AM Radio

AM Radio Using AVR

This code section really gets to show off what timers can do, and we even get to combine a timer with interrupts. Hold on tight! In the radio code, we’ll be using Timer 0 in CTC mode as we did earlier to generate audio in the improved organ code. That is, we have the counter count up to a value that determines its frequency, and then clear the counter and start over at zero.This code section really gets to show off what timers can do, and we even get to combine a timer with interrupts. Hold on tight! In the radio code, we’ll be using Timer 0 in CTC mode as we did earlier to generate audio in the improved organ code. That is, we have the counter count up to a value that determines its frequency, and then clear the counter and start over at zero.simply turn on and off the carrier signal to modulate it, generating the audio on top.


Code:
/*
Plays a simple tune, broadcasts it in the AM radio band.
*/
// ------- Preamble -------- //
#include <avr/io.h> /* Defines pins, ports, etc */
#include <util/delay.h> /* Functions to waste time */
#include <avr/power.h>
#include <avr/interrupt.h>
#include "pinDefines.h"
#include "scale16.h"
#define COUNTER_VALUE 3 /* determines carrier frequency */
// From f = f_cpu / ( 2* N* (1 + OCRnx) )
// Good values for the AM band from 2 to 6: pick one that's clear
// Divide by two b/c we're toggling on or off each loop;
// a full cycle of the carrier takes two loops.
// 8Mhz / (2 * 1 * (1+2)) = 1333 kHz
// 8Mhz / (2 * 1 * (1+3)) = 1000 kHz
// 8Mhz / (2 * 1 * (1+4)) = 800 kHz
// 8Mhz / (2 * 1 * (1+5)) = 670 kHz
// 8Mhz / (2 * 1 * (1+6)) = 570 kHz
// 8Mhz / (2 * 1 * (1+7)) = 500 kHz
static inline void initTimer0(void) {
TCCR0A |= (1 << WGM01); /* CTC mode */
TCCR0A |= (1 << COM0B0); /* Toggles pin each time through */
TCCR0B |= (1 << CS00); /* Clock at CPU frequency, ~8MHz */
OCR0A = COUNTER_VALUE; /* carrier frequency */
}
static inline void initTimer1(void) {
TCCR1B |= (1 << WGM12); /* CTC mode */
TCCR1B |= (1 << CS11); /* Clock at CPU/8 frequency, ~1MHz */
TIMSK1 |= (1 << OCIE1A); /* enable output compare interrupt */
}
ISR(TIMER1_COMPA_vect) { /* ISR for audio-rate Timer 1 */
ANTENNA_DDR ^= (1 << ANTENNA); /* toggle carrier on and off */
}
static inline void transmitBeep(uint16_t pitch, uint16_t duration) {
OCR1A = pitch; /* set pitch for timer1 */
sei(); /* turn on interrupts */
do {
_delay_ms(1); /* delay for pitch cycles */
duration--;
} while (duration > 0);
cli(); /* and disable ISR so that it stops toggling */
ANTENNA_DDR |= (1 << ANTENNA); /* back on full carrier */
}
int main(void) {
// -------- Inits --------- //
clock_prescale_set(clock_div_1); /* CPU clock 8 MHz */
initTimer0();
initTimer1();
// ------ Event loop ------ //
while (1) {
transmitBeep(E3, 200);
_delay_ms(100);
transmitBeep(E3, 200);
_delay_ms(200);
transmitBeep(E3, 200);
_delay_ms(200);
transmitBeep(C3, 200);
transmitBeep(E3, 200);
_delay_ms(200);
transmitBeep(G3, 400);
_delay_ms(500);
transmitBeep(G2, 400);
_delay_ms(2500);
} /* End event loop */
return (0); /* This line is never reached */
}