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

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

@@ -0,0 +1,56 @@
+/*
+   Direct-digital synthesis
+   Phasing saw waves demo
+
+*/
+
+#include "fatSaw.h"
+
+int main(void) {
+
+  uint16_t accumulators[NUMBER_OSCILLATORS];
+  uint8_t waveStep;
+  int16_t mixer = 0;
+  uint8_t i;
+
+  // -------- Inits --------- //
+
+  clock_prescale_set(clock_div_1);                  /* CPU clock 8 MHz */
+  initTimer0();
+  SPEAKER_DDR |= (1 << SPEAKER);                     /* speaker output */
+  LED_DDR |= (1 << LED0);
+
+  // Init all to same phase
+  for (i = 0; i < NUMBER_OSCILLATORS; i++) {
+    accumulators[i] = 0;
+  }
+
+  // ------ Event loop ------ //
+  while (1) {
+
+                                   /* Load in the PWM value when ready */
+    loop_until_bit_is_set(TIFR0, TOV0); /* wait until overflow bit set */
+    OCR0A = 128 + mixer;           /* signed-integers need shifting up */
+    TIFR0 |= (1 << TOV0);                   /* re-set the overflow bit */
+
+                              /* Update all accumulators, mix together */
+    mixer = 0;
+    for (i = 0; i < NUMBER_OSCILLATORS; i++) {
+      accumulators[i] += BASEPITCH;
+      waveStep = accumulators[i] >> 8;
+
+      // Add extra phase increment.
+      // Makes shifting overtones when
+      // different frequency components add, subtract
+      if (waveStep == 0) {                   /* roughly once per cycle */
+        accumulators[i] += PHASE_RATE * i;          /* add extra phase */
+      }
+
+      mixer += fullSaw15[waveStep];
+    }
+    mixer = mixer >> OSCILLATOR_SHIFT;
+                                 /* Dividing by bitshift is very fast. */
+
+  }                                                  /* End event loop */
+  return 0;                            /* This line is never reached */
+}