Foundations — Watchdog timers — purpose, feeding, types

Before you can read a single line of watchdog code or plug numbers into the timeout formula, you need to own about a dozen small ideas. The parent note the watchdog topic assumes you already know what a "counter", a "clock", a "prescaler", and a "reset" are. This page builds every one of them from nothing, in the order they depend on each other, so that when a symbol like $f_{clock}$ or $N_{max}$ shows up, you already know exactly what picture it points at.

1. A clock — the heartbeat that makes everything move

Picture a metronome for a piano student: tick… tick… tick… at perfectly even spacing. The chip has exactly this, but millions of ticks per second.

The single number that describes a clock is its frequency.

Why the topic needs it: the watchdog is driven by a clock. How fast the watchdog's counter changes is set entirely by this $f$. In the parent formula it appears as $f_{clock}$ — the frequency of the clock feeding the watchdog specifically (which, as we'll see, is often a different, independent clock from the CPU's — see Hardware Timer Peripherals).

$\text{time for one tick}=\frac{1}{f}$

2. A counter — remembering how many ticks have passed

Picture a numbered dial that the clock nudges one notch every tick. Left alone, a down-counter slides steadily toward zero.

Why the topic needs it: $N_{max}$ is where a fresh watchdog counter starts before counting down. "Feeding" the watchdog means slamming the counter back up to $N_{max}$. The bigger $N_{max}$, the longer the slide to zero, so it directly sets how long you have before a reset.

3. The prescaler — slowing the clock down on purpose

Here's a problem. A chip clock ticks millions of times per second. A 16-bit counter only holds up to $65535$. At full speed it would count down to zero in a tiny fraction of a second — far too fast to be useful. We need a way to make the counter tick slower.

Picture a turnstile that only opens on every 64th person: the crowd (fast clock) arrives quickly, but the room beyond (the counter) fills slowly.

Why the topic needs it: the prescaler is the tuning knob. It is exactly the prescaler in the parent's timeout formula. Small $P$ → fast counting → short timeout (fast recovery but risk of false resets). Large $P$ → slow counting → long timeout (tolerant of slow operations). Everything about choosing a watchdog timeout is choosing $P$.

4. Feeding — the act that keeps you alive

Picture pushing a boulder that is slowly rolling toward a cliff (zero) back up the hill. As long as you push often enough, it never rolls off.

Why the topic needs it: feeding is the whole interaction between your software and the watchdog. The parent's entire discussion of "feeding strategies" and "feeding too early / too late" is about when in your program you perform this one action.

5. Reset and interrupt — what happens at zero

Why the topic needs it: "reaches zero → reset or interrupt" is the whole point of the watchdog's existence. Without understanding what a reset actually does to a running program, "it resets the system" is meaningless.

6. Timeout period and the feasibility window

Why the topic needs it: these three symbols are the entire vocabulary of the "Window Watchdog" section. $T_{feed}$ is just when your code actually calls the feed instruction, measured from the last feed.

7. Why "independent clock" matters — the failure story

This is why the parent stresses "separate clock source" — and why a watchdog is a genuine Safe State Design tool, not just a fancy timer.

Prerequisite map

Read it bottom-up: clock and bits make a counting counter, the prescaler sets its speed, that gives the timeout, feeding resets it, and hitting zero fires the reset — which is the watchdog.

Equipment checklist

Cover the right side and answer each before moving to the parent note.