Z8F0113HJ005EG Datasheet, PDF (106/245 Pages) Zilog, Inc. – High-Performance 8-Bit Microcontrollers

Z8 Encore! XP^® F0823 Series
Product Specification
91
Watchdog Timer
The Watchdog Timer (WDT) protects against corrupt or unreliable software, power faults,
and other system-level problems which can place Z8 Encore! XP F0823 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 WDT is a retriggerable one-shot timer that resets or interrupts F0823 Series devices
when the WDT reaches its terminal count. The Watchdog Timer uses a dedicated 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. Per-
form 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 down counter 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 59 provides
information about approximate time-out delays for the minimum and maximum WDT
reload values.
Table 59. 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
PS024315-1011
PRELIMINARY
Watchdog Timer