Freescale Solutions for Electrocardiograph and Heart Rate Monitor
Contents
1. CFM STOP Length 0x00 Figure B 9 Stop CFM packet The CFM packet for stop does not require an error code it only has to be received If this packet has not been received the request has been rejected or not taken and must be sent again in order to stop the measurements Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 39 How to Reach Us Home Page www freescale com Web Support http www freescale com support USA Europe or Locations Not Listed Freescale Semiconductor Inc Technical Information Center EL516 2100 East Elliot Road Tempe Arizona 85284 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 English 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 For Literature Requests Only Freescale Semiconductor Literature Distribution Center2. Freescale Semiconductor Hardware description 12 bit digital to analog converter DAC Inter integrated circuit PC Universal serial bus USB connectivity Multiply accumulate unit MAC only in MCF51MM ColdFire V1 and 508 cores respectively In addition the MED EKG board counts with the MC56F8006 16 bit digital signal controller DSC which performs the digital filtering of the signal Main DSC features are 3 analog comparators ACMP 2 x 12 bit ADC 6 PWM outputs PC JTAG ONCE for programming The DSC communicates with the microcontroller through the PC 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 0 Freescale Semiconductor Hardware description Host PC with MED EKG GUI USB DSC ADC gt MICROCONTROLLER MODULE 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 26527 for reference Optional block digital filtering can be performed internally Figure 8 MED EKG system block diagram Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 9 Hardware description 3 2 Signal acquisitio
3. 1 800 441 2447 or 303 675 2140 Fax 303 675 2150 LDCForFreescaleSemiconductor hibbertgroup com Document Number AN4323 Rev 0 06 2011 Information in this document is provided solely to enable system and software implementers to use Freescale Semiconductor 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 and all 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 performance 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 f
4. Rev 0 18 Freescale Semiconductor Software architecture Timer interrupt Y Decrement count Count 0 Yes New sample TRUE Y Remove timer Cr D 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 initialization This part of the software 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 C Peripherals initialization Y Initialize DAC Y Initialize OPAMPs and TRIAMPs Y Initialize ports Y Initialize ECG Timer End Figure 18 ECG initialization flow chart Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 19 Software architecture 4 5 Measurement and conditioning The software described in this section is the most important of the project because it 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
5. Rev 0 Freescale Semiconductor 29 Getting started and running the MED EKG demo Project s title Ecg for MCF51MM mcp 1 FSL Open Source Files Link Order Targets 1 Make sure that the selected target is CFV1 FSL Open Source 3 Sources af 2 Click on the Debug icon B 410 48 m Libraries B main c 602 13 m app 5875 1185 m 5244 855 m Figure 29 CodeWarrior programming interface 6 When the microcontroller has been programmed refer to Section 5 2 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 heart signal can be monitored using the MED EKG GUI which shows both the signal through time and the heart rate of the person The GUI is shown in Figure 31 with an example of the signal displayed when the demo is running Follow the steps below to perform ECG measurement 1 Once the microcontroller is programmed and the TWR SER module is connected to the computer via USB the next step is to connect the electrodes Figure 30 is an example of the correct way to do this In this case we use the Welch Allyn ECG Lead Wires although it is possible to use the embedded electrodes or build your own leads as mentioned before Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applicat
6. 0 28 Freescale Semiconductor 2 4 Getting started and running the MED EKG demo Ensure that the host computer has CodeWarrior v6 3 and the latest service pack for MCF51MM256 1f necessary Otherwise download and install them Connect the modules of the tower system elevators TWR SER TWR MCF51MM256 or TWR S08MM128 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 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 Mini USB cables Tower System Figure 28 USB connections Open CodeWarrior v6 3 and program the microcontroller with the ECG for MCF51MM or ECG for S08MM project included in the compressed folder Figure 29 shows an example of how to do this if you are using the TWR MCF51MM For the 508 128 the target must be HCS08 FSL Open Source BDM Those options are placed in the left side of the Code Warrior screen Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications
