RM0312 datasheet(93/275 Pages) STMICROELECTRONICS

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RM0312

Window watchdog (WWDG)

14.1

Introduction

The window watchdog is used to detect the occurrence of a software fault, usually

generated by external interference or by unforeseen logical conditions, which causes the

application program to abandon its normal sequence. The watchdog circuit generates an

MCU reset on expiry of a programmed time period, unless the program refreshes the

contents of the downcounter before the T6 bit becomes cleared. An MCU reset is also

generated if the 7-bit downcounter value (in the control register) is refreshed before the

downcounter has reached the window register value. This implies that the counter must be

refreshed in a limited window.

14.2

WWDG main features

•

Programmable free-running downcounter

•

Conditional reset

–

Reset (if watchdog activated) when the downcounter value becomes less than

0x40

–

Reset (if watchdog activated) if the downcounter is reloaded outside the window

(see Figure 17)

•

Hardware/software watchdog activation (selectable by option byte)

•

Optional reset on HALT instruction (configurable by option byte)

14.3

WWDG functional description

If the watchdog is activated (the WDGA bit is set) and when the 7-bit downcounter (T[6:0]

bits) rolls over from 0x40 to 0x3F (T6 becomes cleared), it initiates a reset cycle pulling low

the reset pin. If the software refreshes the counter while the counter is greater than the

value stored in the window register, then a reset is generated.

Figure 16. Watchdog block diagram

RESET

WDGA

6-BIT DOWNCOUNTER (CNT)

WATCHDOG CONTROL REGISTER (wWDG_CR)

WATCHDOG WINDOW REGISTER (WWDG_WR)

comparator

T6:0 > W6:0

CMP

= 1 when

Write WWDG_CR

WDG PRESCALER

DIV 12288

fMASTER

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