Hi shankara,

if the UART reacts different, it seems, you have some code, which access the T2 control registers.
On the 89C51 it was ignored, but on the 89C52 it can change T2 to be used as baud rate generator instead T1.

The 89C52 / 89S8252 was fully downward compatible.
Only acessing SFRs, Flash or RAM adresses, which not present on the 89C51 can cause different working.


Peter