...He just replaced the AT89S52 with AT89C52 and the problem was solved.

I do not want to go deeper into this. But here goes the story...

A few years back, I had designed an Industrial process Timer, used in mainly Injection Moulding machines in the Ejector Section. This is a AT89C52 Based product.

Due to (temporary) unavailabitly of AT89C52, I suggested my customer to use AT89S52. The Hex file was the same that was used for AT89C52. But almost 16 products came back for noise problem. He verified it that it was noise problem. So he purchased AT89C52 at a higher price from the market and fired the same Hex file, placed the AT89C52 back into the very same products (so no question about decoupling, PCB design, etc..) and the problem dissapeared. It just proved that AT89C52 is better than AT89S52 as far as noise is concerned. Same was done for P89C52X2BN, Even this IC failed terribly, AT89S52 was better than P89C52X2BN, And AT89C52 was better than AT89S52.

Now my customer is happy using AT89C52 (even at higher $$), and since all are happy, I do not want to dig further and waste my time after it. So I said that I do not want to go deeper into it.

Anyway thanks for your replies..
from Kiran V Sutar, Mumbai, INDIA