Thanks for the reply MorganS 
, but i have got something else to ask.
okay framing my question in different way now.
I want to read the time between two input impulse sensed by using interrupt function of arduino and the time can be read by starting a counter on rising edge and storing the value just after a new rising edge comes and reseting the counter. The resolution of the time depends on the bits of counter whether it is 16 or 32.
so far i am not able to use the internal counters directly.
Could you please help me with the sketch?