What is an external watchdog IC, and when should you use one instead of the internal watchdog?

An external watchdog IC is a separate chip on the PCB — typically a small SOT-23 or SOIC-8 package — that monitors the MCU by expecting a periodic signal transition (toggle or pulse) on an input pin. If the MCU fails to toggle the pin within the configured timeout, the external watchdog asserts the MCU's hardware reset line (or cuts power via a load switch). Examples include the MAX6369 family, TPS3823, STM6601, and ADM6316.

You need an external watchdog when the internal watchdog's assumptions break down: (1) the internal watchdog shares a failure mode with the MCU itself — a power supply glitch that puts the MCU into a partially functional state may corrupt the IWDG configuration registers while leaving the CPU running erratically, or a brownout may halt the LSI oscillator that clocks the IWDG; (2) the MCU enters a latch-up condition (triggered by ESD, radiation, or overvoltage on an I/O pin) where it draws excessive current and is electrically stuck — no internal watchdog can recover from this because the entire digital core is non-functional; (3) safety standards require a monitoring path that is physically independent of the device being monitored — this is common at ASIL C/D and SIL 3/4, where a single IC failure must not disable both the application and its monitor.

External watchdogs also provide capabilities that internal watchdogs typically lack: power-on reset generation with proper voltage supervisory (the IC monitors VDD and holds the MCU in reset until power is stable), brownout detection with configurable thresholds, reset pulse width guarantees (ensuring all peripherals see a long enough reset pulse to properly initialize), and cascaded timeout architectures for multi-processor systems. Some external watchdogs offer window mode (configurable minimum and maximum feed times), providing the same timing integrity check as the STM32 WWDG but as a fully independent device with its own oscillator and power domain.