International Journal of Advances in Science and Technology (IJAST)

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1 Signal detection and FFT calculation using ATmega644 microcontroller D. Sarkar 1, A.Chowdhury 2 1,2 Department of Electronics & Communication Engineering, NIT Agartala, India ABSTRACT: Detection of a signal is often necessary in different kinds of experiment. Previously analog type oscilloscopes were widely used for this purpose. Now digital oscilloscopes are taking their place. But steel they have some limitations and they are very expensive. In this paper an Atmega644 avr microcontroller based signal detection unit is designed, which is interfaced with a 128x64 KS0108 controller based graphic LCD, push buttons, LM741 OPAMP, RS-232 serial port. A fixed point FFT algorithm is also implemented inside the microcontroller for spectrum viewing of the applied signal. The detected signal can be seen on the graphical LCD, if the GLCD (Graphic LCD Mode) is chosen, else in PC mode, a LabVIEW based GUI application is designed for displaying the detected signal on PC screen. The microcontroller embedded software is successfully compiled in WinAVR GNU GCC compiler. The design is successfully simulated in Proteus VSM circuit simulator. Keywords ADC, ATmega644, FFT, WinAVR, GLCD, KS0108, oscilloscope, Proteus, Signal, LabVIEW. I. INTRODUCTION Digital Signal Processing gives a way to access analog signal after converting that to digital signal by means of analog to digital converter. Microprocessors and Microcontrollers can access and process digital signals. This paper represents a scheme of analog signal detection and processing platform based on low cost microcontroller. ATmega644 [1] microcontroller is chosen for the design. The device here can perform real-time signal detection and FFT [2] measurement. There are several other researches has been performed in this field. Real-time FFT calculation can be used in several applications like: speech processing, image processing, bioelectrical impedance measurement, proton magneto meter tuning [3], ECG power line interface removal [4], and data acquisition [5]. [6] Presents a hardware software co-design, which performs FFT calculation based microcontroller and FPGA together. They communicate between each other and that has provided some speed improvement according to the article. In article [7] a multipurpose instrument is proposed which is capable of performing signal processing and measurement. The device uses a microcontroller which communicates with an android platform via bluetooth and uses processing power of the android device. The work done here is quite different from [1], [2]. A 256 point custom FFT algorithm [8],[9],[10],[11] is implemented inside ATmega644 microcontroller which performs FFT calculation by itself and hence it requires less power. The device here can also perform signal measurement. Measured signal and FFT can be viewed in a graphical LCD. The device can also use the processing power of a PC via RS-232 communication. In that case it transfers the signal values continuously to a LabVIEW [12] implemented program, which then measures the FFT for that signal. The work done here is inspired from [13] which is a PIC microcontroller based oscilloscope. II. WORKING PRINCIPLE Basic signal detection using Digital Signal Processing (DSP) is shown in Figure 1. In this method, the signal to be measured or detected is 1 st applied to the input of a differential amplifier for amplification purpose to strengthen the input signal. After that the signal is driven to the input of a sampling circuit to create standard samples of the applied signal. Here Nyquist sampling rate should be maintained or otherwise the signal cannot be detected properly. Then the standard samples from the sampling circuit are applied to the input of an encoder circuit and that translates them into stream of binary numbers. So, this is basically nothing but an Analog to Digital converter (ADC), which converts the analog input values to equivalent digital output. After that the converted digital values are driven to the input of a processing circuitry, which stores the digital values in a wave table array, which to be processed using algorithms to display, measure and control purpose. Fig.1. Basic signal detection using DSP method III. SIGNAL DETECTOR USING ATMEGA

