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AVR314 STK500 Size/128 AT90S4414 1982B STK200 CH-1705 - Datasheet Archive

8-bit Microcontroller Features · · · · · · Generation of Sine Waves Using PWM

AVR314 AVR314: DTMF Generator 8-bit Microcontroller Features · · · · · · Generation of Sine Waves Using PWM (Pulse-Width Modulation) Combine Different Sine Waves to DTMF Signal Assembler and C High-level Language Code STK500 STK500 Top-Module Design 260 Bytes Code Size/128 Size/128 Bytes Constants Table Size Use of Lookup Tables Application Note Introduction This application note describes how DTMF (Dual-Tone Multiple Frequencies) signaling can be implemented using any AVR microcontroller with PWM and SRAM. Applications such as phones are using DTMF signals for transmitting dialing information. There are two frequencies added together to generate a valid DTMF signal, a low frequency (fb) and a high frequency (fa). Table 1 shows how the different frequencies are mixed to form DTMF tones. Figure 1. DTMF Generator VCC VCC PD5 GND AVR AT90S4414 AT90S4414 1 2 3 A PB7 PB0 PB4 4 5 6 B 7 8 9 C PB5 * 0 # D PB7 PB6 GND PB3 PB2 XTAL1 PB1 PB0 GND XTAL2 8 MHz 22 pF GND 22 pF GND Rev. 1982B 1982BAVR05/02 1 Table 1. DTMF Tone Matrix fb/fa 1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D The rows of the matrix shown in Table 1 represent the low frequencies while the columns represent the high frequency values. For example, this matrix shows that digit 5 is represented by a low frequency of fb = 770 Hz and a high frequency of fa = 1336 Hz. The two frequencies are transformed to a DTMF signal using equation 1: f ( t ) = A a sin ( 2 f a t ) + A b sin ( 2 f b t ) ) (1) where the ratio between the two amplitudes should be: Ab / Aa = K 0, 7 < K < 0, 9 (2) Theory of Operation Starting from a general overview about the usage of the PWM, it will be shown how the PWM allows to generate Sine Waves. In the next step, an introduction is given in how frequencies that are different from the ground frequency of the PWM can be generated. After closing the theoretical introduction with the DTMF signal itself, the implementation will be described. Generating Sine Waves According to the relation between high level and low level at the output pin of the PWM, the average voltage at this pin varies. Keeping the relation between both levels constant generates a constant voltage level. Figure 2 shows the PWM output signal. Figure 2. Generation of a Constant Voltage Level V VH V AV VL t x 2 y AVR314 AVR314 1982B 1982BAVR05/02 AVR314 AVR314 While equation 3 shows how to calculate the voltage level: xV H + yV L V AV = -x+y (3) A sine wave can be generated if the average voltage generated by the PWM is changed every PWM cycle. The relation between high and low level has to be adjusted according to the voltage level of the sine wave at the respective time. Figure 3 visualizes this scheme. The values for adjusting the PWM can be calculated every PWM cycle or stored in a lookup table (LUT). Figure 3 also shows the dependency between frequency of the ground sine wave and the amount of samples. The more samples (Nc) are used, the more accurate the output signal gets. At the same time the frequency sinks. Equation 4 shows this correlation: f CK / 510 fl f = - = -Nc Nc f: fL T: fCK: Nc: (4) Sine wave frequency (1/T) PWM frequency (fCK / 510) Period of ground sine wave Timer Clock Number of samples (12 in Figure 3) The PWM frequency is dependent on the PWM resolution. For an 8-bit resolution, the Timer TOP value is 0xFF (255). Because the timer counts up and down this value has to be doubled. In dividing the Timer Clock fCK by 510 the PWM frequency can be calculated. According to this coherence a Timer Clock of 8 MHz generates a PWM frequency of 15.6 kHz. Modifying the Frequency Figure 3. Generating a Sine Wave with PWM of the Sine Wave V 1 2 3 4 5 6 7 1/fl 8 9 10 11 12 t T 3 1982B 1982BAVR05/02 Let's assume that the sinusoid samples for adjusting the PWM are not read in a sequentially manner from the lookup table but just every second value. At the same sample frequency an output signal with twice the frequency is generated (see Figure 4). Figure 4. Doubling the Output Frequency (XSW = 2) V 1 1/fl 2 3 4 5 6 7 8 9 10 11 12 t T/2 T In using not every second sample but every third, fourth, fifth. it is possible to generate Nc different frequencies in the range from [1/T Hz . 0 Hz]. Note: for high frequencies it will not be a sine wave anymore. The step-width between samples is specified by XSW. Equation 5 describes this relation: Nc f Nc 510 X SW = f - = -fl F CK (5) How to calculate the actual value with which the PWM has to be adjusted every PWM cycle (Timer overflow) is shown in equation 6. Based on the value of the previous cycle (X'LUT) the new value (XLUT) is calculated in adding the step-width (XSW). X LUT = X'LUT + X SW X'LUT: Adding the Two Different Frequencies to a DTMF Signal old position in lookup table XLUT: (6) new position in lookup table A DTMF signal has to be generated according to equations (1) and (2). Since this is easy to obtain with simple shift register operations a K-Factor of K = 3/4 has been chosen. By using equation (6) the lookup table position of the next value for adjusting the PWM can be calculated as follows: 3 f ( XLUT ) = f ( X LUTa ) + - f ( X LUTb ) 4 (7) with X