Hi Babar,

I have never seen any 89C51 on its limit. E.g. most 12MHz devices running fine until 16MHz.

But you can try to use a 6MHz crystal, to check, if then the software works equal on both devices.

Have you really the exact same software burned into both ?

E.g., if you use the 89C51 with external code RAM, then you must add a LJMP 800Bh at address 000Bh to redirect the interrupt into the RAM. But this modify your interrupt response time.
And, if you not watch it, like Rauf, this was the reason.


Peter