Hi Parviz,

I apologize for misreading your post. You wrote S, I saw C.

I took a quick look at the datasheets for the S51, S52 and the S53 and S8252. (Don't you just love datasheets two years old, and still called "Preliminary"?) The difference between the two chip's programming algorithm that jumps out at me is the S53's and S8252's autoerase function, whereas the S51 and S52 require a chip erase command to be executed first.

Take a look at the software doing the programming. The programming serial interface is the same, so I would guess it's not the hardware.

Dennis