Generating DTMF Tones Using Z8 Encore! MCU

Transcription

1 Application Note Generating DTMF Tones Using Z8 Encore! MCU AN Abstract This Application Note describes how Zilog s Z8 Encore! MCU is used as a Dual-Tone Multi- (DTMF) signal encoder to generate DTMF tones. Z8 Encore! microcontrollers have built-in timers and direct memory access (DMA) with three independent channels (DMA0, DMA1, and DMA2). The timers have Pulse-Width Modulation (PWM) capability and the PWM register values are loaded using DMA. These features of Z8 Encore! are used to generate DTMF signals with less processor overhead. Note: The source code associated with this Application Note is available under Z8 Encore! Applications Library in Application Sample Libraries on Z8 Encore! Microcontrollers Z8 Encore! products are based on the ez8 CPU and introduce Flash memory to Zilog s extensive line of 8-bit microcontrollers. The Flash memory in-circuit programming capability allows faster development time and program changes in the field. The high-performance register-to-register based architecture of the ez8 core maintains backward compatibility with Zilog s popular Z8 MCU. Featuring ez8 CPU, the new Z8 Encore! microcontrollers combine a 20 MHz core with Flash memory, linear-register static random access memory (SRAM), and an extensive array of onchip peripherals. These peripherals make the Z8 Encore! MCU suitable for a variety of applications including motor control, security systems, home appliances, personal electronic devices, and sensors. Discussion This section discusses Dual-Tone Multi- (DTMF) basics and its implementation using microcontroller. DTMF is an international signaling standard for touch-tone telephones. The DTMF encoder encodes standard telephone digits into sum of sinusoid frequency pair corresponding to a frequency table. The DTMF decoder takes a digital signal as input and converts to the decoded digit. The DTMF standard developed by Bell Laboratories is used in touch-tone telephones and voice mail systems. Each telephone digit corresponds to High and Low tone that are transmitted simultaneously. Four frequencies in the High and Low group provide 16 possible combinations; 12 digits are used in most of the telephones and 4 digits are used in military applications. A DTMF detector must conform to the International Telecommunication Union (ITU) formerly known as International Telegraph and Telephone Consultative Committee (CCITT) recommendations. Table 1 lists the frequencies corresponding to each digit. Table 1. DTMF Table Low-Group Frequencies High-Group Frequencies A B C 941 * (E) 0 # (F) D Copyright 2008 by Zilog, Inc. All rights reserved.

2 Theory of Operation The DTMF tone is a combination of two sinusoids of different frequencies. Pulse-width modulation (PWM) method is the easiest and best way to generate a sine wave from a microcontroller. Sine Wave Generation Using PWM In PWM method, the duty cycle determines the amplitude of the sine wave and the number of samples determines the frequency of the sine wave. A sine wave is generated when the PWM output is passed through a low pass filter (LPF). To generate a DTMF tone, two timers in PWM mode are used for each frequency and their outputs are together passed through the LPF. Figure 1 displays the relation between signal frequency (1/T), sampling period (Ts), and number of samples (N). Ts Figure 1. Sine Wave Generation Using PWM From Figure 1, ( Number of samples (N)) ( Sampling Period (Ts)) = Signal Period (T) Therefore, N Fs = 1 F...(1) Fs = NF where, Fs is the sampling frequency and F is the signal frequency. PWM duty cycle registers are loaded using either of the following methods: Timer Interrupt Direct Memory Access Timer Interrupt In the Timer Interrupt method, the PWM registers are loaded at every timer interrupt (or as determined by the number of samples). The timer interrupts occur at the rate of the sampling frequency and servicing of these interrupts keeps the processor busy. Direct Memory Access In the Direct Memory Access (DMA) method, DMA is used to load PWM duty cycle register. The usage of DMA reduces the processor overhead. The user interface with this application provides a feature to calculate the processor overhead. For more information, see Testing on page 6. User application seamlessly integrates with the DTMF generation application as the DMA allows enough processing time for other application. AN Page 2 of 11

