ST52T400 datasheet(54/94 Pages) STMICROELECTRONICS

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ST52T400/T440/E440/T441

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8 WATCHDOG TIMER

8.1 Functional Description

The Watchdog Timer (WDT) 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 WDT circuit

generates an MCU reset on expiry of a pro-

grammed time period (Timeout), unless the pro-

gram refreshes the WDT before the end of the

programmed time period itself.

16 different time delays can be selected by using

the WDT configuration register REG_CONF2 as

in Table 8.2.

WDT is activated by the assembler instruction

WDTRFR

At the end of the programmed time delay, WDT

starts a reset cycle pulling the reset pin low.

During normal operation, when WDT is active, the

application program has to refresh this peripheral

at regular intervals to prevent an MCU reset. WDT

refresh is performed by the WDTRFR assembler

instruction.

To stop WDT during the user program executions

instruction WDTSLP has to be used.

WDT working frequency is equal to Master Clock

frequency divided by a fixed Prescaler with a divi-

sion factor of 500, to obtain WDT CLK signal that

is used to fix the WDT Timeout period (Figure

8.1).

With a Master Clock of 20MHz, for instance, a

WDT Timeout period can be defined between

0.025ms and 234.375ms, depending on WDT

REG_CONF2 values.

Timeout delay values at different Master Clock fre-

quencies can be calculated as the product of WDT

clock number pulses (Table 8.2) by WDT CLK

period (Table 8.4).

Warning: changing the REG_CONF2 value when

the WDT is active, a WDT reset is generated and

the CPU is restarted. To avoid this side effect, use

the WDTSLP instruction before changing the

REG_CONF2.

Table 8.1 Watchdog Timing range (CLKM=20

MHz)

WDT Timeout period (ms)

min

0.025

max

234.375

Figure 8.1 Watchdog Block Diagram

REG_CONF 2

RESET

WDTRFR

PRES CLK = CLK MASTER

WDTSLP

PRESCALER

WDT

RESET

GENERATOR

RESET

WTD CLK

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