Started following some tutorial on att85 usi. Downloaded example code from make avr book.

Make AVR Examples/Chapter13_Advanced-PWM-Tricks/dialTone/dialTone.c

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+/* Direct-digital synthesis */
+
+// ------- Preamble -------- //
+#include <avr/io.h>		/* Defines pins, ports, etc */
+#include <util/delay.h>		/* Functions to waste time */
+#include <avr/interrupt.h>	
+#include <avr/power.h>
+#include "pinDefines.h"
+#include "macros.h"
+#include "fullSine.h"
+
+
+static inline void initTimer0(void){
+  set_bit(TCCR0A, COM0A1);	/* PWM output on OCR0A */
+  set_bit(SPEAKER_DDR, SPEAKER); /* enable output on pin */
+
+  set_bit(TCCR0A, WGM00);	/* Fast PWM mode */
+  set_bit(TCCR0A, WGM01);	/* Fast PWM mode, pt.2 */
+  
+  set_bit(TCCR0B, CS00);	/* Clock with /1 prescaler */
+}
+
+static inline void pollButton(void){
+  if (bit_is_clear(BUTTON_PIN, BUTTON)) {
+    set_bit(SPEAKER_DDR, SPEAKER); /* enable output on pin */
+    clear_bit(LED_PORT, LED0);
+  }
+  else {
+    clear_bit(SPEAKER_DDR, SPEAKER); /* disable output on pin */
+    set_bit(LED_PORT, LED0);
+  }
+}
+
+static inline void initLED(void){
+  set_bit(LED_DDR, LED0);	/* LED on for diagnostics */
+  set_bit(LED_PORT, LED0);
+  _delay_ms(100);
+  clear_bit(LED_PORT, LED0);
+}
+
+int main(void){
+
+  volatile  uint16_t accumulator0;  
+  volatile  uint16_t accumulator1;  
+  volatile  uint16_t tuningWord0;
+  volatile  uint16_t tuningWord1;
+  volatile  uint16_t mixer;
+
+  // -------- Inits --------- //
+  
+  clock_prescale_set(clock_div_1);                  /* CPU clock 8 MHz */
+  initLED();
+  initTimer0();
+  
+  set_bit(BUTTON_PORT, BUTTON);	/* pullup on button */
+  set_bit(SPEAKER_DDR, SPEAKER); /* speaker output */
+  
+  tuningWord0 = 440*2;		/* Dial tone frequencies */
+  tuningWord1 = 350*2;
+  
+  // ------ Event loop ------ //
+  while(1){		       
+    
+    loop_until_bit_is_set(TIFR0, TOV0); /* wait until overflow bit set */
+    set_bit(TIFR0, TOV0);		/* writing set should reset... */
+
+    //  Note that this bit has to be pretty fast... we have only 256 cycles to
+    //  set the next value in OCR0A, or we may hear a glitch
+
+    accumulator0 += tuningWord0;	/* take tuningWord steps forward */
+    accumulator1 += tuningWord1;	/* take tuningWord steps forward */
+
+    mixer = fullSine[(uint8_t) (accumulator0 >> 8)]; /* add together */
+    mixer += fullSine[(uint8_t) (accumulator1 >> 8)];
+    OCR0A = 128 + (mixer >> 1);		/* divide by 2, the fast way */
+    
+    pollButton();
+    
+
+  } /* End event loop */
+  return 0;		      /* This line is never reached  */
+}
+
+
+