3 Lookup Table Formation The lookup table is used to load the PWM timer reload and duty cycle registers. The PWM duty cycle registers are loaded at the rate of the sampling frequency. For DTMF generation, sinusoids of different frequencies are required. A single lookup table is used for this purpose and the values are scaled to achieve the desired output frequency. Calculating Timer Reload Value The following example describes the method of obtaining PWM timer reload value for different frequencies. Let the base frequency be 697 Hz, the system clock be MHz, and samples per cycle be 104 s. The time period is calculated as, Tbase = = ms From Equation (1), N Ts = T Ts = T N == μs where, Ts is the time period of PWM. Therefore, the timer reload register value is calculated as, Reload = ( Ts System Clock Prescalar) == = 254 Reload = 0 FE (hexadecimal representation) This corresponds to a sampling frequency of khz (1/Ts). The time periods for all other frequencies are expressed as multiple of the time period of the base frequency. The multiplier values are scaled by 256 to avoid floating point operations. Using these multiplier values the timer reload value for other frequencies is calculated. Consider the example of 770 Hz signal. Time period = = ms In terms of the base signal, Multiplier value = Tbase = = The multiplier value is scaled by 256. Therefore, Multiplier value = = = 232 The timer reload value for this signal (expressed as a multiple of the base frequency) is given by, Timer reload value = ( ) 256 = 230 Therefore, the frequency is given by, F = System clock ( Number of samples Reload) = ( ) = Hz Percentage Error = ( ) = 0.073% Calculating PWM Values The PWM values are calculated as follows: The sine value corresponding to the samples is given by, sin( n) = sin( 2 π n N) AN Page 3 of 11

4 For example, for sin(10), sin( 2 π n N) = sin( 2 π ) = Note: One is added to all the sine values to make the sine value positive as negative PWM values are not acceptable. Therefore, sin( 2 π n N) = = Let the minimum PWM value be 15, then the corresponding PWM value is, PWM value = ( Timer reload value Minimum PWM value) 2 + Minimum PWM value = ( ) = 202 The PWM values are also scaled according to the multiplier before loading to the PWM registers. For example, for 770 Hz frequency, the multiplier value calculated above is 232. Therefore, the PWM value corresponding to sample 10 for 770 Hz is given by, PWM value = = 183 Note: For detailed calculations, refer to the spread sheet provided in the Application Sample Library on The spread sheet automatically calculates all the necessary values for a given number of samples, system clock, and timer prescale. AN Page 4 of 11

5 Developing the DTMF Encoder Using Z8 Encore! MCU Low Pass Filter Design The PWM outputs are passed through a LPF to demodulate and get the DTMF signal. A simple resistor-capacitor (RC) filter is used for this purpose. The RC values are calculated as follows: The maximum frequency in the DTMF signaling is 1633 Hz. For example, if the cut-off frequency (Fo) of an resistor-capacitor (RC) filter is 3000 Hz, then, Fo = 1 2 π RC Let C = 0.2 µf Therefore, R = 1 2 π = = 270 Ω 270 T1out (PC1) /\/\/\/\ T2out (PC7) /\/\/\/\ 270 Oscilloscope/Line in on PC 0.2 F GND Figure 2. Low Pass Filter Software Implementation This section describes the software to generate DTMF signals using timer and DMA of Z8 Encore! MCU. The input to the application is a digit whose DTMF signal is required. The index to the specific frequencies is obtained from the DTMF frequency table defined in sine_table.asm file. Each index refers to the multiplication factors for that frequency. The first value in the _SineTable is the timer compare value of the base frequency. The remaining values represent the first half of the sine wave. The second half is generated by the software as a mirror image of the first half. During operation, these values are transferred to the Timer PWM low register via DMA. The timer reload value is calculated by multiplying the base timer compare value with the multiplication factor. Only the most significant byte (MSB) is considered. For more information on the software flow, see the flow charts in Appendix B Flowcharts on page 9. Loading PWM Register Using DMA The DMA of Z8 Encore! is used to load the PWM registers. The Z8 Encore! XP F64XX Series DMA controller consists of three independent DMA channels (DMA0, DMA1, and DMA_ADC). DMA0 and DMA1 transfer data between the onchip peripherals and the register file. The third channel (DMA_ADC) controls the Analog-to- Digital Converter (ADC) operation and transfers SINGLE-SHOT mode ADC output data to the register file. In this application, Timer2 is associated with DMA0 to generate Low tone and Timer1 is tied to DMA1 to generate High tone. The source (start address) for the DMAs is the start address of the corresponding sine table. The sine tables thus formed are scaled and mirrored. The destination (peripheral) address of DMA0 and DMA1 are the PWM Low byte registers of Timer2 and Timer1 respectively. AN Page 5 of 11

