Thanks for the sugestion, but I've already done that for hours.

I've done a lot of measurements on hardware, and it all seems to be ok.

This is my ISP Cable - works fine with AT89S8252 and Atmel AT89ISP Software. So, I don't think this is the problem. My flat cable length is small, only 20cm. (This schematic is used in lab classes, in a university).

This is my schematic.

I've just tried to use an external oscillator (10MHz) instead of my crystal (I've tried a 22.1184MHz and a 12MHz crystal), to check if it could be a problem with Fuse4, but it didn't work too.

I've seem that many people are using other programmers, so I guess my next step will be building another JTAG and trying other softwares. (Even though I don't think this will help much, because if my cable works with AT89S8252, it should work with AT89S8253).

I'm aware of the differences between the 2 processors. I've read datasheets, the "Migrating to AT89S8253" article, and a lot of stuff. But maybe there's something I am missing. Didn't think it was going to be so hard to pass from the 8252 to 8253.

The people who are programming the AT89S8253, could you please send me your considerations?

Thank you very much.
Leonardo Müller.