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

Make AVR Examples/Chapter13_Advanced-PWM-Tricks/dds_saw15/dds_saw15.c

@@ -0,0 +1,56 @@
+                                           /* Direct-digital synthesis */
+
+// ------- Preamble -------- //
+#include <avr/io.h>                        /* Defines pins, ports, etc */
+#include <util/delay.h>                     /* Functions to waste time */
+#include <avr/power.h>
+
+#include "pinDefines.h"
+
+#include "fullSaw15.h"
+
+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 */
+}
+
+int main(void) {
+
+  uint16_t accumulator = 0;
+  uint16_t accumulatorSteps = 880;                    /* approx 440 Hz */
+  uint8_t waveStep;
+  int8_t pwmValue;
+
+  // -------- Inits --------- //
+
+  clock_prescale_set(clock_div_1);                  /* CPU clock 8 MHz */
+  initTimer0();
+  BUTTON_PORT |= (1 << BUTTON);                    /* pullup on button */
+
+  // ------ Event loop ------ //
+  while (1) {
+
+    if (bit_is_clear(BUTTON_PIN, BUTTON)) {
+
+      SPEAKER_DDR |= (1 << SPEAKER);                 /* enable speaker */
+      accumulator += accumulatorSteps;          /* advance accumulator */
+      waveStep = accumulator >> 8;           /* which entry in lookup? */
+      pwmValue = fullSaw15[waveStep];                 /* lookup voltage */
+
+      loop_until_bit_is_set(TIFR0, TOV0);        /* wait for PWM cycle */
+      OCR0A = 128 + pwmValue;                     /* set new PWM value */
+      TIFR0 |= (1 << TOV0);                  /* reset PWM overflow bit */
+    }
+
+    else {                                       /* button not pressed */
+      SPEAKER_DDR &= ~(1 << SPEAKER);              /* disable speaker */
+    }
+
+  }                                                  /* End event loop */
+  return 0;                            /* This line is never reached */
+}