#include <Wire.h>  // Comes with Arduino IDE

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address

/* Pin to interrupt map: 

 * D0-D7 = PCINT 16-23 = PCIE2 = pcmsk2 

 * D8-D13 = PCINT 0-5 = PCIE0 = pcmsk0 

 * A0-A5 (D14-D19) = PCINT 8-13 = PCIE1 = pcmsk1 

 */ 

#define interrupt_pin2 8 //Define Analog Pin (analog pins are 16-21) 

volatile unsigned long t=0;
volatile unsigned long ot=0;

ISR(PCINT0_vect) {
  if (PINB & 1) {
    ot=t;
    t=millis();
  }
} 

//actualizar o visor (aqui calibrado para um maximo de 3000 rpm, basta mudar o valor)
#define max_rpm 3000
void printVu(int r) {
  int i=map(r, 0, max_rpm, 0, 20);
  for(int n=0;n<20;n++) {
     lcd.print((char)(n<i?255:' '));
  }
}

void setup() { 
 Serial.begin(115200);
 lcd.begin(20,4);   // initialize the lcd for 20 chars 4 lines, turn on backlight
 lcd.backlight(); // finish with backlight on  
 lcd.print("RPM with PCINT");
 MCUCR = (1<<ISC01) | (1<<ISC00); 
 PCICR |= (1<<PCIE0); 
 PCMSK0 |= (1<<PCINT0); 
 pinMode(interrupt_pin2, INPUT); //Make pin an input 
 digitalWrite(interrupt_pin2,HIGH); //Enable pullup resistor on Analog Pin 
 interrupts(); 
} 

unsigned int rpm=0;

void loop() {
 //lcd.setCursor(0,3);lcd.print(millis()-t);lcd.print("     ");
 if((millis()-t)>300) rpm=0;
 else if (t!=ot) {
   rpm=60000.0/(float)(t-ot);
 }
 Serial.print(rpm);Serial.print(" ");Serial.println(t-ot);
 lcd.setCursor(0,1);
 lcd.print(rpm);
 lcd.print(" rpms    ");
 lcd.setCursor(0,2);
 printVu(rpm);
}