What's the difference between an independent watchdog and a window watchdog?

An independent watchdog (IWDG on STM32) runs from its own dedicated low-speed clock, separate from the main clock tree, so it keeps counting even if the system clock fails or is misconfigured, maximally robust, and once started it generally can't be stopped. You must kick it before it times out; kicking any time before the deadline is fine. A window watchdog (WWDG) adds a lower bound: you must kick within a window, not too late and not too early. Kicking too early triggers a reset just like kicking too late. The reason to forbid early kicks is that healthy firmware has a known timing cadence; if it suddenly starts kicking much sooner than expected, it's likely looping incorrectly or executing an unintended path, so the window watchdog detects both "stopped making progress" and "running abnormally," a stronger correctness guarantee than a simple timeout. You use IWDG for robust liveness against total hangs, and WWDG when you also want to catch timing/flow anomalies.