LUTa = X'LUTa + X SWa X LUTb = X'LUTb + X SWb 3 f ( X LUT ) = f ( X'LUTa + X SWa ) + - f ( X'LUTb + X SWb ) ( 8 ) 4 4 AVR314 AVR314 1982B 1982BAVR05/02 AVR314 AVR314 Implementation of the DTMF Generator In this application a DTMF tone generator is built using one of the 8-bit PWM outputs (OC1A) and a sinusoid table with Nc = 128 samples each with n = 7 bits. The following equations show this dependency and shows how the elements of the LUT are calculated: 2 x f ( x ) = 63 + 63 × sin - 128 x [ 0 .127 ] ( 9) The advantage in using 7 bits is that the sum of the high and low frequency signals fits in one byte. To support the whole DTMF tone set, we have to calculate eight XSW values, one for each DTMF frequency, and place them in a table. To achieve a higher accuracy, the following solution has been implemented: The XSW values calculated after equation 5 need only five bytes. To use all eight bytes to have a lower rounding error, this value is multiplied by eight. The pointer to the lookup table is saved in the same manner. But here two bytes are needed to store the actual value times eight. This means that three right shifts and a module operation with Nc have to be executed before using them as pointers to the sine values in the lookup table. Equation 10 shows the complete dependencies: 1 8 Nc f 510 X LUTa, b = ROUND - X'LUTa, bExt + - (10) 8 F CK XLUTa,b: Current position of element in LUT (actual format) X'LUTa,bExt: Previous position of element in LUT (extended format) Figure 5. Schematics of the STK500 STK500 Top-Module PB4 4 5 6 B 7 8 9 C PB5 * PB7 0 # D PB3 PB1 J700 (Expand2) 1 2 3 A PB2 GND AUXO0 CT6 CT4 CT2 BSEL2 REF PE2 PE0 GND VTG PC6 PC4 PC2 PC0 PA6 PA4 PA2 PA0 GND PB0 GND AUXI0 CT7 CT5 CT3 CT1 NC RST PE1 GND VTG PC7 PC5 PC3 PC1 PA7 PA5 PA3 PA1 GND PB6 GND AUXI1 DATA7 DATA5 DATA3 DATA1 SI SCK XT1 VTG GND PB7 PB5 PB3 PB1 PD7 PD5 PD3 PD1 GND GND AUXO1 DATA6 DATA4 DATA2 DATA0 SO CS XT2 VTG GND PB6 PB4 PB2 PB0 PD6 PD4 PD2 PD0 GND J701 (Expand1) The PWM signal is put out on the OC1A pin (PD5). An additional output filter will help to achieve a good sinusoid. If the PWM frequency is decreased, it can be necessary to implement a steeper filter to obtain a good result. 5 1982B 1982BAVR05/02 The connection with the keypad is shown in Figure 1. The functionality of the keypad determines how the pressed key has to be evaluated. It has to be done in two steps: 1. Determination of the row of the pressed key - define low nibble of PORTB as output/zero value - define high nibble of PORTB as input/pull up - low bit in high nibble determines row 2. Determination of the column of the pressed key - define high nibble of PORTB as output/zero value - define low nibble of PORTB as input/pull up - low bit in low nibble determines column Note: On the STK200 STK200 there are serial resistors between the PORTB header pins and the pins BP5, PB6 and PB7 of the part itself (please see the schematics of the STK200 STK200 for more details). This will cause problems if the keypad is connected to the PORTB header. Figure 6 visualizes the functionality of the routine to detect a pressed key. Dependent on which key is pressed it determines the step width value. The interrupt routine uses this values to calculate the PWM settings for the two Sine Waves of the DTMF tone. The interrupt routine is shown in Figure 7 and Figure 8. The interrupt routine calculates the output compare value for the next PWM cycle. The interrupt routine first calculates the position of the next sample value in the LUT and read the value stored there. The position of the sample in the LUT is determined by the step-width. The step-width itself is determined by the frequency which is to be generated. Combining the sample values of the both DTMF frequencies using formula 7 gives the final output compare value of the PWM. Figure 6. Main Function Main Check Keypad Key Pressed ? No Step-width a,b = 0 Yes Set Step-width a,b According the Pressed Key 6 AVR314 AVR314 1982B 1982BAVR05/02 AVR314 AVR314 Figure 7. Interrupt Service Routine Timer Overflow ISR Timer1_OVF XLUTaEXT = XLUTaEXT ' + XSW OCR_RelVal a = GetSample (XLUTaEXT ) XLUTbEXT = XLUTbEXT ' + XSW OCR_RelVal b = GetSample (XLUTbEXT ) OCR = OCR_RelVal a + 3/4 OCR_RelVal b Return Figure 8. Function "GetSample" GetSample XLUTa,b = (XLUTa,bExt + 4) / 8 XLUTa,b < 128 OCR_RelVal = f(X LUTa,b ) Return 7 1982B 1982BAVR05/02 Atmel Headquarters Atmel Operations Corporate Headquarters Memory 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 487-2600 Europe Atmel Sarl Route des Arsenaux 41 Case Postale 80 CH-1705 CH-1705 Fribourg Switzerland TEL (41) 26-426-5555 FAX (41) 26-426-5500 Asia Room 1219 Chinachem Golden Plaza 77 Mody Road Tsimhatsui East Kowloon Hong Kong TEL (852) 2721-9778 FAX (852) 2722-1369 Japan 9F, Tonetsu Shinkawa Bldg. 1-24-8 Shinkawa Chuo-ku, Tokyo 104-0033 Japan TEL (81) 3-3523-3551 FAX (81) 3-3523-7581 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 436-4314 RF/Automotive Theresienstrasse 2 Postfach 3535 74025 Heilbronn, Germany TEL (49) 71-31-67-0 FAX (49) 71-31-67-2340 Microcontrollers 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 436-4314 La Chantrerie BP 70602 44306 Nantes Cedex 3, France TEL (33) 2-40-18-18-18 FAX (33) 2-40-18-19-60 ASIC/ASSP/Smart Cards 1150 East Cheyenne Mtn. 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