Hi Swapna,

The hardware within the 8051 is clearing or initializing the SFRs when Reset is active. If you don't give it sufficient time (see your particular data sheet for this value, it might not always be two cycles) it will not be finished and the results are not guaranteed--or rather they are guaranteed to be wrong. For example, the PC might not be reset to 0, so it will start executing code where ever it happens to be.

Only if you like headaches should you dare short time the reset line. Remember, even if it happened to work, it might not work all the time! It might fail after it's in your customer's hands (or while you are demonstrating it to your teacher).

Better to give it a lot of time, and best to use a proper supervisor chip that will guarantee a reliable reset.

Dennis