ECE 4760: Laboratory 2

DTMF generator.

You will construct a standard Dual Tone Multi-Frequency (DTMF) generator like those used on any tonch-tone phone.

Procedure:

You will use a keypad to make a DTMF dialer with audio output to a TV. 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 speaker audio input (30 kohm). You should set the RC time constant to be bigger than the the period of the PWM, but not big enough to excessively attenuate the DTMF frequencies. The output of the lowpass filter will go to the audio input of the speakers. The PWM signal will be DTMF sine wave bursts generated using Direct Digital Synthesis (DDS) technique. A short example shows how you might implement a DDS. For this lab, you will need to modulate the amplitude, and 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. This example shows how you might amplitude modulate a sine wave, but may be too slow for two sine waves. You will have to try it.

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 OC0A (port pin B.3). In PWM mode, each PWM cycle completion is signaled by the overflow interrupt. The timer0 overflow 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. A demonstration keypad scanning code for testing your connections is here. An example of a debounce state machine for a single STK500 pushbutton (not the keypad) is included here for reference.

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.

Hints on debugging are here.

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, the tones are played. You must debounce the keypad.
   • Pressing the * button erases the entire stored phone number.
   • When the # button is pressed, the DTMF signal for each entered digit is produced for 100 millseconds to simulate dialing of the digits. There should be at least 30 millseconds of silence between each digit.
   • Each tone burst should ramp up to full amplitude over a time of about 5 milliseconds and ramp down again in about the same amount of time. Ramping the amplitude will decrease the broad band Fourier components which tend to add an annoying click to the sounds. The ramp need not be linear, but could be.
   • Frequency accuracy should be +/-1%. Duration accuracy should be +/-3 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. A table of DTMF frequencies:

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

2. Connect the circuit to a speaker audio input (green 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 the sections mentioned in the policy page, and :
   • 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.