watchdog timer design

**Watchdog Timer and System Health Monitor Design** is **the dedicated hardware subsystem that continuously monitors processor operation and system health indicators, automatically triggering corrective actions (reset, interrupt, or safe-state transition) when software execution hangs, thermal limits are exceeded, or supply voltages drift outside specification** — providing the autonomous safety net that enables reliable operation in unattended and safety-critical systems. **Watchdog Timer Architecture:** - **Basic Watchdog**: a free-running down-counter clocked by an independent oscillator (not derived from the main CPU clock); software must periodically write a specific value to the watchdog register (kick/pet/feed) before the counter reaches zero; if the software fails to respond (hung, crashed, stuck in infinite loop), the counter expires and asserts a system reset - **Windowed Watchdog**: extends the basic watchdog by defining both a minimum and maximum time window for the kick; the software must respond neither too early nor too late; early kicks indicate runaway execution (software looping too fast); this catches a broader class of software malfunctions than a simple timeout - **Independent Watchdog**: uses a completely separate clock source (dedicated RC oscillator or crystal) and power domain from the main CPU; continues operating even if the CPU clock fails; essential for automotive ASIL-D and aerospace applications where the watchdog itself must be immune to the failure modes it monitors - **Multi-Stage Watchdog**: provides multiple escalating timeout levels; first timeout generates a non-maskable interrupt (NMI) giving software a chance to recover; second timeout asserts a warm reset; third timeout (if warm reset fails) triggers a cold power-cycle reset **System Health Monitoring:** - **Temperature Monitoring**: on-die thermal sensors (BJT-based or ring oscillator-based) measure junction temperature at multiple locations; hardware comparators trigger interrupts when temperature approaches the thermal throttle threshold (typically 100°C) and force shutdown above the critical threshold (typically 125°C) - **Voltage Monitoring**: on-chip ADC or comparator circuits monitor VDD core, VDD I/O, and other supply rails; under-voltage detection prevents operation below the minimum voltage for reliable logic switching; over-voltage detection prevents gate oxide stress and reliability degradation - **Clock Monitoring**: a clock supervisor circuit checks that the main clock is running within the expected frequency range; loss-of-clock detection triggers failsafe mode using the backup oscillator; frequency out-of-range indicates PLL malfunction - **Memory Health**: periodic ECC scrubbing of SRAM and flash checks for accumulated bit errors; crossing a correctable error threshold indicates aging or radiation damage that may require preventive maintenance or safe shutdown **Design Considerations:** - **Kick Sequence**: simple single-write kicks are vulnerable to accidental writes from runaway software; robust watchdog designs require a specific multi-step unlock sequence before the kick is accepted, ensuring that only intentional software action can reset the timer - **Reset Behavior**: the watchdog reset output must be clean (glitch-free) and held for sufficient duration (typically >100 μs) to ensure all chip blocks properly initialize; the reset cause is recorded in a persistent status register so that software can identify watchdog-triggered resets at boot - **Testability**: the watchdog must be testable during manufacturing without waiting for the actual timeout period; test modes provide accelerated timeouts and direct access to the counter and status registers - **Power Consumption**: the independent watchdog and its oscillator operate continuously, even in low-power sleep modes; power consumption must be minimized (typically <1 μA total) to avoid significantly impacting battery-powered device standby time Watchdog timer and system health monitor design is **the essential autonomous safety infrastructure in every microcontroller and SoC — providing the hardware-level failure detection and recovery mechanism that keeps systems running reliably when software encounters unexpected conditions, from consumer electronics to life-critical automotive and medical devices**.