7. General Options Debugger selection C C Compiler Assembler Output Converter Custom Build Build Actions i Run to Setup macros CI Use macro file s Device description file C Overide default TOOLKIT_DIR CONFIG debugger Freescale iok 4 Third Party Driver Figure 25 Selecting appropriate debugger 7 To load the microcontroller with the project click on the Download and debug icon The screen of Figure 26 shows the location of the icon Embedded Workbench IDE File Edit View Project Tools Window Help Dae Ae s 5n 4 Y X x E 9 GHW BD Flash Debug 40 Files CDC Flash Debug Ka0 E app class driver Ha G MED_EKG 5 Mie Drivers O startup B readme tt E Output Figure 26 IAR screen 8 After programming the Kinetis K53 refer to Section 5 2 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 with Flexis MM The steps for the TWR MCF51MM and the TWR S08MM 128 are listed below 1 Download the software for TWR MCF51MM TWR S08MM128 from www freescale com unzip the folder and install the included GUI Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev
8. Monitor Applications Rev 0 6 Freescale Semiconductor 3 Hardware description 2 8 8 HEART RATE BEATS PER MINUTE 120 8 80 20 25 30 35 40 45 50 55 60 65 Recovery Zone Target Heart Rate Zone Fat Burning Zone lll Anaerobic Threshold Zone Figure 7 Exercise zones Hardware description 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 K50 and the TWR MCF51MM and 508 128 from Flexis MM family offer many advantages in this regard The features of Kinetis K50 and some of the peripherals in its integrated measurement engine are Ultra low power operation 2 operational amplifiers OPAMP 2 trans impedance amplifiers TRIAMP 2 x 12 bit DAC 2 x 16 bit SAR ADC up to 31 channels with programmable gain amplifiers PGA Inter integrated circuit PC USB connectivity ARM Cortex core with digital signal processor DSP instructions TWR MCF51MM and TWR S08MM128 provide the next useful features and peripherals Ultra low power operation 2 operational amplifiers OPAMP 2 trans impedance amplifiers TRIAMP 16 bit SAR analog to digital converter ADC 4 differential channels and up to 12 external single ended channels Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0
9. Rem dad aw ace T 3 4 system implementation 8 32 Signal EY REOS g 3 3 Instrumentation 10 di PINGING essere e eres Rx Ry RA ER ER Ra 12 3 5 Digital signal controller 08 15 4 Software 15 41 Freescale USB 16 AD Stateimachines c2 c05h2eh2s bax 18 4 3 Timer configuration and functioning 18 44 ECGinilialization xen LR 19 4 5 Measurement and conditioning 20 5 Getting started and running the MED EKG demo 26 5 1 Hardware settings and connections 26 5 2 Graphical user interface for the MED EKG 30 6 Reference 32 Appendix A Comparison between Freescale microcontrollers 33 Appendix B COMMUNICANON DOC scs aseo oeste x oer peor d Rie 34 2 z freescale semiconductor Heart rate and electrocardiograph fundamentals 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 2 He
10. 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 limit frequencies these are described in the next section 3 4 1 Band pass filter 0 5 Hz 250 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 cut frequencies are described below e High limit 250 Hz C 22nF 294 1 1 246 06Hz 2 2 294 22 7 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 12 Freescale Semiconductor Hardware description e Lowlimit 0 5 Hz C 0 33F R 1M 1 1 0 482Hz 2mRC 1 0 0 33 f Filter implementation is shown in Figure 12 Vref1 6V R17 R12 Vref1 6V To notch filter From internal amplifier INTERNAL OPAMP 2 c a ee a Vref1 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 J2 J5 Operation 1 2 Negative input of the amplifier is connected direc
11. 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 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 below 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 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 Oncethe 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 e The successive samples are compared in order to detect when a big variation occurs by checking the difference between samples against a predefined threshold e Heartrate is calculated with eac
