C++ | Tachometer Arduino Using OLED 128x64

#include <Arduino.h>
#include <Wire.h>
#include <Adafruit_SSD1306.h>// You may have to edit library for 128x64, //default is 128 x 32.
#define OLED_WIDTH 128
#define OLED_HEIGHT 64
#define OLED_ADDR   0x3C // A very common address for these displays.

// -------------------------------------------
// Tachometer Arduino Using OLED 128x64
// www.ardukode.blogspot.com
// -------------------------------------------

Adafruit_SSD1306 display(OLED_WIDTH, OLED_HEIGHT);
float value=0;
float rev=0;
int rpm;
int oldtime=0;
int time;

void isr(){         //interrupt service routine
  rev++;
}

void setup(){
 display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDR);
 display.clearDisplay();
 digitalWrite(2 ,HIGH);// Instead of using a pull up resistor //using pin number 2
 attachInterrupt(0,isr,RISING);  //attaching the interrupt
}

void loop(){
  delay(2000);// 2 second delay
  detachInterrupt(0);           //detaches the interrupt while calculating
  time=millis()-oldtime;        //finds the time
  rpm=(rev/time)*60000;         //calculates rpm
  oldtime=millis();             //saves the current time
  rev=0;
  display.clearDisplay();
  display.setTextSize(1);
  display.setTextColor(WHITE);
  display.setCursor(10, 0);// Vertical, Horizontal.
  display.println("Digital Tachometer");
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(0, 25);// Vertical, Horizontal.
  display.println(rpm);
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(85,25);
  display.println("RPM");
  display.setTextSize(1);
  display.setTextColor(WHITE);
  display.setCursor(0, 55);// Vertical, Horizontal.
  display.println("ardukode.blogspot.com");
  display.display();
  attachInterrupt(0,isr,RISING);
}
// avrdude -DV -patmega328p -Pnet:192.168.4.1:23 -carduino -b115200 -U flash:w:firmware.hex:I