RM0312 datasheet(98/275 Pages) STMICROELECTRONICS

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Independent watchdog (IWDG)

RM0312

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Independent watchdog (IWDG)

15.1

Introduction

The independent watchdog peripheral can be used to resolve processor malfunctions due to

hardware or software failures. It is clocked by the 38 kHzLSI internal RC clock source, and

thus stays active even if the main clock fails.

15.2

IWDG functional description

Figure 18 shows the functional blocks of the independent watchdog module.

When the independent watchdog is started by writing the value 0xCC in the key register

(IWDG_KR), the counter starts counting down from the reset value of 0xFF. When it reaches

the end of count value (0x00) a reset signal is generated (IWDG RESET).

Once enabled, the independent watchdog can be configured through the IWDG_PR, and

IWDG_RLR registers. The IWDG_PR register is used to select the prescaler divider feeding

the counter clock. Whenever the KEY_REFRESH value (0xAA) is written in the IWDG_KR

watchdog reset is prevented.

The IWDG_PR and IWDG_RLR registers are write protected. To modify them, first write the

KEY_ACCESS code (0x55) in the IWDG_KR register. The sequence can be aborted by

writing 0xAA in the IWDG_KR register to refresh it.

Refer to Section 15.3: IWDG registers for details on the IWDG registers.

Figure 18. Independent watchdog block diagram

Hardware watchdog feature

If the hardware watchdog feature has been enabled through the IWDG_HW option byte, the

watchdog is automatically enabled at power-on, and generates a reset unless the key

option byte description in the datasheet.

WDG reset

prescaler

8-bit down-counter

IWDG_PR

IWDG_RLR

reload register

7-bit

IWDG_KR

key register

LSI clock

(38 kHz)

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