12. 2 bit DAC Timers 2x4 channel TPM with PWM 2 x 4 channel TPM with PWM 3 x 12 channel Timer Communication Interfaces 2 x SCI 2 x SPI USB Device 2 2 x SCI 2 x SPI USB Device Host OTG 2 MiniFlexBus 6 x SCI 3 x SPI USB OTG Ethernet Secure Digital Host Controller SDHC FlexBus PRACMP 2 x OPAMP 2 x TRIAMP PRACMP 2 x OPAMP 2 x TRIAMP 3 x ACMP 2 x OPAMP 2 x TRIAMP 2 x Programmable Gain Amplifier PGA Random Number Generator RNG LCD Driver Touch Sensing Interface Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 33 Communication protocol Appendix B Communication protocol The application communicates with the GUI on the PC using the Freescale USB Stack with PHDC with the device acting as a communication device class CDC The device communicates via a serial interface similar to the RS232 communications standard but emulating a virtual COM port Once the device has been connected and a proper driver has been installed the PC will recognize it as a Virtual COM Port and it will be accessible for example using HyperTerminal Communication is established using the following parameters Bits per second 115200 Data bits 8 Parity None 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 the
13. 4 2 State machines The software for the MED EKG includes the functionality of state machines These state machines provide the demo with some kind of memory meaning that the device can switch from one task to another remembering the state it was in before the change or the next state to be taken up This type of execution emulates parallelism and ensures that the process follows a well defined sequence of steps or states It is this state machine 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 gt State 1 State 1 me N N State 2 State 2 1 Task E N State 3 State 3 Task amp 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 The timer has to be forced to generate interrupts each millisecond depending on the bus clock frequency Figure 17 contains a flow chart explaining the basic functioning of the timer Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications
14. ART_MEASUREMENT X X 0x06 BPM MEASUREMENT X X 0x07 BPM MEASUREMENT COMPLETE OK X 0x08 BPM MEASUREMENT ERROR X 0x09 BPM START LEAK TEST X X BPM LEAK TEST X X 0x0B BPM_LEAK_TEST_COMPLETE X 0x0C BPM_SEND_PRESSURE_VALUE_TO_PC X 0x28 Electro Cardiograph Opcode ECG_HEART_RATE_START_MEASUREMENT X X 0x0D ECG_HEART_RATE_ABORT_MEASUREMENT X X OxOE ECG HEART RATE MEASUREMENT COMPLETE OK X OxOF ECG HEART RATE MEASUREMENT ERROR X 0x10 ECG DIAGNOSTIC MODE START MEASUREMENT X X 0x12 ECG DIAGNOSTIC MODE STOP MEASUREMENT X X 0x13 ECG_DIAGNOSTIC_MODE_NEW_DATA_READY X 0x14 Thermometer TMP_READ_TEMEPRATURE X X 0x15 Height scale HGT_READ_HEIGHT X X 0x16 Weight scale WGT_READ_WEIGHT X X 0x17 Spirometer Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 36 Freescale Semiconductor Communication protocol SPR DIAGNOSTIC MODE START MEASURMENT X X Ox1C SPR DIAGNOSTIC MODE STOP MEASURMENT X X Ox1D SPR DIAGNOSTIC MODE NEW DATA READY X Ox1E Pulse Oximetry POX START MEASURMENT X X 0 21 ABORT MEASURMENT X X 0x22 POX MEASURMENT COMPLETE OK X 0x23 POX MEASURMENT ERROR X 0x24 POX_DIAGNOSTIC_MODE_START_MEASURMENT X X 0x25 POX DIAGNOSTIC MODE STOP MEASURMENT X X 0x26 POX DIAGNOSTIC MODE NEW DATA READY X 0x27 0x21 System commands SYS_CHECK_DEVICE_CONNECTION X X 0x29 SYS R
15. ESTART SYSTEM X Ox2A Data Length and Data String The data length and data string bytes are the data quantity count and the data itself In other words the data length byte represents the number of bytes contained in the data string The data string 1s the information to be sent just the data therefore the data length byte must not count the packet type byte the command opcode byte or itself Functional Description Communication starts when the host sends a REQ packet indicating that the device should start a new measurement The host must send a REQ packet type to start transactions Figure B 3 REQ START Figure B 3 Start packet sent by host The start opcode can be any opcode related with start a measurement For example if we wanted to start ECG in diagnostic mode the data packet will look like Figure B 4 REQ START 0x52 0x12 Figure B 4 Starting ECG in diagnostic mode Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 37 Communication protocol 0x52 is the HEX code for a 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 Length 0x01 Figure B 5 Confirmation packet structure CFM START Error The error byte will indicate the device status The table below
16. Freescale Semiconductor Application Note Document Number 4323 Rev 0 06 2011 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications by Jorge Gonz lez Guadalajara M xico 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 K53 module a member of the Kinetis K50 microcontroller family or in the Flexis MM family s TWR MCF51MM or 508 128 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 Freescale Semiconductor Inc 2011 All rights reserved Contents 1 Introduction iii 1 2 Heartrate and electrocardiograph fundamentals 2 2 1 Heart functional description 2 2 2 Electrical activity of the 3 2 3 Heart signals and the QRS complex 4 2 4 Biological electrical potentials 5 4 5 Hbartrate Taltjes oci erem Ee PR ot dd bet SETA 6 3 Haidwaiedesernplioisz siue er
