Brownout and Power-Loss Safety
A supply does not fail like a light switch. Pull the plug, drop a battery connector, or let a coin cell finally reach the end of its discharge curve, and VDD does not fall to zero — it decays, over a span set by whatever capacitance is still holding charge on the rail and whatever the circuit is still drawing from it. That decay is the entire subject of this page, because it is the only thing standing between "the supply is gone" and "the core has actually stopped running," and the honest answer to "how long do I have" is: a lot less than most designs assume, and the amount is calculable rather than a matter of taste.