3 docs tagged with "flash"

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.

Almost every mechanism that makes a computer fast makes it less predictable, and the trade is usually invisible in source code. Caches, prefetchers, branch speculation, bus arbitration and dynamic memory allocation are all bets on history repeating: they are fast when the recent past resembles the present and slow when it does not. In a throughput-oriented system that is an unambiguously good bargain. In a system with deadlines it means the number you have to defend — the worst case — is set by the unlucky path through every one of them, while every measurement you take is dominated by the lucky one.

Flash is not memory that happens to be non-volatile. It is a device with an asymmetric write model, and every design decision about storing settings on an MCU comes out of that asymmetry: you can clear a bit at any time, cheaply, one word at a time — but you cannot set a bit back to 1 without erasing an entire sector, which takes up to two seconds, during which the processor cannot fetch instructions from the same flash it is executing from.