ST52T410 datasheet(50/84 Pages) STMICROELECTRONICS

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ST52T410/ST52T420/E420

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9 WATCHDOG TIMER

9.1 Operational 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 cause the application program to

abandon its normal sequence. The WDT circuit

generates

MCU

reset

expiry

programmed time period, unless the program

refreshes the WDT before the end of the

programmed time delay.

16 different delays can be selected by using the

WDT configuration register.

After the end of the delay programmed by the

configuration register if the WDT is activated (by

using the assembler instruction WDTSFR), it starts

a reset cycle pulling the reset pin low.

Once the WDT has been activated the application

program has to refresh this peripheral (by the

WDTSFR

instruction) at regular intervals during

normal operation in order to prevent an MCU reset.

In order to stop the WDT during user program

execution the instruction WDTSLP has to be used.

Figure 9.1 Watchdog Block Diagram

The working frequency of the WDT (PRES CLK in

the Figure 9.1) is equal to the clock master. The

clock master is divided by 500, obtaining the WDT

CLK signal, which is used to fix the timeout of the

WDT.

According to the WDT configuration register

values, a WDT delay may be defined between 0.1

ms and 937.5 mS when the clock master is 5 MHz.

By changing the clock master frequency the

timeout delay can be calculated according to the

configuration register values REG_CONF 2, as

described in the following section.

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 9.1 Watchdog Timing range (CLK=5

MHz)

WDT timeout period (ms)

min

0.1

max

937.5

REG_CONF 2

RESET

WDTRFR

PRES CLK = CLK MASTER

WDTSLP

PRESCALER

WDT

RESET

GENERATOR

RESET

WTD CLK

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