ST6208CB3 datasheet(23/70 Pages) STMICROELECTRONICS

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ST62T08C/T09C ST62T10C/T20C/E20C

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 Digital Watchdog Register (DWDR).

This register is set to 0FEh on Reset: 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 Watch-

dog timer period. This time period can be set to the

user’s requirements by setting the appropriate val-

ue for bits T0 to T5 in the DWDR register. The SR

bit must be set to “1”, since it is this bit which gen-

erates the Reset signal when it changes to “0”;

clearing this bit would generate an immediate Re-

set.

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 17.

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 17. Watchdog Counter Control

OSC

÷12

RESET

VR02068A

÷28

111

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