hi,

I haven't found any publishing, which at least gives a hint how to handle reset line of AT89S52 at power-up and power-down with ISP-programming.


Kai, I may suggest next way:
- use external power supervisor with "manual reset" input;
- connect positive reset output of SV to RST pin of MCU. If MCU has watchdog built-in and reset output of SV is not open-emitter then current limit resistor should be used as well;
- connect reset line from ISP programmer cable to manual reset input of SV (probably via invertor if this input is active low and ISP software does not allow to change reset active level).

Now MCU is good protected. Power supervisor protects MCU from power-up and brown-out problems, ISP software still be able to reset MCU. Moreover, most power SV which have manual reset input, provide reset-delay and so reset timings will be in safe conditions even if ISP software fails with it.

Regards,
Oleg