Z8F042ASJ020EG2156 Datasheet, PDF (108/280 Pages) Zilog, Inc. – High-Performance 8-Bit Microcontrollers

Z8 Encore! XP^® F082A Series
Product Specification
93
Watchdog Timer
The Watchdog Timer (WDT) protects against corrupt or unreliable software, power faults
and other system-level problems which may place the Z8 Encore! XP F082A Series
devices into unsuitable operating states. The features of Watchdog Timer include:
• On-chip RC oscillator
• A selectable time-out response: reset or interrupt
• 24-bit programmable time-out value
Operation
The Watchdog Timer is a one-shot timer that resets or interrupts the Z8 Encore! XP F082A
Series devices when the WDT reaches its terminal count. The Watchdog Timer uses a ded-
icated on-chip RC oscillator as its clock source. The Watchdog Timer operates in only two
modes: ON and OFF. Once enabled, it always counts and must be refreshed to prevent a
time-out. Perform an enable by executing the WDT instruction or by setting the WDT_AO
Flash option bit. The WDT_AO bit forces the Watchdog Timer to operate immediately
upon reset, even if a WDT instruction has not been executed.
The Watchdog Timer is a 24-bit reloadable downcounter that uses three 8-bit registers in
the eZ8 CPU register space to set the reload value. The nominal WDT time-out period is
described by the following equation:
WDT Time-out Period (ms)
=
-^W----^D----^T----^R----^e---^l-^o---^a--^d-----^V---^a---^l-^u---^e-
10
where the WDT reload value is the decimal value of the 24-bit value given by
{WDTU[7:0], WDTH[7:0], WDTL[7:0]} and the typical Watchdog Timer RC oscillator
frequency is 10 kHz. The Watchdog Timer cannot be refreshed after it reaches 000002H.
The WDT reload value must not be set to values below 000004H. Table 58 provides infor-
mation about approximate time-out delays for the minimum and maximum WDT reload
values.
Table 58. Watchdog Timer Approximate Time-Out Delays
WDT Reload Value
(Hex)
000004
FFFFFF
WDT Reload Value
(Decimal)
4
16,777,215
Approximate Time-Out Delay
(with 10 kHz typical WDT oscillator frequency)
Typical
400 s
Description
Minimum time-out delay
28 minutes
Maximum time-out delay
PS022828-0413
PRELIMINARY
Watchdog Timer