Hi mahesh!

From your post I could make out that you are using timer 0 as input for the pulses. I'll give you a probable solution.(I haven't tried it practically, its just a theoretical solution.)

* Use timer 0 as a 16 bit counter to count external pulses.
* Use timer 1 as a 16 bit timer and configure it for a 100ms delay.
* Clear th0 and tl0 to 00h.
* Start timer 1.
* In the timer 1 ISR, obtain the value of th0 and tl0 registers.
* Now, this count is the number of pulses in 100ms i.e. 0.1sec. Hence, when multiplied the count by 10d it will give the frequency of the input signal.

IMPORTANT NOTE: The 8051 samples the T0 or T1 pin in S5,P2(State 5, Phase 2) of every machine cycle. So in order to increment the count by 1 in the counter the input to the T0 or T1 pin must be high for one machine cycle and low for one machine cycle. Thus, the minimum time period of the input signal should be two machine cycles.(i.e. maximum frequency that can be measured at the T0 or T1 pins is crystal frequency divided by 24.)
SOURCE OF ABOVE INFORMATION:
The 8051 microcontroller-Architecture, Programming and Applications.(Second Edition)
Author: Kenneth J Ayala