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

Make AVR Examples/Chapter18_Using-Flash-Program-Memory/talkingVoltmeter/talkingVoltmeter.c

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+//  Talking Voltmeter Example
+
+#include <inttypes.h>
+#include <avr/io.h>
+#include <avr/interrupt.h>
+#include <util/delay.h>
+#include <avr/pgmspace.h>
+#include <avr/power.h>
+#include "pinDefines.h"
+#include "USART.h"
+
+#include "talkingVoltmeter.h"
+
+void startSampleTimer(void) {
+  sampleNumber = 0;                   /* back to start of sample table */
+  TCCR2B = (1 << CS21);                         /* turn on timer clock */
+                /* Two clock options above end up ~8kHz on 8MHz system */
+}
+void stopSampleTimer(void) {
+  TCCR2B = 0;                         /* disable sample-playback clock */
+  OCR0A = 128;                              /* idle PWM at mid-voltage */
+  lastout = 0;                                 /* start at 0 next time */
+}
+void speak(void) {
+  startSampleTimer();
+  loop_until_bit_is_clear(TCCR2B, CS21);            /* Wait until done */
+}
+
+void updatePWMAudio(void) {
+  OCR0A = out + 128;                        /* re-center for 0-255 PWM */
+  lastout = out;                                  /* update last value */
+  sampleNumber++;                                 /* on to next sample */
+}
+void unpackByte(uint8_t dataByte) {
+                           /* Select pairs of bits from byte, save out */
+  differentials[0] = (dataByte >> 6) & 0b00000011;
+  differentials[1] = (dataByte >> 4) & 0b00000011;
+  differentials[2] = (dataByte >> 2) & 0b00000011;
+  differentials[3] = (dataByte & 0b00000011);
+}
+
+/* Timer 2 controls sampling speed.
+   ISR reads new data, loads PWM values into OCR0A */
+ISR(TIMER2_COMPA_vect) {
+  /* Since we can decode 4 2-bit values at once, need to know where
+     we are in the 4-step mini-cycle. */
+  uint8_t cycle = sampleNumber & 0b00000011;       /* keep last 2 bits */
+  uint16_t tableEntry;
+  uint8_t packedData;
+
+  if (cycle == 0) {                        /* at first sample, re-load */
+    tableEntry = sampleNumber >> 2;           /* where we are in table */
+    if (tableEntry < thisTableLength) {
+                         /* read the next byte from the selected table */
+      packedData = pgm_read_byte(&thisTableP[tableEntry]);
+      unpackByte(packedData);    /* split up byte into differentials[] */
+    }
+    else {                                   /* at end of table, done. */
+      stopSampleTimer();
+    }
+  }
+          /* Decode the differences: current value = last + difference */
+  out = lastout + dpcmWeights[differentials[cycle]] - (lastout >> 4);
+  updatePWMAudio();
+}                               // end  ISR (TIMER2_COMPA_vect)
+
+void printString_Progmem(const char *stringP) {
+  char oneLetter;
+  while ((oneLetter = pgm_read_byte(stringP))) {
+    transmitByte(oneLetter);
+    stringP++;
+  }
+}
+
+
+int main(void) {
+  uint16_t voltage;
+  uint8_t volts;
+  uint8_t tenths;
+  uint8_t vcc = 51;                               /* 10x VCC, in volts */
+
+  clock_prescale_set(clock_div_1);                            /* 8 MHz */
+  initTimer0();
+  initTimer2();
+  sei();                                             /* for timer2 ISR */
+  initADC();
+  initUSART();
+
+  printString_Progmem(PSTR("\r\n--=( Talking Voltmeter )=--\r\n"));
+
+  selectTable(INTRO);
+  speak();
+
+  while (1) {
+
+    ADCSRA |= (1 << ADSC);                                /* start ADC */
+    loop_until_bit_is_clear(ADCSRA, ADSC);
+
+    voltage = ADC * vcc + vcc / 2;      /* vcc/2 to make rounding work */
+    voltage = voltage >> 10;              /* divide by 10-bits for ADC */
+                   /* "voltage" is now actually 10x real-world voltage */
+    volts = voltage / 10;
+    tenths = voltage % 10;
+
+    transmitByte('0' + volts);                /* serial output as well */
+    selectTable(volts);                 /* 0 points to ZERO_TABLE, etc */
+    speak();
+
+    transmitByte('.');
+    selectTable(POINT);
+    speak();
+
+    transmitByte('0' + tenths);
+    selectTable(tenths);
+    speak();
+
+    printString_Progmem(PSTR("  volts\r\n"));
+    selectTable(VOLTS);
+    speak();
+
+    _delay_ms(SPEECH_DELAY);
+
+  }                                                       /* end while */
+  return 0;
+}