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

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

@@ -0,0 +1,74 @@
+                                           /* 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 "fullSine.h"
+#include "USART.h"
+
+volatile uint16_t accumulator;
+volatile uint16_t tuningWord;
+
+static inline void initTimer0(void) {
+  TCCR0A |= (1 << COM0A1);                      /* PWM output on OCR0A */
+  SPEAKER_DDR |= (1 << SPEAKER);               /* enable output on pin */
+
+  TCCR0A |= (1 << WGM00);                             /* Fast PWM mode */
+  TCCR0A |= (1 << WGM01);                       /* Fast PWM mode, pt.2 */
+
+  TCCR0B |= (1 << CS00);                    /* Clock with /1 prescaler */
+  TIMSK0 |= (1 << TOIE0);                        /* Overflow interrupt */
+  sei();                                           /* Enable interrupt */
+}
+
+ISR(TIMER0_OVF_vect) {
+
+  accumulator += tuningWord;          /* take tuningWord steps forward */
+                                              /* lookup and set output */
+  OCR0A = 128 + fullSine[(uint8_t) (accumulator >> 8)];
+
+}
+
+static inline void pollButton(void) {
+  if (bit_is_clear(BUTTON_PIN, BUTTON)) {
+    SPEAKER_DDR |= (1 << SPEAKER);             /* enable output on pin */
+    LED_PORT &= ~(1 << LED0);
+  }
+  else {
+    SPEAKER_DDR &= ~(1 << SPEAKER);          /* disable output on pin */
+    LED_PORT |= (1 << LED0);
+  }
+}
+
+int main(void) {
+
+  // -------- Inits --------- //
+
+  clock_prescale_set(clock_div_1);                  /* CPU clock 8 MHz */
+  initTimer0();
+  initUSART();
+
+  BUTTON_PORT |= (1 << BUTTON);                    /* pullup on button */
+
+  LED_DDR |= (1 << LED0);                    /* LED on for diagnostics */
+  LED_PORT |= (1 << LED0);
+  _delay_ms(100);
+  LED_PORT &= ~(1 << LED0);
+
+  SPEAKER_DDR |= (1 << SPEAKER);
+
+  tuningWord = 880;
+  transmitByte((uint8_t) tuningWord >> 2);       /* approx value in Hz */
+
+  // ------ Event loop ------ //
+  while (1) {
+
+    pollButton();
+
+  }                                                  /* End event loop */
+  return 0;                            /* This line is never reached */
+}