17. MED EKG board Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 11 Hardware description From right electrode To band pass filter and 05 ADC From left electrode DscPwmOutput R38 from 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 c r R33 402 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 Again the output of this 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 good noise rejection ratio it is still necessary to filter some noise from the signal Some causes for noise are electric installation muscle contractions respiration electromagnetic interference and electromagnetic emissions from electronic components A band pass filter helps in attenuating those sources of noise A low limit rejects signals caused by respiration and muscle contractions A high limit avoids the pass of electromagnetic high frequency interferences As can
18. art rate and electrocardiograph fundamentals This section provides an introduction 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 l Left pulmonary Right pulmonary arteries arteries Left pulmonary veins Left atrium Right pulmonary veins Right atrium Semilunar valves Atrioventricular mitral valve Atrioventricular tricuspid valve Left ventricle Chordae tendineae Septum Right ventricle Inferior vena cava Figure 1 Cardiac conduction system Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 2 Freescale Semiconductor Heart rate and electrocardiograph fundamentals The cardiac cycle is described next and co
19. e feedback voltage and how much to change it Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 22 Freescale Semiconductor Software architecture 4 5 3 Heart rate It was already mentioned in Section 4 5 1 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 is not reliable because eventually some pulses take less or more time to occur and the measurement could change drastically 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 Calculate heart rate median Y Copy HR array Y Order data of copy No Median counter 4 Obtain median HR sum HR sum median Calculate average Increment median counter current heart rate HR sum 0 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 microcontro
20. et 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 they contain the information of the requested measurement Data quantity is determined by the data length byte and data is interpreted depending on the kind of measurement specified For example for MED EKG demo Data 2 to Data n 1 contains the data to be graphed Each point on the graph is represented by a 16 bit signed number This means that each of the 2 data bytes in the packet is one point in the graph The first byte is the most significant part of the long 16 bits and the second byte is the less significant part The sign of the long data is set using one s complement The last byte contains the heart rate measurement This byte must be taken as it is an Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 38 Freescale Semiconductor Communication protocol unsigned char data that contains the number of beats per minute Figure B 7 shows a typical MED EKG demo indication packet IND M READY Length Packet ID Packet ID 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 Table B 8 shows the stop command structure REQ STOP Figure B 8 Stop command structure Immediately after this the device must acknowledge with a CFM packet like the shown in Figure B 9
21. h new pulse detected in the base ofa counter incremented with each sample taken between successive peaks This value is called Real Time Heart Rate because it is obtained immediately 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 0 20 Freescale Semiconductor Software architecture C State measuring New sample ready No New sample FALSE Y Read ADC feedback value Read ADC ECG signal value Start timer Control algorithm Yes Time without pulses 2 5s Y Heart rate 0 No Peak detected Valid pulse conditions No Calculate and store new real time heart rate in HR array Y Calculate heart rate median No Y Yos Store current heart rate in ECG buffer Serial communication new data ready Figure 19 Measurement flow chart Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 21 Software arch
22. imit Increase gain Amplitude gt high limit Decrease gain Return lt Figure 23 Automatic gain control flow chart Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 25 Getting started and running the MED EKG demo 5 Getting started and running the MED EKG demo So far this application note has analyzed the general implementation and functioning of the MED EKG demo with the advantages of Freescale MCUS This section explains how to use the demo with the help of the graphical user interface GUI You can find the MED EKG GUI in the same folder as the software from Freescale s website Previous sections have mentioned that 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 www freescale com 5 1 Hardware settings and connections The contents of this section are intended as a guide in the implementation and operation ofthe MED EKG demo Instructions are provided for both the Kinetis K50 and Flexis MM tower modules 5 1 1 MED EKG demo with Kinetis K50 The following steps have to be completed in order to run the MED EKG demo in the TWR K53 1 Download the software for the demo from the website It co
