// Reads analog input and sends over serial.
// Reports the time the input was sampled.
// Uses precise timing via interrupts.
// Requires > 57600 baud serial.

// The Arduino clock frequency is 1 cycle 6.25e-8 seconds.
// Timer1 is 16 bits. It overflows every (2^16 - 1) * (6.25e-8 seconds).
// That is, every 4.0959375 milliseconds. So that's our sampling time.

// We get reliable 4 ms samples if we use > 28800 baud serial. Lower baud rate doesn't work.

// avr-libc library includes
#include <avr/io.h>
#include <avr/interrupt.h>

void setup() {
  // initialize serial communication
  Serial.begin(57600);
  
  // Initialize Timer1
  cli(); //disable global interrupts
  TCCR1A = 0;
  TCCR1B = 0;
  
  // Enable Timer1 overflow interrupt:
  TIMSK1 = (1 << TOIE1);
  // Set CS10 bit so timer runs at clock speed:
  TCCR1B |= (1 << CS10);
  sei(); // enable global interrupts
}

ISR(TIMER1_OVF_vect){
  // Serial.println(millis()); 
  Serial.println(analogRead(A0));    
}

void loop() {    
  // do nothing since the script is interrupt driven
}