80C186EC/80C188EC Microprocessor User's Manual

WATCHDOG TIMER UNIT
12.2.2 Watchdog Timer Reload Value
The Watchdog Timer Reload Value is controlled by the WDTRLDL and WDTRLDH registers
in the Peripheral Control Block. These two registers make up the 32-bit reload value.
The Watchdog Timer Reload Value cannot be modified after the Watchdog Timer is reloaded using
the reload instruction sequence. Locking the WDT Reload Value prevents errant code from
affecting Watchdog Timer operation.
The WDT Reload Value should be calculated based on the design of the system software. If the
system is executing a simple control loop, the Reload Value should be slightly longer than the
longest path through the loop. If the Watchdog Timer is reloaded during the timer tick service,
the Reload Value should be slightly longer than the timer tick interval. In general, determining
the Reload Value involves analysis of the system software and some amount of experimentation.
wdt_data
segment
wdt_key DB 0AAH, 055H
wdt_data
wdt_code
ends
segment
assume cs:wdt_code
mov
mov
mov
ax, wdt_key
ds, ax
si, offset wdt_key
;ES:SI points to reset value for
;WDTCLR
mov dx, WDTCLR ;I/O address of WDTCLR
cld
;clear direction flag (autoincrement)
mov cx, 2 ;2 bytes will be written
lock rep outsb es:[si] ;LOCKed reload sequence
;The WDT down counter
;has been reloaded.
wdt_code
ends
Example 12-1. Reload Sequence (Peripheral Control Block Located in I/O Space)
12-4