In datasheet of AT89S52 you will find this passage



By "asynchronous" I mean a reset, which immediately resets all port pins when asserting the reset input high, without the need of oscillator allready running. A synchronous reset, on the other hand, needs the reset pin asserted high for at least two machine cycles while the oscillator is running. This will result in a brief period where the port pins are undefined.

I have taken a scope plot from one of my AT89S52 board:




The upper line is Vcc, showing the typical "waviness" due to mains rectification. The lower line is some bidirectional port pin. You will notice, that the port pin is "0" allthough the micro is reseted by a state-of-the-art reset controller (MAX1232). Only after start-up of oscillator (not shown in this plot) the port pin goes high, indicating that the AT89S52 provides no asynchronous reset.

Eric, as far as I know there are many many 8051 derivatives today, even modern ones, which do not provide an asynchronous reset.

Well, here it's only about 4msec that the port pin is undefined. But I have seen systems showing much longer start-up times of oscillator presenting a potential risk of dangerous shorts.

Kai