23. ions Rev 0 30 Freescale Semiconductor Getting started and running the MED EKG demo These sides connected to the patches These connectors are for the MED EKG board J12 E n 2 oil a f o white electrode MED EKG Board Figure 30 Electrode connection 2 Pressthe reset button on the microcontroller board When the demo is used for the first time you have to install the driver for the USB CDC Virtual COM The default path for the driver is c Program Files Freescale MED EKG Driver 3 Goto the device manager in Windows and search for the port assigned to the USB CDC Virtual COM This port assignment is requested by the GUI Open the Graphical User Interface Start All programs gt Freescale MED EKG 5 Select the appropriate COM port number from the list displayed in the menu located on the upper right side of the GUI 6 Press the start button 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 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 31 Reference documents These buttons are used to start Heart Rate value is This box is used to select the port and stop ECG measurement shown at this place corresponding to the Virtual COM Med EKG z freescale F Display Raw Data The raw signal data coming from the microcontroller i
24. is to obtain the potential difference between two signals and amplify it Vo Electrically unrelated tissue S Signal ye 1 Noise V 55 0 n S2 n S4 5 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 0 Freescale Semiconductor 5 Heart rate and electrocardiograph fundamentals 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 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 situations in which heart rate is measured is during exercise when an athlete wants to know how hard he or she is training Figure 7 correlates average heart rate with physical condition and age Freescale Solutions for Electrocardiograph and Heart Rate
25. itecture 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 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 Control algorithm Samples count lt samples Yes Y Store sample in array No Y Samples count 0 Y Order data Y Obtain median Y Low limit median high limit Yes Compensate DAC output 1 6 V offset with DAC Y Figure 20 Baseline control flowchart Below is a brief explanation of the algorithm Each sample of the baseline signal is stored in an array Oncethe 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 th
26. ller 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 appropriate register The following flowchart Figure 22 depicts the gain control Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 23 Software architecture The microcontroller is continuously checking for the state of the buttons since it is part of an infinite loop C Gain control SWA pressed Turn on D9 Decrease gain SW2 pressed Turn on D10 Increase gain a Both SW2 and SW4 released Yes Turn LEDs off Figure 22 Gain control flowchart 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 Section 4 5 4 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
27. mes with the GUI and a jumper setting file Install the GUI on your computer 2 Download the IAR Embedded Workbench from the IAR Systems website www 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 0 26 Freescale Semiconductor Getting started and running the MED EKG demo TWR SER TWR K53N512 MED EKG AFE TWR ELEV Secondary Figure 24 Parts of the tower to be assembled 4 Openthe system and search for the project in the software folder The name 18 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 folder 6 Verify that the selected debugger is micro in the project options panel Project Options Debugger as shown in Figure 25 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 27 Getting started and running the MED EKG demo Options for node USB CDC Category Factory Settings
28. mplemented with Figure 2 which is a scheme of the blood flow in the circulatory system The cycle consists of three principal stages 1 Atrial systole Atria contract and throw 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 2 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 3 Diastole At this point all the parts of the heart are relaxed to allow the arrival of new blood Pulmonary Vein Superior and Inferior Vena Cava Pulmonary Artery Mitral Valve Aortic Valve Tricuspid Valve I Right Pulmonary Valve side I Figure 2 Blood circulation scheme 2 2 Electrical activity of the heart There is some electrical activity in the heart related to depolarization and re polarization of myocardial cells that accompanies the blood flow This electrical activity is the source of our ability to measure the heartbeat with electronic means 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 is a brief pause and the current flows thr
