IWDG·WWDG 워치독 구현 — Independent vs Window 비교

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LSI = 32 kHz (typ)
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Prescaler = 4, 8, 16, 32, 64, 128, 256
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Reload (RLR) = 12-bit (0 ~ 4095)
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Timeout = (RLR + 1) × prescaler / 32000
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Examples:
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  PR=4,  RLR=4095 → 4096 × 4   / 32000 = 0.512 s
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  PR=64, RLR=4095 → 4096 × 64  / 32000 = 8.19  s
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  PR=256, RLR=4095 → 4096 × 256 / 32000 = 32.77 s (max)

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void iwdg_init_1sec(void) {
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    IWDG->KR = 0x5555;        // unlock
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    IWDG->PR = 4;             // /64 → ~500 Hz
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    IWDG->RLR = 500;          // 500 / 500 = 1 sec
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    IWDG->KR = 0xAAAA;        // refresh (start counter)
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    IWDG->KR = 0xCCCC;        // start
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}
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static inline void iwdg_kick(void) {
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    IWDG->KR = 0xAAAA;
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}
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// main
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iwdg_init_1sec();
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while (1) {
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    do_work();
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    iwdg_kick();   // 100 ms마다 — 1 sec 한참 전
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}

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#define TASKS 4
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static volatile uint32_t task_alive_mask;
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#define ALL_TASKS_MASK ((1u << TASKS) - 1)
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void task_alive(int id) {
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    task_alive_mask |= (1u << id);
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}
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// 최상위 supervisor
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void watchdog_task(void) {
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    if (task_alive_mask == ALL_TASKS_MASK) {
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        iwdg_kick();
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        task_alive_mask = 0;   // 다음 주기
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    }
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    // 다른 task가 죽으면 mask가 안 차고 IWDG가 reset
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}

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void wwdg_init(void) {
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    RCC->APB1ENR |= RCC_APB1ENR_WWDGEN;
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    // PCLK1 = 42 MHz, prescaler /8 = 5.25 MHz → /4096 = 1.28 kHz
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    // counter 0x40~0x7F (64 step), 1 step = 0.78 ms
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    // window W = 0x60 (=96) → 0x40 (=64): 32 steps = 25 ms
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    // total timeout: 0x7F (=127) → 0x3F = 64 steps = 50 ms
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    WWDG->CFR = (1u << 8)             // WDGTB = /8
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              | (3u << 7)             // EWI off
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              | 0x60;                 // W = 0x60
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    WWDG->CR = (1u << 7) | 0x7F;      // WDGA = 1, counter = max
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}
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static inline void wwdg_kick(void) {
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    WWDG->CR = (WWDG->CR & ~0x7F) | 0x7F;   // refresh — 0x7F write
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}

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DBGMCU->APB1FZ |= DBGMCU_APB1_FZ_DBG_IWDG_STOP
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                | DBGMCU_APB1_FZ_DBG_WWDG_STOP;

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typedef enum {
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    RESET_POR,       // power-on
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    RESET_PIN,       // NRST pin
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    RESET_SOFT,      // NVIC_SystemReset()
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    RESET_IWDG,      // independent watchdog
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    RESET_WWDG,      // window watchdog
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    RESET_LPWR,      // low-power management
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    RESET_BOR,       // brown-out
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} reset_cause_t;
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reset_cause_t get_reset_cause(void) {
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    uint32_t csr = RCC->CSR;
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    RCC->CSR |= RCC_CSR_RMVF;  // clear flags
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    if (csr & RCC_CSR_IWDGRSTF) return RESET_IWDG;
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    if (csr & RCC_CSR_WWDGRSTF) return RESET_WWDG;
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    if (csr & RCC_CSR_SFTRSTF)  return RESET_SOFT;
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    if (csr & RCC_CSR_PORRSTF)  return RESET_POR;
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    if (csr & RCC_CSR_PINRSTF)  return RESET_PIN;
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    if (csr & RCC_CSR_BORRSTF)  return RESET_BOR;
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    if (csr & RCC_CSR_LPWRRSTF) return RESET_LPWR;
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    return RESET_POR;
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}

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iwdg_init_1sec();
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GPIOA->BSRR = (1u << 5);   // LED on
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delay_ms(500);
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GPIOA->BSRR = (1u << 21);  // LED off
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delay_ms(500);
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GPIOA->BSRR = (1u << 5);   // LED on
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while (1);   // hang here — watchdog should reset after 1 sec