the code to be able to calculate the frequency of external pulse at P3_2 is as below:

bit flag = 0;
unsigned int timercounts = 0;
float time = 0.0;
unsigned int rpm = 0;

void main(void){
  TMOD = 0x09;   //sets timer0 in 16-bit mode
  //timer0 will run only when INT0(3.2) is high
  TH0 = 0;
  TL0 = 0;
  TR0 = 0;
  IPL0 = 0x01;  //highest priority to external interrupt 0
  IPH0 = 0x01;
  IEN0 = 0x81;    //enabling external interrupt
  while(1);
}

void externalinterrupt(void) interrupt 0    //invoked on every positive edge
{
   if(!flag){
   TR0 = 1;    //enable timer0
   flag = !flag;
  }
   if(flag){
   TR0 = 0;   //disable timer0
   IEN0 = 0x00;    //disable all interrupts
   flag = !flag;
   timercounts = (TH0*256) + TL0;
   time = timercounts/921583;
   rpm = (unsigned int)(30/time);
   }
}


however the output is quite bizzare as it shows a constant no. even if the input pulse is changed....pls suggest where i may have gone wrong