2 ATmega644 is an 8 bit microcontroller from AVR microcontroller s family. It has lots of advanced features and with 64K bytes of In-System Programmable Flash Program memory, the developer gets lots of opportunity to make a good and reliable embedded design. The signal detector designed in this paper uses internal analog to digital converter (ADC) unit of the ATmega644. The internal ADC supports 10 bit size and has 8 multiplexed input ADC channel. For this design channel- 0 (ch0) is chosen as the input ADC channel. Block diagram of the design is shown in Figure 2. The input signal is applied to LM741 [14] based unity gain buffer which has supply voltage +5 volt and -5 volt, so that it attenuates signal containing peak to peak voltage more than +5 volt and thus it saves the microcontroller in case a very large amplitude signal is applied. The ATmega644 is interfaced here with a KS0108 controller based 128x64 size graphical LCD (LGM12641BS1R). It is configured here in 8 bit mode. For the controlling purpose of the signal generator some interrupt driven push buttons are interfaced with the microcontroller. Push button algorithms are written inside the interrupt service routine of external interrupt INT0, INT1 and INT2. In the welcome screen of LCD, signal detector mode or FFT view mode can be chosen using push button. For FFT view, a 256 point FFT algorithm is written, which is based on Butterfly FFT algorithm.there are two modes of operation for the signal detector. The user can switch the signal detector to work in PC mode or in GLCD (Graphical LCD) mode. If the mode chosen is GLCD, then after pressing the detect button the user can see the signal in the LCD. If the amplitude and frequency detect button is pressed after observing the signal on LCD, then the approximate value of frequency and amplitude will be displayed on LCD. The signal detector generally remains in PC mode of operation. In case of FFT view mode, the detector generally remains in GLCD mode of operation. To work with PC mode, it must need to be connected with PC by using a serial RS-232 port. For the design to be controlled by PC, a LabVIEW based Graphical User Interface (GUI) is designed. For PC control, first we need to choose the COM port using the GUI and baud rate should be chosen 9600bps, and then on pressing the read button, it will start plotting the signal in the waveform chart available in the GUI. Fig. 2. ATmega644 based signal detector block diagram IV. HARDWARE S AND INTERFACING A. ATmega644 microcontroller ATmega644 is an 8-bit high performance microcontroller of Atmel s Mega AVR family with low power consumption. Atmega644 is based on enhanced RISC (Reduced Instruction Set Computing) architecture with 131 powerful instructions. Most of the instructions execute in one machine cycle. Atmega644 can work on a maximum frequency of 20MHz. ATmega644 has 64 KB programmable flash memory, static SRAM of 4 KB and EEPROM of 2048 Bytes. The write/erase cycle of flash memory and EEPROM is 10,000 and 100,000, respectively. ATmega644 is a 40 pin microcontroller. There are 32 I/O (input/output) lines which are divided into four 8-bit ports designated as PORTA, PORTB, PORTC and PORTD. ATmega644 has various in-built peripherals like USART, ADC, Analog Comparator, SPI, JTAG etc. Each I/O pin has an alternative task related to in-built peripherals. ATmega644 pin diagram is shown in Figure 3. Fig. 3 ATmega644 pin description B. 128x64 Graphical LCD interface The 128x64 graphic LCD plays an important role in this design. Proteus VSM model of KS0108 controller based 128x64 graphic LCD, LGM12641BS1R is chosen for this purpose. Generally 128x64 size graphic LCD comes with 20 pins. Pin no 19 & 20 is for the back light led anode and cathode. LGM12641BS1R does not require back light, so only 18 no of pins are there. The LCD has 8 data pins, DB0 to DB7. As the LCD here connected is in 8 bit mode, so all the data pins are connected to ATmega644 PORTC pins, PC0 to PC7. The detail interfacing technique is shown in Figure 4. Here CS1 & CS2 are chip select for IC1 and IC2, GND and VCC are pin for ground and supply voltage, V o is the contrast adjust pin, DI is for data/instruction mode selection, R/W & E are read/write selection and enable pin. RST should be connect to supply voltage +VCC and Vout is the negative voltage output from the LCD. 236

