ZUSBOPTSC01ZACG datasheet(110/282 Pages) ZILOG

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PS022827-1212

P R E L I M I N A R Y

Watchdog Timer

Z8 Encore! XP® F082A Series

Product Specification

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:

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 10kHz. 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)

WDT Reload Value

(Decimal)

Approximate Time-Out Delay

(with 10kHz typical WDT oscillator frequency)

Typical

Description

000004

400

s

Minimum time-out delay

FFFFFF

16,777,215

28 minutes

Maximum time-out delay

WDT Time-out Period (ms)

WDT Reload Value

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