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

Make AVR Examples/Chapter12_Analog-to-Digital-Conversion-II/voltmeter/voltmeter.c

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+/* ADC Voltmeter
+ * Continuously outputs voltage over the serial line.
+ */
+
+// ------- Preamble -------- //
+#include <avr/io.h>
+#include <util/delay.h>
+#include <avr/interrupt.h>
+#include <avr/sleep.h>                           /* for ADC sleep mode */
+#include <math.h>                           /* for round() and floor() */
+
+#include "pinDefines.h"
+#include "USART.h"
+
+/* Note: This voltmeter is only as accurate as your reference voltage.
+ * If you want four digits of accuracy, need to measure your AVCC well.
+ * Measure either AVCC of the voltage on AREF and enter it here.
+*/
+#define REF_VCC 5.053
+                               /* measured division by voltage divider */
+#define VOLTAGE_DIV_FACTOR  3.114
+
+
+// -------- Functions --------- //
+void initADC(void) {
+  ADMUX |= (0b00001111 & PC5);                      /* set mux to ADC5 */
+  ADMUX |= (1 << REFS0);                  /* reference voltage on AVCC */
+  ADCSRA |= (1 << ADPS1) | (1 << ADPS2);    /* ADC clock prescaler /64 */
+  ADCSRA |= (1 << ADEN);                                 /* enable ADC */
+}
+
+void setupADCSleepmode(void) {
+  set_sleep_mode(SLEEP_MODE_ADC);            /* defined in avr/sleep.h */
+  ADCSRA |= (1 << ADIE);                       /* enable ADC interrupt */
+  sei();                                   /* enable global interrupts */
+}
+
+EMPTY_INTERRUPT(ADC_vect);
+
+uint16_t oversample16x(void) {
+  uint16_t oversampledValue = 0;
+  uint8_t i;
+  for (i = 0; i < 16; i++) {
+    sleep_mode();                   /* chip to sleep, takes ADC sample */
+    oversampledValue += ADC;                        /* add them up 16x */
+  }
+  return (oversampledValue >> 2);          /* divide back down by four */
+}
+
+void printFloat(float number) {
+  number = round(number * 100) / 100; /* round off to 2 decimal places */
+  transmitByte('0' + number / 10);                       /* tens place */
+  transmitByte('0' + number - 10 * floor(number / 10));        /* ones */
+  transmitByte('.');
+  transmitByte('0' + (number * 10) - floor(number) * 10);    /* tenths */
+                                                   /* hundredths place */
+  transmitByte('0' + (number * 100) - floor(number * 10) * 10);
+  printString("\r\n");
+}
+
+int main(void) {
+
+  float voltage;
+
+  // -------- Inits --------- //
+  initUSART();
+  printString("\r\nDigital Voltmeter\r\n\r\n");
+  initADC();
+  setupADCSleepmode();
+
+  // ------ Event loop ------ //
+
+  while (1) {
+
+    voltage = oversample16x() * VOLTAGE_DIV_FACTOR * REF_VCC / 4096;
+    printFloat(voltage);
+    /*  alternatively, just print it out:
+     *  printWord(voltage*100);
+     *  but then you have to remember the decimal place
+     */
+    _delay_ms(500);
+
+  }                                                  /* End event loop */
+  return 0;                            /* This line is never reached */
+}