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

Make AVR Examples/Chapter13_Advanced-PWM-Tricks/adsr/adsr.h

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+
+/*Include file for ADSR synth project. */
+
+#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 "macros.h"
+#include "scale.h"
+#include "fullSaw7.h"		/* a 7-harmonic bandlimited sawtooth */
+                   /* see generateWavetables.py if you're interested */
+#include "fullTriangle.h"
+#include "USART.h"
+
+#define FULL_VOLUME     31 	/* 5-bit volumes */
+
+// Default envelope values (slightly percussive)
+// Play around with these!  
+#define ATTACK_RATE    8	/* 0-255 */
+#define DECAY_RATE     120	/* 0-255 */
+#define SUSTAIN_LEVEL  25	/* 0-255 */
+#define SUSTAIN_TIME   4000	/* 0-65535 */
+#define RELEASE_RATE   200	/* 0-65535 */
+
+// Compute these constants
+#define ATTACK_TIME   ATTACK_RATE * FULL_VOLUME
+#define DECAY_TIME   (ATTACK_TIME + (FULL_VOLUME-SUSTAIN_LEVEL) * DECAY_RATE)
+#define RELEASE_TIME     DECAY_TIME + SUSTAIN_TIME
+
+// -------------- Init Routines --------------- //
+
+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 initLEDs(void){
+  uint8_t i;
+  LED_DDR = 0xff;	/* All LEDs for diagnostics */
+  for (i=0; i<8; i++){
+    set_bit(LED_PORT, i);
+    _delay_ms(100);
+    clear_bit(LED_PORT, i);
+  }
+}
+
+static inline uint16_t lookupPitch(char i){
+  switch(i){
+  case 'a':
+    return(G1);
+  case 's':
+    return(A1);
+  case 'd':
+    return(B1);
+  case 'f':
+    return(C2);
+  case 'g':
+    return(D2);
+  case 'h':
+    return(E2);
+  case 'j':
+    return(F2);
+  case 'k':
+    return(G2);
+  case 'l':
+    return(A2);
+  case ';':
+    return(B2);
+  case '\'':
+    return(C3);   
+  }	 
+  // Default value -- if press some other key
+  return(C1);
+}
+