EOD

Cricket Call Generator

Introduction:

The goal of this project was to build a field-portable cricket-call generator. The cricket-call generator will be used to attract crickets and for song discrimination tests.

Specifications:

The generator has to be able to store dozens of songs.
A song is specified by:
- burst duration
- number of bursts
- burst spacing
- song duration (repeat time)
- carrier frequency
Playback consists of bursts of sinewaves.
Low power. Current from a 6 volt battery should be a few milliamps.
Song selection is by pushbutton curently, but will be by thumbwheel switch.
Song duration 0-4 seconds
Carrier frequency 950 to 16000 Hz.
Burst duration/spacing 0-4 seconds

We used an Atmel STK-200 development board with an AT90s8515 flash-programmable microcontroller (mcu) on-board. A port of the mcu was used to drive an 8-bit digital-to-analog converter (Analog Devices AD557). Another port was used to detect a button push. The button starts playback of the selected song. A third port was used to run LEDs indicating which song is being played.

Program:

There are four processes which run on the microcontroller:

The main program loop detects a button push and sets up the song by copying parameters from flash to registers. The RESET entry point initializes the ports and timers, then sets up song #1.
The timer 1A compare-match ISR (interrupt service routine) occurs when a new song is ready to start. The ISR initializes burst counters and table pointers, then does nothing else until the next repitition of the song. Timer 1 ticks once every 64 microseconds.
The timer 1B compare-match ISR occurs whenever a burst starts or ends. The ISR sets the burst variables, then sets it's own next occurance by keeping track of the next burst start/end time.
The timer zero overflow ISR occurs at every sinewave sample time. This ISR emits a table look-up sample to the DAC. It also performs a multply to ramp up/down the waveform at the beginning and end of each burst. Timer 0 ticks at 4 MHz.
AVR assembler code is the program running on the mcu.

Schematics:

The DAC schematic shown in the EOD project was used again here.