... and I'm not at all certain that your particular MCU (AT89S8253) uses that, you want a PULLDOWN resistor, which will pull RESET to an inactive level once the capacitor is charged. If you want a NEGATIVE-going nRESET pulse, then you need a PULLUP resistor, in which case, the charge time of the capacitor will determine how long the nRESET pulse remains low. It's never that simple, however, as, during power-on, other factors are at work, e.g. the risetime of Vcc, the discharge current through the resistor, and, of course, the particular details of the mode in which you'd like your MCU to wake up.

The best place to start is the MCU datasheet. That will give you SOME information. Then, of course, the specifics of the ISP circuit writeups you've found (You have found 'em, haven't you?) will be at play, too.

Some, and in fact, most, old hands with various 805x flash-based MCU's will tell you that you should use a RESET generator or "supervisor" IC in order to handle RESET/nRESET. While I'm not in that camp, a dozen or so seasoned experts who are can't be wrong about the fact it helps to use one of those.

Datasheets from manufacturers who build their own version of a supervisor IC generally include 'em in their MCU app-notes. OTOH, those who don't, well, they don't.

If you can find a solid specification for exactly how long the RESET/nRESET pin on your particular MCU must be held high or low, then, with the aid of a calculator and a schmidt-trigger inverter, you can fabricate a circuit that produces a sufficiently long active-high/low pulse of appropriate voltage and stability during both power-up/power-down conditions and occasional brownouts, should they occur. Keep in mind that odd things can happen when Vcc is outside the recommended range, particularly when other on-board circuits are behaving oddly as well. Remember, too, that different portions of your application circuit may have different requirements of Vcc and RESET/nRESET. You must meet them all.

RE