ST72141K datasheet(76/132 Pages) STMICROELECTRONICS

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ST72141K

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WATCHDOG TIMER (Cont’d)

8.2.3 Functional Description

The counter value stored in the CR register (bits

T6:T0), is decremented every 12288 machine cy-

cles, and the length of the timeout period can be

programmed by the user in 64 increments.

If the watchdog is activated (the WDGA bit is set)

and when the 7-bit timer (bits T6:T0) rolls over

from 40h to 3Fh (T6 become cleared), it initiates a

reset cycle pulling low the reset pin for typically

500ns.

The application program must write in the CR reg-

ister at regular intervals during normal operation to

prevent an MCU reset. The value to be stored in

the CR register must be between FFh and C0h

(see Table 38 . Watchdog Timing (fCPU = 8

MHz)):

– The WDGA bit is set (watchdog enabled)

– The T6 bit is set to prevent generating an imme-

diate reset

– The T5:T0 bits contain the number of increments

which represents the time delay before the

watchdog produces a reset.

Table 38. Watchdog Timing (fCPU = 8 MHz)

Notes: Following a reset, the watchdog is disa-

bled. Once activated it cannot be disabled, except

by a reset.

The T6 bit can be used to generate a software re-

set (the WDGA bit is set and the T6 bit is cleared).

If the watchdog is activated, the HALT instruction

will generate a Reset.

8.2.4 Low Power Modes

8.2.5 Interrupts

None.

8.2.6 Register Description

CONTROL REGISTER (CR)

Read /Write

Reset Value: 0111 1111 (7F h)

Bit 7= WDGA

Activation bit.

This bit is set by software and only cleared by

hardware after a reset. When WDGA = 1, the

watchdog can generate a reset.

0: Watchdog disabled

1: Watchdog enabled

Bit 6:0 = T[6:0]

7-bit timer (MSB to LSB).

These bits contain the decremented value. A reset

is produced when it rolls over from 40h to 3Fh (T6

becomes cleared) if WDGA=1.

STATUS REGISTER (SR)

Read /Write

Reset Value*: xxxx xxxx0

Bit 0 = WDOGF

Watchdog flag.

This bit is set by a watchdog reset and cleared by

software or a power on/off reset. This bit is useful

for distinguishing power/on off or external reset

and watchdog reset.

0: No Watchdog reset occurred

1: Watchdog reset occurred

* Only by software and power on/off reset

CR Register

initial value

WDG timeout period

(ms)

Max

FFh

98.304

Min

C0h

1.536

Mode

Description

WAIT

No effect on Watchdog.

HALT

Immediate reset generation as soon as

the HALT instruction is executed if the

Watchdog is activated (WDGA bit is

set).

WDGA

----

WDOGF

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