89S8252 does not support RS232 serial code download (but dows SPI serial download). If you still want to use RS232 download, you should add additional hardware to your design or take a different 8051 processor (DALLAS has this feature as far as I know).

As additional hardware, you can add AT80C2051 additional uC that interfaces your PC with RS232 and AT89S8252 by SPI interface. (this is a small 20 pin 8051 compatible device). AT89C2051 can share the same clock with AT89S8252. If you just want to make a programmer make an adaptor using RS232 --> SPI convertor (AT89c2051).


Hope this helps a bit. Franc