Z8F082ASH020SC Datasheet, PDF (97/244 Pages) Zilog, Inc. – Z8 Encore XP-R F08xA Series with eXtended Peripherals

Z8 Encore! XP^® F08xA Series
Product Specification
79
Watch-Dog Timer
Overview
The Watch-Dog Timer (WDT) protects against corrupt or unreliable software, power
faults, and other system-level problems which may place the Z8 Encore! XP^® F08xA
Series devices into unsuitable operating states. The Watch-Dog Timer includes the follow-
ing features:
• On-chip RC oscillator
• A selectable time-out response: reset or interrupt
• 24-bit programmable time-out value
Operation
The Watch-Dog Timer (WDT) is a retriggerable one-shot timer that resets or interrupts the
Z8 Encore! XP^® F08xA Series devices when the WDT reaches its terminal count. The
Watch-Dog Timer uses a dedicated on-chip RC oscillator as its clock source. The Watch-
Dog 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 instruc-
tion or by setting the WDT_AO Flash Option Bit. The WDT_AO bit forces the Watch-
Dog Timer to operate immediately upon reset, even if a WDT instruction has not been
executed.
The Watch-Dog 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 Watch-Dog Timer RC oscillator
frequency is 10KHz. The Watch-Dog Timer cannot be refreshed after it reaches 000002H.
The WDT Reload Value must not be set to values below 000004H. Table 55 provides
information about approximate time-out delays for the minimum and maximum WDT
reload values.
PS024705-0405
PRELIMINARY
Watch-Dog Timer