ST62T80B datasheet(24/78 Pages) STMICROELECTRONICS

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ST62T80B/E80B

DIGITAL WATCHDOG (Cont’d)

The Watchdog is associated with a Data space

tion 0D8h) which is described in greater detail in

Section 3.3.1 . This register is set to 0FEh on Re-

set: bit C is cleared to “0”, which disables the

Watchdog; the timer downcounter bits, T0 to T5,

and the SR bit are all set to “1”, thus selecting the

longest Watchdog timer period. This time period

can be set to the user’s requirements by setting

the appropriate value for bits T0 to T5 in the

DWDR register. The SR bit must be set to “1”,

since it is this bit which generates the Reset signal

when it changes to “0”; clearing this bit would gen-

erate an immediate Reset.

It should be noted that the order of the bits in the

DWDR register is inverted with respect to the as-

sociated bits in the down counter: bit 7 of the

DWDR register corresponds, in fact, to T0 and bit

2 to T5. The user should bear in mind the fact that

these bits are inverted and shifted with respect to

the physical counter bits when writing to this regis-

ter. The relationship between the DWDR register

bits and the physical implementation of the Watch-

dog timer downcounter is illustrated in Figure 14.

Only the 6 most significant bits may be used to de-

fine the time period, since it is bit 6 which triggers

the Reset when it changes to “0”. This offers the

user a choice of 64 timed periods ranging from

3,072 to 196,608 clock cycles (with an oscillator

frequency of 8MHz, this is equivalent to timer peri-

ods ranging from 384

µs to 24.576ms).

Figure 14. Watchdog Counter Control

OSC

÷12

RESET

VR02068A

÷28

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