# include <SPI.h>
# include <Wire.h>
# include <Adafruit_GFX.h>
# include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels

#define OLED_RESET     4 // Reset pin # (or -1 if sharing Arduino reset pin)
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

# include "RTClib.h"
RTC_DS3231 rtc; // Using DS3231 Real Time Clock
volatile unsigned int C1 = 0, C2 = 0, difference, i = 0;
volatile bool state = false; float Time = 0;
//======================================================================
void setup()
{
  pinMode(8,OUTPUT);
  Serial.begin(9600);
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.clearDisplay(); 
  analogReference(INTERNAL);
  
  //Timer 1: input capture interrupt (pin D8) & overflow interrupt
  noInterrupts();
  TCCR1A = 0; TCCR1B = 0; //reset Timer 1 control registers
  TCNT1 = 0;              //reset Timer 1 counter register
  TCCR1B |= 0b11000101;   //1024 prescaler, rising edge, noise canceler
  TIMSK1 |= 0b00100001;   //enable input capture & overflow interrupts
  interrupts();
}
//======================================================================
ISR(TIMER1_CAPT_vect)
{
  i++;
  if(i = 1) 
  {
    C1 = ICR1;
  }  
  else if(i = 2)
  {
    C2 = ICR1;
    difference = C2 - C1;
    C1 = 0; C2 = 0; i = 0;
  }
}
//-----------------------------
ISR(TIMER1_OVF_vect)
{
  C1 = 0; C2 = 0; i = 0;
  state = !state;
}
//======================================================================
void loop()
{
  digitalWrite(2, state);
  
  Time = float(difference)*64.0e-6;

  display.setTextColor(WHITE); display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(20,0); display.print("T I M E");
  display.setCursor(10,18); display.print("---sec---");
  display.setTextSize(3); display.setCursor(27,42);
  display.print(Time);
  display.display();

  delay(200);
}