Z8F082ASH020SC datasheet(97/244 Pages) ZILOG

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PS024705-0405

P R E L I M I N A R Y

Watch-Dog Timer

Z8 Encore! XP® F08xA Series

Product Specification

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:

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.

WDT Time-out Period (ms)

WDT Reload Value

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