Watchdog Timer Calculator

This tool calculates the actual timeout periods (in milliseconds and seconds) for both the Independent Watchdog (IWDG) and Window Watchdog (WWDG) timers on STM32 microcontrollers. It uses user-provided clock, prescaler, and reload/window values to derive precise timeout windows — critical for ensuring timely system reset or recovery without false triggers.

IWDG is a free-running, low-power timer with no window constraint—ideal for simple system recovery. WWDG requires refreshes within a strict time window to prevent reset, enabling detection of both early and late software hangs. Their distinct architectures demand independent parameter handling and timeout formulas.

Common IWDG prescalers are powers of 2 from 4 to 256 (e.g., 4, 8, 16, 32, 64, 128, 256). The reload value ranges from 0x000 to 0xFFF (0–4095), where higher values increase timeout. Default settings like 64 prescaler + 4095 reload yield ~1.1 s timeout at 32 kHz LSI clock.

IWDG typically runs on the low-speed internal RC oscillator (LSI ≈ 32 kHz), while WWDG uses the APB1 clock (often derived from HCLK via PCLK1, e.g., 42 MHz). This makes WWDG much faster and more precise—but also more sensitive to clock configuration errors in RCC setup.

A window value that’s too high (e.g., close to 0x7F) narrows the refresh window excessively, increasing risk of accidental reset. Too low (e.g., < 0x40) widens the window but reduces hang-detection capability. STM32 requires WWDG window values between 0x40 and 0x7F for valid operation.

Yes — by computing exact timeout margins, you can size your reload values to comfortably exceed worst-case execution time (including debug halts). For debugging, consider disabling IWDG/WWDG in startup code or using ST-Link’s “halt watchdog” option — but always re-enable for final firmware validation.

Measure the actual reset interval using an oscilloscope on a GPIO toggled just before watchdog refresh — or use a logic analyzer to capture the reset pin assertion. Account for interrupt latency and instruction cycles; discrepancies >5% may indicate incorrect clock assumptions or register write timing issues.

For IWDG: ~100 µs (prescaler=4, reload=0x001 @ 32 kHz) up to ~32 s (prescaler=256, reload=0xFFF @ 32 kHz). For WWDG: ~10 µs (PSC=1, W=0x7F @ 42 MHz) to ~2.5 ms (PSC=8, W=0x40 @ 42 MHz). Values outside these ranges are either unsupported or unsafe per STM32 datasheets.

LSI frequency varies ±10% across temperature/voltage — so a nominal 32 kHz IWDG clock may actually be 28.8–35.2 kHz. Use the calculator with worst-case clock values (e.g., 28 kHz) when designing for robustness, and consider trimming LSI via RCC_ICSCR if precision is required.

This calculator provides the hardware foundation — but effective watchdog usage requires software discipline: placing refresh calls in main loop exits, using multiple independent checks (e.g., task health flags), and avoiding refresh in error-handling paths. Never refresh blindly — only when core functionality is confirmed healthy.