Mohit:

I am always using the Maxim reset circuit, MAX810
There are different types of this, dependning on threshold value,
MAX810M - 4,38V, 810T - 3,08V, 810R - 2,63V
MAX809 are same but inverted reset. (not to 8051)
It's encapsulated in SOT23

This generatess a proper start-reset for aprox. 300mS

It will cost a little, but I can guaranee, it's the best.

Also, if there is watchdog-timer in circuit , I can certainly
suggest that you shall use that.