Produced below is the coding for RPM display by the reciprocal method.
Only the main is given to keep the posting short.

The RPM display is quite fine with an averaging of
about 5 samples( that code not appearing here )

Problem is to clear the display when the RPM=0.The code gets stuck
at any of the points [A] to [D]and the display holds the last
value before the rpm became zero.

Any ideas ? ( The MCU is 89v51rd2 and T2 is unused )

  
void main(void)
  {
    union merge {tChar hilo[4]; tLong regval;} t1val;                 
    ...
    ...
    while(1)
    {
    msDelay(100);               // One update every 0.1sec  
     
    while (RPMCLK == 0);        // [A]wait to clear the partial low of clock
    while (RPMCLK == 1);        // [B]wait to clear the partial high of clock
        
    TR1 = 1;                    // start timer_1 to count MCU clock pulses

    while (RPMCLK == 0);           [C] start counting the 1micro pulses
    while (RPMCLK ==1);	       //  [D] continue counting for 1 full period
 
    TR1 = 0 ;                  // stop counting. 

    tmpResult = ovrfloCnt * 65536 ;  // Get the total period of overflows
		
    t1val.hilo[3] = TL1;       // (LSB) merge TL1 and TH1 into a long int... 
    t1val.hilo[2] = TH1;       //     ... to get the residual count
    t1val.hilo[1] = 0;
    t1val.hilo[0] = 0;         // (MSB)	
		
    t1val.regval = t1val.regval + tmpResult;				

    tmpResult = 2000000 / t1val.regval; // Transfer function to get RPM
      
    sprintf(dispBuffer, "%04lu", tmpResult ); // Display the value..

    ...
    ...
  }
 }

Thanks

Raghu