3 Fig x64 LCD interface with ATmega644 C. RS-232 Serial interface RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between DTE (data terminal equipment) and DCE (data circuitterminating equipment, originally defined as data communication equipment). It s an asynchronous serial data transmission protocol. In real circuit mode a MAX232 IC (first created in 1987 by Maxim Integrated Products) is required to make interface between RS-232 to TTL logic. In Proteus simulator, one inbuilt virtual COM port model (COMPIM) is available, that has been used to interface with microcontroller serial communication pin, TXD0 and RXD0. The interface is shown in Figure 5. view mode, the device normally stays in GLCD (Graphical LCD) mode. The signal detector also has two modes, PC mode and GLCD (Graphical LCD) mode. For signal detector, the device generally stays in PC mode for viewing output on the LabVIEW GUI window. However, modes can be switched with a specific pushbutton click. There is another loop that checks for X level (time) and Y level (amplitude) adjustment. By pressing push buttons it can be done. For PC mode, the rest of the control is given to the LabVIEW based GUI design. In GLCD mode, if the detect signal button is pressed it will display the applied signal to the 128x64 graphical LCD. After that on pressing frequency and voltage detect button, the voltage and frequency of the applied signal will be displayed on the LCD. On the other hand for FFT view mode, with the pressing of detect push button, it will plot the calculated FFT of the applied input signal samples to LCD or to LabVIEW waveform chart depending on the mode selection. Fig. 6. MCU software programming flowchart Fig. 5. RS-232 interface with ATmega644 V. SOFTWARE DESIGN A. Microcontroller embedded Software Design Microcontroller Software design is the essential and important part of a new embedded design. The embedded software for the signal detector is written in Embedded C language and compiled in WinAVR. WinAVR includes the GNU GCC compiler for the AVR target for C and C++, built for Windows by Eric Weddington. The software written here is interrupt driven peripheral controlled. External interrupt INT0, INT1, INT2 are used for real time push button control, that includes mode selection, X level & Y level adjustment, frequency & voltage detect operation. The programming flowchart for the ATmega644 microcontroller based signal detector is shown in Figure 6. The microcontroller embedded software designed here contains two separate algorithms to work in FFT view mode and Signal detector mode. In FFT B. FFT algorithm implementation in embedded C The Fast Fourier Transform (FFT) was 1st proposed by Cooley and Tukey in FFT is a highly efficient way for computing the Discrete Fourier Transform (DFT) of a finite series and requires less number of computations than that of direct computation of DFT. The FFT is based on decomposition and breaking the transform into smaller transforms and combining them to get the total transform. The DFT of a sequence can be calculated using the formula: Substituting W N =, we have 0 k N-1 (1) 0 k N-1 (2) 237

4 If x(n) is an N-point sequence, where N is assumed to be a power of 2. Decimating the sequence into two sequences of length N/2, where one sequence consisting of the even-indexed values x e (n) and the other of odd-indexed values x o (n) of x(n). X (k) = X e (k) + X o (k) 0 k N/2-1 (4) Now for k N/2, X (k) = X e (k) - X o (k) (5) So, it has seen that calculation of X(k) requires less numbers of multiplication and addition if the main input sequence is decomposed into even and odd indexed sequences and then combining their separate DFT values to get the total transform. Here, each N/2 size sequences further can be decomposed into N/4 size and the computations required now will be much more lesser than the previous case. The FFT algorithm implemented here works on this principle. At first the applied input signal is sampled using ATmega644 internal ADC and stored into an array. The FFT algorithm implemented here takes only 1 st 256 number of samples and passes them to the FFT calculation function, which returns the calculated samples to an output array. From the output array, the FFT is displayed to graphical LCD or send to PC. C. LabVIEW based Graphical User Interface (GUI) Graphical User Interface (GUI) gives an interactive and effective system for an user to easily control and use a design. There are various software s for creating GUI applications. LabVIEW 8.5 is chosen here for the GUI application design, because of its very good data acquisition feature. LabVIEW data acquisition component VISA gives the option to communicate between several devices having serial communication ability, hence it is used here for the design. The application wizard is shown in Figure 7. (3) Fig. 7. GUI application for the function generator D. RS-232 Communication protocol RS232 is an asynchronous serial communication protocol widely used in computers and digital systems. In the ATmega644 signal detector, RS-232 protocol is used for serial communication with the PC based software. It is called asynchronous because there is no separate synchronizing clock signal as there are in other serial protocols like SPI and I2C. The protocol is such that it automatically synchronizes itself. In RS232 there are two data lines RX and TX. TX is the wire in which data is sent out to other device. RX is the line in which other device put the data it needs to send to the device. As there is no "clock" line so for synchronization accurate timing is required so transmissions are carried out with certain standard speeds. The speeds are measured in bits per second. Number of bits transmitted is also known as baud rate. In this design 9600 bps (bits per second) baud rate is chosen. To transmit a single byte we need two extra bits they are start and stop bit. Thus to send a byte a total of ten bits are required so we are sending 960 bytes per second. VI. SIMULATION The simulation of the ATmega644 based signal detector is performed in Proteus VSM circuit simulator, which has wide range of microcontrollers and accessories. The signal applied to the input of the ATmega644 ADC channel is given from Proteus VSM signal generator. Figure 8 shows the VSM signal generator, where a sine wave is selected having 3.40 volt peak to peak amplitude and centre frequency of 3Hz and Figure 9 shows the welcome screen of the graphical LCD. From there scope (signal detection mode) or FFT view mode can be selected using push button click. Figure 10 shows the graphical LCD plot of the signal with detected voltage and frequency displayed on the LCD screen. Similarly 3 other types of waveforms rectangular, sawtooth and triangular are detected and shown in Figure 11, Figure 12 and Figure 13. Figure 14 shows the detected FFT spectrum of an applied input sine wave in FFT view mode. So, this was for the GLCD mode of operation. Now, to test the PC mode of operation a 1 volt peak to peak sinusoidal signal is given to the input ADC channel of ATmega644. Then after pressing the read button on the LabVIEW based GUI application, it starts plotting the signal in the waveform chart and the detected amplitude is approximately equal to 1 volt. The detected signal on the LabVIEW GUI window is shown in Figure 15. Similarly other 3 types of signal mentioned previously is successfully detected and shown in Figure 16, Figure 17 and Figure 18. On the other hand for FFT view mode, Figure 19 shows the detected sine wave FFT spectrum on LabVIEW GUI. The full Proteus simulator window and LabVIEW GUI (PC mode) window during the time of simulation for GLCD mode and PC mode is shown in Figure 20 & Figure

