ST & T UPSD3212C - Manual (Page 46)

uPSD3212A, uPSD3212C, uPSD3212CV
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WATCHDOG TIMER
The hardware Watchdog Timer (WDT) resets the
uPSD321x Devices when it overflows. The WDT is
intended as a recovery method in situations where
the CPU may be subjected to a software upset. To
prevent a system reset the timer must be reloaded
in time by the application software. If the processor
suffers a hardware/software malfunction, the soft-
ware will fail to reload the timer. This failure will re-
sult in a reset upon overflow thus preventing the
processor running out of control.
In the Idle Mode the watchdog timer and reset cir-
cuitry remain active. The WDT consists of a 22-bit
counter, the Watchdog Timer RESET (WDRST)
SFR and Watchdog Key Register (WDKEY).
Since the WDT is automatically enabled while the
processor is running. the user only needs to be
concerned with servicing it.
The 22-bit counter overflows when it reaches
4194304 (3FFFFFH). The WDT increments once
every machine cycle.
This means the user must reset the WDT at least
every 4194304 machine cycles (1.258 seconds at
40MHz). To reset the WDT the user must write a
value between 00-7EH to the WDRST register.
The value that is written to the WDRST is loaded
to the 7MSB of the 22-bit counter. This allows the
user to pre-loaded the counter to an initial value to
generate a flexible Watchdog time out period.
Writing a “00” to WDRST clears the counter.
The watchdog timer is controlled by the watchdog
key register, WDKEY. Only pattern 01010101
(=55H), disables the watchdog timer. The rest of
pattern combinations will keep the watchdog timer
enabled. This security key will prevent the watch-
dog timer from being terminated abnormally when
the function of the watchdog timer is needed.
In Idle Mode, the oscillator continues to run. To
prevent the WDT from resetting the processor
while in Idle, the user should always set up a timer
that will periodically exit Idle, service the WDT, and
re-enter Idle Mode.
Table 32. Watchdog Timer Key Register (WDKEY: 0AEH)
Table 33. Description of the WDKEY Bits
7 6 5 4 3 2 1 0
WDKEY7 WDKEY6 WDKEY5 WDKEY4 WDKEY3 WDKEY2 WDKEY1 WDKEY0
Bit Symbol Function
7 to 0
WDKEY7 to
WDKEY0
Enable or disable Watchdog Timer.
01010101 (=55h): disable watchdog timer. Others: enable watchdog timer