Hi, I am new to 8051 programming. I have written and downloaded the following short codes onto a AT89C2051 using the Advantec LabTool-48 Universal Programmmer:

(...defines obmitted)
ORG 0000h
loop: MOV P1,#00
CLR P3.3
SJMP loop
END

I tested the micro with an oscilloscope and it works fine (all pins on port 1 and P3.3 at 0V).

BUT when I downloaded the same program onto a AT89S8252 (44 pin PLCC package) all pins remain high at ~5V. The 8252 chips I have are freshly bought from the supplier. The LabTool programmer (it can check for the device ID and bad pins to see if the chip is blown) doesn't complain about poor pin contacts and it programs the 8252 quite happily. I also used the LabTool programmer to subsequently read the program memory back out from the 8252 and the codes were correct. The reset pin does get a high signal for a short while (using the normal power on reset circuit). The same 11.0592MHz crystal is used and so the oscillator outputs for both micros looked that same on the scope too.

So what could make the 8252 not run the program that can be run on a 2051? Could a chip still be programmable with the output ports dead at 5V? Has anyone had similar problems before? ?_?