5 Fig. 12. Detected sawtooth wave on graphical LCD Fig 8. Proteus VSM signal generator Fig. 13. Detected triangular wave on graphical LCD Fig. 9. LCD screen at MCU startup Fig. 14. FFT view of sine wave input Fig. 10. Detected sine wave on graphical LCD Fig. 15. Detected sine wave on LabVIEW GUI Fig. 11. Detected rectangular wave on graphical LCD Fig. 16. Detected triangular wave on LabVIEW GUI 239

6 Fig. 17. Detected sawtooth wave on LabVIEW GUI Fig. 18. Detected rectangular wave on LabVIEW GUI Fig. 19. FFT view of sine wave input on LabVIEW GUI Fig. 20. Full Proteus simulator window for GLCD mode Fig. 21. Full simulation windows for PC mode of operation VII. CONCLUSION In this paper, a new kind of signal detection unit is designed using ATmega644 microcontroller, 128x64 size graphical LCD, push buttons and RS-232 for software viewing, that gives a very user friendly interface for the signal detector, which is economic, requires less power and having good stability. Moreover future modification of the signal detector is also possible. VIII. FUTURE WORK The ATmeg a644 microcontroller based signal detector designed here can be interfaced with a high speed external analog to digital converter (ADC) for wide range of signal detection capability and the LabVIEW based GUI application can be added with more application features like, frequency calculation, power spectral density (PSD) calculation, noise & distortion analysis etc. REFERENCES [1] Atmel Corporation, Atmega644 Microcontroller datasheet [online]. Available: [2] Wikipedia, Fast Fourier transform [online]. Available: [3] Tan Chao, Dong Hao-bin, "The study of Proton magnetometer sensor fast tuning algorithm based on FFT," IEEE Electric Information and Control Engineering (ICEICE), 2011 International Conference on, vol., no., pp.3747,3750,15-17 April 2011 [4] F. Shirbani, S.K. Setarehdan, "ECG power line interference removal using combination of FFT and adaptive non-linear noise estimator," IEEE Electrical Engineering (ICEE), st Iranian Conference on, pp.1,5, May [5] F. Bianchi et al, "A multichannel data acquisition system for bolometer detectors based on microcontroller Cortex M3 architecture," Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC),2012 IEEE, pp.1031,1034, Oct Nov [6] N. Govil, S. Roy Chowdhury, "High performance and low cost implementation of Fast Fourier Transform algorithm based on Hardware Software co-design," Region 10 Symposium, 2014 IEEE, pp.403,407, April [7]. R. Katona, D. Fodor, "Texas instruments MSP430 microcontroller based portable multi-purpose instrument for android platforms," IEEE Education and Research Conference (EDERC), th European Embedded Design in, pp.1,5, Sept [8] R.H. Barnett, S. Cox and L. O Cull, Embedded C Programming and the Atmel AVR 2nd Edition, July 5, [9] B. Reh, An Introduction to programming an Atmega microcontroller, July 30, [10] Wavetable Synthesis [online]. Available: _Synthesis. [11] P. R. Babu, Digital Signal Processing, 4 th edition, July

7 [12] National Instruments, LabVIEW Completed User Manual for Everyone, NI-Corporate Headquaters, Austin, TX, [13] S.A. Cholewiak. (2006, August 21). PIC18F2550 KS0108 Graphical LCD Oscilloscope Videos [Online]. Available: /08/21/pic18f2550- ks0108- graphical-lcd-oscilloscope-videos/ [14] Texas Instruments, LM741 OPAMP datasheet [online]. Available: 241