Z8F6421PM020SC Datasheet, PDF (111/299 Pages) Zilog, Inc. – High Performance 8-Bit Microcontrollers Z8 Encore-R 64K Series

Z8 Encore!^® 64K Series
Product Specification
91
Watch-Dog Timer
Overview
The Watch-Dog Timer (WDT) helps protect against corrupt or unreliable software, power
faults, and other system-level problems which may place the Z8 Encore!^® into unsuitable
operating states. The Watch-Dog Timer includes the following features:
• On-chip RC oscillator
• A selectable time-out response:
• WDT 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
64K Series devices when the WDT reaches its terminal count. The Watch-Dog Timer uses
its own dedicated on-chip RC oscillator as its clock source. The Watch-Dog Timer has
only two modes of operation—ON and OFF. Once enabled, it always counts and must be
refreshed to prevent a time-out. An enable can be performed by executing the WDT
instruction or by setting the WDT_AO Option Bit. The WDT_AO bit enables the Watch-Dog
Timer to operate all the time, 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
given 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 once it reaches 000002H.
The WDT Reload Value must not be set to values below 000004H. Table 46 provides
information on approximate time-out delays for the minimum and maximum WDT reload
values.
PS019915-1005
Watch-Dog Timer