Asia Pacific Freescale Semiconductor China Ltd Exchange Building 23F No 118 Jianguo Road Chaoyang District Beijing 100022 China 86 10 5879 8000 support asia freescale com Document Number AN4323 Rev 1 04 2013 Information in this document is provided solely to enable system and software implementers to use Freescale Semiconductors products There are no express or implied copyright licenses granted hereunder to design or fabricate any integrated circuits or integrated circuits based on the information in this document Freescale Semiconductor reserves the right to make changes without further notice to any products herein Freescale Semiconductor makes no warranty representation or guarantee regarding the suitability of its products for any particular purpose nor does Freescale Semiconductor assume any liability arising out of the application or use of any product or circuit and specifically disclaims any liability including without limitation consequential or incidental damages Typical parameters that may be provided in Freescale Semiconductor data sheets and or specifications can and do vary in different applications and actual performan
41. utions for Electrocardiograph and Heart Rate Monitor Applications Rev 1 04 2013 20 Freescale Semiconductor Inc Software architecture Afterwards the microcontroller decides between keeping the signal on the same level or modifying the voltage delivered to the MED EKG board through the DAC output Such process correspond to the function PerformControlAlgorithm and can be observed in Figure 20 B Control algorithm i oe jum A count amp samples _ T E t s pa Ze No Store sample in array Samples count 0 lt median lt high limit T m m s Si e N I T Return jw k ra Figure 20 Baseline control flowchart Below is a brief explanation of the algorithm Each sample of the baseline signal is stored in an array Once the determined quantity of samples have been stored in the array the software orders the values within the same array in ascending order and obtains the middle or median value The median of the values is compared with several ranges to determine whether or not it is necessary to change the feedback voltage and how much to change it 4 5 3 Heart rate It was already mentioned in State measuring that heart rate is calculated with each sample of the ECG signal That value corresponds to a real time heart rate but it 1s not reliable because eventually some pulses take less or more time to occur and the measurement could change dr
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