29. n The first step in obtaining a heart signal is the physical interface with human The embedded electrodes of the MED EKG board are one way to achieve this A signal can be detected just by placing the fingers of both hands on the electrodes Figure 9 shows the MED EKG board with the mentioned electrodes 1 Fingertip electrodes 2 External electrodes connector 3 Medical connector Figure 9 MED EKG development board connectors As shown in Figure 9 a connector is provided to use external electrodes because contact is more secure and there is less input noise that with on board electrodes which are very sensitive to any small movement Users can either purchase a three lead cable required for that connector or read AN4223 Connecting Low Cost External Electrodes to MED EKG available at www freescale com 3 3 Instrumentation amplifier As mentioned before signals detected by electrodes have a low voltage in the order of hundreds of microvolts making the amplification of those signals necessary An instrumentation amplifier consists of three operational amplifiers see Section 2 4 Biological electrical potentials two of them are for buffering input signals and the third one is for differential signal gain This kind of amplifier has a very high common mode rejection ratio to eliminate noise and also counts with high accuracy which makes it ideal for instrumentation systems Users can decide to use either the integrated instrume
30. n responds with a confirmation packet indicating to the 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 that the device should stop The following block diagram describes the data flow Host sends start request v Device sends start confirmation Device sends measurement data Yes desea Device sends stop request confirmation Figure B 1 Communication protocol data flow Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 34 Freescale Semiconductor Communication protocol Packets sent between host and device have a specific structure Each packet is divided into four main parts packet type command opcode data length and the data itself The image below shows the packet structure Packet Command Data DataO Dat Type Opcode Length ie l Y Data Packet Figure B 2 Packet structure 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 REQ packet This is a request packet This kind of packet is used by the host to request the device to perform some action such as starting or stopping a measurement A REQ packet is usually composed of 2 bytes packet type and command opcode Da
31. ntation amplifier or the one implemented with the microcontroller s internal amplifiers It is important to mention that reference electrodes are connected to a reference voltage because an acquired signal has negative values and the amplifiers used can t drive negative output signals Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 10 Freescale Semiconductor Hardware description 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 R1 From Right Electrode To the band pass filter O INTERNAL OPAMP 1 R19 From Left Electrode Vref1 6V INTERNAL 100K TRIAMP 1 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 Connections J3 and J4 Gain 1 2 G 100K 100 2 3 G 100K 10 10K The output of the amplifier is sent to a band pass filter 3 3 2 External instrumentation amplifier EL8172FSZ The EL8172 is an instrumentation amplifier implemented as an integrated circuit IC whose gain is configurable through external resistors Figure 11 shows the use of this IC in the
32. o stop a measurement 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 is full the microcontroller sends an INDICATION packet and the data as well Figure 151sa diagram showing the basic functioning ofthe USB protocol and a more detailed explanation can be found in the appendix B at the end of this document Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 16 Freescale Semiconductor Request start measurement Software architecture Serial Communication Protocol request received Evaluate request Request stop measurement Currently measuring Currently measuring No state changes Initialize ADCs State measuring Start timer Stop timer State idle Y Y Send confirmation Send confirmation Serial Communication Protocol new data ready Currently measuring Continue Copy buffer to OutBuffer Send indication ECG data Figure 15 Communication with host PC Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 17 Software architecture
