ST6253C datasheet(28/83 Pages) STMICROELECTRONICS

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ST6253C ST6263C ST6263B ST6260C ST6260B

DIGITAL WATCHDOG (Cont’d)

3.3.1 Digital Watchdog Register (DWDR)

Address: 0D8h

—

Read/Write

Reset status: 1111 1110b

Bit 0 = C: Watchdog Control bit

If the hardware option is selected, this bit is forced

high and the user cannot change it (the Watchdog

is always active). When the software option is se-

lected, the Watchdog function is activated by set-

ting bit C to 1, and cannot then be disabled (save

by resetting the MCU).

When C is kept low the counter can be used as a

7-bit timer.

This bit is cleared to “0” on Reset.

Bit 1 = SR: Software Reset bit

This bit triggers a Reset when cleared.

When C = “0” (Watchdog disabled) it is the MSB of

the 7-bit timer.

This bit is set to “1” on Reset.

Bits 2-7 = T5-T0: Downcounter bits

It should be noted that the register bits are re-

versed and shifted with respect to the physical

counter: bit-7 (T0) is the LSB of the Watchdog

downcounter and bit-2 (T5) is the MSB.

These bits are set to “1” on Reset.

3.3.2 Application Notes

The Watchdog plays an important supporting role

in the high noise immunity of ST62xx devices, and

should be used wherever possible. Watchdog re-

lated options should be selected on the basis of a

trade-off between application security and STOP

mode availability.

When STOP mode is not required, hardware acti-

vation without EXTERNAL STOP MODE CON-

TROL should be preferred, as it provides maxi-

mum security, especially during power-on.

When STOP mode is required, hardware activa-

tion and EXTERNAL STOP MODE CONTROL

should be chosen. NMI should be high by default,

to allow STOP mode to be entered when the MCU

is idle.

The NMI pin can be connected to an I/O line (see

Figure 19) to allow its state to be controlled by soft-

ware. The I/O line can then be used to keep NMI

low while Watchdog protection is required, or to

avoid noise or key bounce. When no more

processing is required, the I/O line is released and

the device placed in STOP mode for lowest power

consumption.

When software activation is selected and the

Watchdog is not activated, the downcounter may

be used as a simple 7-bit timer (remember that the

bits are in reverse order).

The software activation option should be chosen

only when the Watchdog counter is to be used as

a timer. To ensure the Watchdog has not been un-

expectedly activated, the following instructions

should be executed within the first 27 instructions:

jrr 0, WD, #+3

ldi WD, 0FDH

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