most of user are suggesting rc circuit should be avoided,agreed but why rc circuit not creating problem with 89c52,89c52 can run for continuse 24 hours in same board 89s52 starts creating problem even in first minute.

I do not think anyone said that RC reset was guaranteed to be the reason for this, but if one is reset and the othere is !reset (you DID check that, I hope) you will definitely have a problem.

The reason RC reset has been hammered is that the more recent your uC is the more sensitive to reset it seems to be. I shall not post this as a fact, but while RC reset has always been problematic, the problems get bigger and bigger for every advance in processor technology.

SO, why wonder "is it the RC?" just install a supervisor with the correct polarity of reset and wonder no more.

Nobody stated that a circuit with RC reset will never work, it is, however a fact that a RC reset will not always work regardless of which uC you use. You may never have heard about it, your customer switched the unit on twice to get it going and did not bother to report it. Your unit screwed up when a brownout happened, your customer did not expect your unit to operate correctly afterwards and thus did not tell you. Maybe you never had a brownout, maybe you have a PS with a very fast risetime (for now, till the components age), maybe, maybe, maybe. The whole point in using a supervisor is that it removes the "maybe" and replaces it with "it is".

Erik