33. o 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 buried inside the QRS wave The electrocardiogram signal is really useful in diagnosing cardiac arrhythmias Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 4 Freescale Semiconductor Heart rate and electrocardiograph fundamentals conduction abnormalities ventricular hypertrophy myocardial infarction electrolyte derangements and other heart diseases 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 For the sake of clarification the heart could 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 points 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
34. 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 There is an extra filter called notch filter that helps to eliminate a specific frequency In this case the frequency is 60 Hz The reason to include this filter is to eliminate any noise related to an electrical power source since electrical installations 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 just high and low pass but with the same cut frequencies of the band pass filter The amplifier provides a gain of 3 3 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 14 Freescale Semiconductor Software architecture 2 Vref1 6V Po Low Pass Filter 10K 33K 250 Hz TP5 ger From internal To ECG ADC OPAMP 0 33UF 0 10UF C9 0 10UF 722077 ONES Baseline pm Figure 14 Notch filter The microcontroller uses the signal coming out of the notch filter as a reference upon which to set a base level The microcontroller does this by delivering a feedback signal from the output of the internal DAC to the previously mentioned high pass filter Finally the signal resulting from all these blocks is sen
35. or surgical implant into the body or other applications intended to support or sustain life or for any other application in which the 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 and hold 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 claim alleges that Freescale Semiconductor was negligent regarding the design or manufacture of the part For information 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 The Power Architecture and Power org word marks and the Power and Power org logos and related marks are trademarks and service marks licensed by Power org ARM Cortex M4 is the registered trademark of ARM Limited Freescale Semiconductor Inc 2011 All rights reserved t 2 freescale semiconductor
36. ough 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 1 Atrium begins to depolarize Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor Heart rate and electrocardiograph fundamentals Atrium depolarizes Ventricles begin to depolarize at apex Atrium repolarizes Ventricles depolarize Ventricles begin to repolarize at apex SOUL YN Ventricles repolarize e Y Figure 3 Electrical activity in myocardium NO f T 2 3 Heart signals and the QRS complex The boxes included in Figure 3 represent 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 electrocardiogram 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 t
37. s shown at this area if the checkbox is selected EKG SIGNAL Figure 31 MED EKG GUI 6 Reference documents In addition to this application note the following documents are available at www freescale com e Reference manuals and data sheets for Kinetis K53 K53P144M 100SF2RM K53P144M 100SF2 51 256 MCF51MM256RM MCF51MM256 and MC9SO8MM128 MC9S08MM128RM MC9S08MM 126 e MED EKG board User Manual MED EKGUG e Schematics of the Tower Modules and MED EKG board e Quick Start Guides USB Stack with PHDC User Guide MEDUSBUG Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 32 Freescale Semiconductor Comparison between Freescale microcontrollers Appendix A Comparison between Freescale microcontrollers The demo described in this document has been implemented with three microcontrollers the 51 256 the 9508 128 and the Kinetis K50 Table A 1 is a brief comparative analysis of those MCUs Flash SRAM Table A 1 Freescale MCU comparison 508 8 bit 48 MHz Core 24 MHz Bus 128 KB Flash 12 KB RAM ColdFire V1 with MAC 8 32 bit compatibility 50 33 MHz Core 25 165 MHz Bus 256 KB Flash 32 KB RAM ARM Cortex M4 with DSP SIMD 72 MHz or 100 MHz 64 512 KB Flash 64 128 KB RAM A D converter 16 bit SAR ADC 16 bit SAR ADC 2 x 16 bit SAR ADC D A converter 12 bit DAC 12 bit DAC 2 x 1
