EE 476: Laboratory 2

Talking DTMF generator.

You will construct a standard Dual Tone Multi-Frequency (DTMF) generator like those used on any tonch-tone phone. In addition, as you enter the phone number using a keypad, the digits will be spoken in English.

Procedure:

You will use a keypad to make a DTMF dialer. Analog output will be in the form of a PWM signal which will be filtered using a simple RC circuit to low pass filter the analog signal. Use a big enough resistor so that you don't load the port pin, and small enough to be below the input resistance of the TV audio input (30 kohm). You should set the RC time constant to more 10 times the period of the PWM, but not big enough to excessively attenuate the DTMF or voice signal. The output of the lowpass filter will go to the audio input of the TV. The PWM signal will be either

• DTMF sine wave bursts generated using Direct Digital Synthesis (DDS) technique. A short example shows how you might implement a DDS and amplitude modulate it. For this lab, you will not need to modulate the amplitude, but you will need to run two simulaneous DDS units and sum them. The sine wave table will need to be scaled to prevent overflow. Use the timer0 fast PWM mode with the prescalar set to 1 to produce about 62500 samples/sec.
• Spoken digits taken from a compressed flash table. See the speech synthesis and comression page for more details, the table and example code. The software you need to use is in the DPCM section of the page. Use the timer0 fast PWM mode as above, but only update the OCR0 register every 8 interrupts to produce about 7812 samples/sec.

You will need to control various intervals of time and generate the PWM signal. I suggest the following timing scheme, but you may use any scheme you want. Timer 0 is used to generate the high speed PWM-based D/A signal necessary for the sound synthesis. The output will be through OC0 (port pin B.3). In PWM mode, each PWM cycle completion is signaled by the overflow interrupt. The timer0 OVFL ISR is therefore used to load a new output compare value for the next audio sample. In this ISR you can also increment a counter to count to 1 mSec. The resulting 1 mSec time base will be used to control all other timing (in the main program, not in the interrupt).

You will need to get user input from a keypad with one of two following configurations. Some keypads have the connector on the top, as shown. Some have them on the bottom. Demonstration keypad scanning code is here. See also the keypad library by Richard West.

Connector:  top:8-pin      top:9-pin   bottom
Pin 1  ---- row 1 2 3 A    col 1       col 1
Pin 2  ---- row 4 5 6 B    col 2       col 2
Pin 3  ---- row 7 8 9 C    col 3       col 3
Pin 4  ---- row * 0 # D    col 4       col 4

Pin 5  ---- col 1 4 7 *    row 1       row 1
Pin 6  ---- col 2 5 8 0    row 2       row 2
Pin 7  ---- col 3 6 9 #    row 3       row 3
Pin 8  ---- col A B C D    row 4       row 4
Pin 9  ----              (NO CONNECT--common)

(a) Each switch shorts one row to one column.
(b) Each pin should be connected to one bit of an i/o port.
(c) The i/o port pins will be used both as inputs and outputs. 
    When they are inputs, they should have active pullup resistors.
(d) On the 9-pin models, do not connect the common lead.

1. Write a C program which implements a DTMF generator with the following specifcations:
   • The keypad will be used to enter up to 12 digit telephone numbers. As each digit is pressed, a synthesized voice speaks the digit. If a second digit is pressed before the pervious digit speech is done, the second digit speech should start immediately. You must debounce the keypad.
   • Pressing the * button erases the entire stored phone number.
   • When the # button is pressed, the DTMF signal for each digit is produced for 100 millseconds to simulate dialing of the digits. There should be at least 30 millseconds of silence between each digit.
   • A table of DTMF frequencies.

     Freq	1209Hz	1336Hz	1477Hz
     697Hz	1	2	3
     770Hz	4	5	6
     852Hz	7	8	9
     941Hz	*	0	#

   • Frequency accuracy should be +/-1%. Duration accuracy should be +/-1 mSec. Generate sinewaves using DDS. The sinewaves should be built from 8-bit samples (taken from a 256 entry table). Implement a test mode in which each of 7 DTMF frequencies can be individually measured for frequency accuracy. Holding down the * button (or any button on the STK500) while pressing the buttons 1 through 7 should produce one of seven sine waves for as long as you hold the buttons.

2. Connect the circuit to the TV audio input (white phone plug) and listen the output. This will aid in debugging and is required for the demo.
3. Demo this program to a staff member. Show that all specifications are met by verifying frequencies with an oscilloscope.
4. Your written lab report should include:
   • How you generated the freqencies (DDS resolution, etc) and an measurement of how accurate they are.
   • How you generated the various time intervals required by this project.
   • Other design aspects of the assignment, such as the conversion from user input frequency in Hz to DDS increment.
   • A heavily commented listing of your code.