EE 476: Laboratory 2

Cricket-call generator.

You will construct a cricket-call generator which will produce audio output of a simplified cricket call. A cricket call may be very complex, but we will limit ourselves to the following form.

1. A call is made up by repeating indefinitely a chirp and a silent period. If the duration of the silence is zero, the call is refered to as a trill.
2. A chirp is made up of a certain number of syllables with specific duration and repitition interval.
3. A syllable is made up of a sine wave burst at a certain frequency. The burst starts at low amplitude, which increases quickly to a maximum, sustains the maximum amplitude until just before the end, then decreases amplitude to zero.

The image below summerizes the definition ( from Manfred Hartbauer).

Procedure:

You will use an LCD and a keypad to set the parameters like those shown in the table below. 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 (100 kohm). You should set the RC time constant to about 10 times the period of the PWM. The output of the lowpas filter will go to the audio input of the TV. The PWM signal will be sine wave bursts generated using an amplitude modulated Direct Digital Synthesis (DDS) technique. A short example shows how you might implement a DDS and amplitude modulate it.

You will need to figure out how to generate a variety of time delays and durations. I suggest the following 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 sine wave synthesis. The output will be through OC0 (port pin B.3). Use the PWM mode with the highest speed. This interrupt will 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.

         Conn. on top     Conn. on bottom
Pin 1  -- row 1 2 3 A         Column 1
Pin 2  -- row 4 5 6 B         Column 2
Pin 3  -- row 7 8 9 C         Column 3
Pin 4  -- row * 0 # D         Column 4

Pin 5  -- col 1 4 7 *         Row 1
Pin 6  -- col 2 5 8 0         Row 2
Pin 7  -- col 3 6 9 #         Row 3
Pin 8  -- col A B C D         Row 4

(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.

1. Write a C program which implements a cricket-call generator with the following specifcations:
   • The LCD and keypad will be used to enter arbitrary combinations of the 5 parameters shown below. The four calls shown are typical of real cricket species. The range which can be set is shown in the 5th row. There will be a play and a stop button to start and stop song production. All times are in milliSeconds. Frequencies are in Hz.

     parameter	chirp+ silence duration	number of syllables	syllable duration	syllable repeat interval	burst frequency
     call 0	20	1	10	---	4500
     call 1	250	4	18	30	5000
     call 2	1500	50	8	20	6000
     call 3	1000	5	30	50	3000
     Range	10 to 1000	1 to 100	5 to 100	10 to 100	1000 to 6000

   • The LCD display must have enough information so that the user can determine which parameter is being set and the value of the parameter.
   • 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).
   • Each sine wave burst must be amplitude modulated from zero to full amplitude in 4 mSec at the beginning of a burst and full to zero in 4 mSec at the end. Syllable duration is taken to be the width of the burst at half-amplitude.

2. Connect the circuit to the TV audio input (white phone plug) and listen the calls. 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 times and 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.