38. shows the possible error codes OK 0x00 BUSY 0x01 INVALID OPCODE 0x02 If the error byte received corresponds to OK the device will start sending data as soon as a new data packet is ready If a BUSY error is received the host must try to communicate later If 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 will start sending information related to the requested measurement This is performed using indication packets IND The indication packet structure is shown in Figure B 6 Length Packet ID Packet ID Figure B 6 Indication packet structure The first byte contains the respective HEX code for an indication packet type The second byte contains the opcode for the kind of indication For example if the device is sending an indication packet for ECG DIAGNOSTIC MODE NEW DATA READY the HEX code read in this position will be 0 14 since this is the indication opcode for a new set of data from ECG diagnostic mode The next byte is the length which indicates the quantity of data to be sent The first couple of bytes after the length byte are the packet ID bytes The packet ID is a 16 bit data divided into 2 bytes to be sent It contains the number of packets to be sent The packet ID number data packet is the packet ID of the previous packet 1 For example if the packet ID of the previous packet sent was 0 0009 the pack
39. t 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 notable feature of the software is that it is conformed using state machines which offers more efficiency by emulating parallelism Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 15 Software architecture NOTE The respective software for TWR K53N512 TWR MCF51MM and 508 128 is available on the Freescale website 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 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 e REQ packet The host sends a REQUEST packet either to start or t
40. t to the microcontroller for sampling and measurement 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 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 embedded in the MED EKG board this way users can program their own applications It is possible to avoid using the DSC since the ARM cortex M4 Kinetis K50 and ColdFire V1 51 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 mos
41. ta length and data packet bytes are not required 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 It sends a response indicating whether the command is accepted or if the device 1s busy IND packet This is an indication packet This kind of packet is used to indicate to the host that an event has occurred in the device and data needs to be sent This is used for example when the device has a new set of data to be sent to the GUI The following table shows the respective HEX codes for each packet type REQ 0x52 CFM 0x43 IND 0x69 Command Opcode The command opcode byte indicates the action to be performed for a REQ packet and the kind of confirmation or indication in case of CFM and IND packet types respectively There are different opcodes for each packet type The following table shows the different opcodes NOTE Software related with this application note does not respond to all of these commands Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 35 Communication protocol Glucose Meter GLU START MEASUREMENT X X 0x00 GLU ABORT MEASUREMENT X X 0x01 GLU_START_CALIBRATION X X 0x02 GLU_BLOOD_DETECTED X 0x03 GLU_MEASUREMENT_COMPLETE_OK X 0x04 GLU_CALIBRATION_COMPLETE_OK X 0x05 Blood Pressure Meter BPM_ST
42. tly to Vref 1 6 V and gain is configured by software 2 3 1 2 Amplifies the signal with a gain of 69 2 3 Open of the signal 1 Amplifier acts like a buffer 3 4 2 Band pass filter 0 5 Hz 153 Hz The other band pass filter is placed at the output of the EL8172FSZ and it has cut frequencies of 0 5 and 153 Hz respectively The low frequency is the same as in the first filter which was obtained in Section 3 4 1 Band pass filter 0 5 Hz 250 Hz We now have the math for high frequency High limit 153 2 22nF R 47K 1 1 2aRC 2z ATKQ Q2nF f 153 9Hz Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 Freescale Semiconductor 13 Hardware description 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 deliver to the DSC The low pass filter values have already been used in a previous filter from Section 3 4 1 Band pass filter 0 5 Hz 250 Hz and the gain of the amplifier is simply calculated with the following equation R35 _ 100K R37 5JIK G 1 20 56 Band Pass Filter 0 5 Hz 153 Hz 5 Los Dunn From external instrumentation R29 47K C19 R30 amplifier 0 33UF C18 To DSC Figure 13 Band pass filter from 0 5 to 153 Hz Resistors R25 and R26 help to maintain the offset
43. to maintain the signal between appropriate levels The basic algorithm consists of the following e 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 Ifwe consider a minimum heart rate of 40 bpm a pulse should occur at least every 1 5 seconds 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 window 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 Freescale Solutions for Electrocardiograph and Heart Rate Monitor Applications Rev 0 24 Freescale Semiconductor Software architecture e 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 4 New sample Min sample Sample Max Y Max sample Automatic gain control Counter time window Amplitude Max Min Initialize Max and Min Amplitude lt low l
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