interrupts problem

Projects Programming
1

hey ! i have been given a school project where i have to run a motor on user input speed (accomplished ! ) and also try and verify this speed using a TACHOMETER . i used an H - bridge for motor control . my motor runs on the speed i want it to run at, ( maximum of 271 RPM on 12 V input) but my tachometer gives wrong readings and sometimes even negative values in the serial monitor . I don't understand what the problem is. Here is the code

int motor1Pin = 9; // H-bridge leg 1 
int motor2Pin = 8; // H-bridge leg 2 
int speedPin = 5; // H-bridge enable pin 
int ledPin = 13; 
int speed1 = 0;
int RPM = 0;
float value=0;
int percent;
float rev=0;
int rpm;
int oldtime=50;
int time;

void isr() 

{
rev++;
}

void setup() {

Serial.begin (115200);
pinMode(motor1Pin, OUTPUT); 
pinMode(motor2Pin, OUTPUT); 
pinMode(speedPin, OUTPUT);
pinMode(ledPin, OUTPUT);
digitalWrite(speedPin, HIGH);
attachInterrupt(0,isr,RISING);
digitalWrite(motor1Pin, HIGH); 
digitalWrite(motor2Pin, LOW); 
      
}

void loop()
{
  
  Serial.println("Enter RPM");
  
  while (Serial.available()<= 0)
  {}
  if (Serial.available() > 0)
  {
    RPM = Serial.parseInt();
    if ((RPM >= 0) || (RPM < 272))
    {Serial.print("speed is ");
      Serial.print(RPM);
      Serial.println(" RPM");
    
      
      speed1 = map (RPM , 0 , 271, 0 , 255);
       
      
      Serial.print("PWM value is ");
      Serial.println(speed1);
      Serial.println();
    
  
      
      analogWrite (speedPin, speed1);
    
    
detachInterrupt(0);   

time=millis()-oldtime;         
rpm=(rev/time)*60000;
    
oldtime=millis();             
rev=0;
    
    Serial.println(rpm);
Serial.println(" ");

attachInterrupt(0,isr,RISING);

    }
    else
    {   Serial.println("not possible! ");
      return;

    
  }


      
  }
}
2 
float rev=0;

You count fractional interrupts?

Why isn't it volatile?

int oldtime=50;

And millis() returns . . . what?

3

the old time = 50 takes care of any delays. The millis() returns an integer .I changed rev to volatile .

4

And the reference says

Number of milliseconds since the program started (unsigned long)

5

The millis() returns an integer

That's right, it returns an integer.
But it doesn't return an "int"

6

millis() returns unsigned long, a 32-bit number from 0x00000000 to 0xFFFFFFFF.

Technically an "integer" as it is a whole number, with no decimal points.
Not an "int", as that is signed number from -32768 to 32767 (0x8000 to 0x8FFF are negative, 0x0000 to 0x7FFF are positive)
Or an unsigned int, 0 to 65535

long is similarly a signed number, 32 bits.