ST62T55B Datasheet, PDF (24/74 Pages) STMicroelectronics – 8-BIT OTP/EPROM MCUs WITH A/D CONVERTER, AUTO-RELOAD TIMER, EEPROM AND SPI

ST62T55B ST62T65B/E65B
DIGITAL WATCHDOG (Cont’d)
The Watchdog is associated with a Data space
register (Digital WatchDog Register, DWDR, loca-
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 in13.
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 384µs to 24.576ms).
Figure 13. Watchdog Counter Control
D0
C
D1
SR
RESET
D2
T5
D3
T4
D4
T3
D5
T2
D6
T1
D7
T0
÷2⁸ OSC ÷12
VR02068A
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