The pic devices are not really expected to interface to other proecessors/peripherals so the output characteristics can be slightly wobbly and they can bolt diodes onto the outputs.If you look at the structure of the output pins with the quite strange pseudo-bidirectional format makes it quite difficult to do the same thing,so they rely on parasitic transisotrs.When you model the 8052 in a fpga one of the most tricky parts is getting the outputs to behave correctly