6 Testing The source code present in the associated Application Sample Library consists of a console interface for demonstration of this application. Setup Figure 3 displays the setup for generation of DTMF tones using the Z8 Encore! MCU. JP2 J1 T1out T2out CONSOLE PC-HyperTerminal LPF Oscilloscope DBG Figure 3. Setup for Generation of DTMF Tones Using Z8 Encore! XP F64XX Series Development Board Equipments Used The equipments used for the setup are listed below: Z8 Encore! development kit featuring Z8F KIT development board ZDSII IDE Oscilloscope A PC equipped with HyperTerminal configured to the following settings: baud rate 8 data bits No parity One stop bit No flow control Procedure to Test the DTMF Application Follow the steps below to test the DTMF application: 1. Connect the external LPF to the timer outputs (see Figure 3). 2. Connect the console port output to the COM port configured with the HyperTerminal. 3. Install Z8 Encore! Applications Library available under Application Sample Libraries on 4. Launch ZDS II and open the DTMF.zdsproj project file located in the source folder. 5. Build and download the code to the Z8 Encore! development board. 6. Execute the code. The user interface routine prints a list of menu items in the HyperTerminal window. AN Page 6 of 11

7 7. Select the appropriate menu item and follow the instructions to generate the DTMF signals. Relevant APIs are called automatically and the DTMF signal is viewed on the oscilloscope. The DTMF signal can also be fed to the Line in on a PC and listened through the speaker output for demonstration purposes. Result The DTMF signals as well as the individual sine wave outputs are captured on the oscilloscope. Table 2 lists the percentage error in the output frequencies. Table 2. Percentage Error in the Output Frequencies Observed Low-Tone Expected Low-Tone Low-Tone Difference % Error in Low-Tone Observed High-Tone Expected High-Tone High-Tone Difference % Error in High-Tone Summary This Application Note provides a quick solution to generate DTMF tones using the DMA and timer feature set of the Z8 Encore! MCUs. The PWM mode of the timers combined with the DMA gives the flexibility to integrate the application that use tone generation as a subset. This method greatly reduces processor overhead compared to other microcontrollers where interrupts are used. References The documents associated with ez8 and Z8 Encore! XP available on are provided below: ez8 TM CPU User Manual (UM0128) Z8 Encore! XP F64XX Series Product Specification (PS0199) AN Page 7 of 11

8 Appendix A Glossary Table 3 lists the abbreviations and definitions used in this Application Note. Table 3. List of Abbreviations Abbreviation Definition API Application Programming Interface DBG Debug Pin-on Development Board DMA Direct Memory Access DTMF Dual-Tone Multi- LPF Low Pass Filter MCU Microcontroller Unit PWM Pulse-Width Modulation SRAM Static Random Access Memory AN Page 8 of 11

9 Appendix B Flowcharts Appendix B provides the flowchart for loading the sine table and PWM registers using DMA. The steps followed in the flowchart for loading the sine table (see Figure 4) are same for loading Low and High tone table. START Get the digit(s) whose DTMF is to be generated Calculate the index offset for DTMF frequency table Get the address of the multiplication factor Scale and transfer the timer compare value which determines the frequency of the output waveform Scale and transfer the half sine wave stored in the program space (ROM) to the RAM table. If the tone multiplication factor is 0, then the sine table values are not scaled. Mirror the calculated values for the second half of the sine wave END Figure 4. Flowchart for Loading the Sine Table AN Page 9 of 11

10 START Turn OFF Timer1, Timer2, DMA0, and DMA1 Load DMA start and end address (sine table address) Load DMA destination address (PWM registers) Load timer reload value Start DMA0 and DMA1 DTMF ON time elapsed No Yes Turn OFF Timer1, Timer2, DMA0, and DMA1 END Figure 5. Flowchart for Loading the PWM Registers Using DMA AN Page 10 of 11

11 Warning: DO NOT USE IN LIFE SUPPORT LIFE SUPPORT POLICY ZILOG'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION. As used herein Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. Document Disclaimer 2008 by Zilog, Inc. All rights reserved. Information in this publication concerning the devices, applications, or technology described is intended to suggest possible uses and may be superseded. ZILOG, INC. DOES NOT ASSUME LIABILITY FOR OR PROVIDE A REPRESENTATION OF ACCURACY OF THE INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT. ZILOG ALSO DOES NOT ASSUME LIABILITY FOR INTELLECTUAL PROPERTY INFRINGEMENT RELATED IN ANY MANNER TO USE OF INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED HEREIN OR OTHERWISE. The information contained within this document has been verified according to the general principles of electrical and mechanical engineering. Z8 Encore!, Z8 Encore! XP, and ez8 are trademarks or registered trademarks of Zilog, Inc. All other product or service names are the property of their respective owners. AN Page 11 of 11