Watchdog Timer Reload Value - Intel 80C188EC User 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 us-
ing 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
wdt_data ends
wdt_code segment
assume cs:wdt_code
mov
mov
mov
mov
cld
mov
lock rep outsb es:[si]
wdt_code ends
Example 12-1. Reload Sequence (Peripheral Control Block Located in I/O Space)
12-4
0AAH, 055H
ax, wdt_key
ds, ax
si, offset wdt_key
dx, WDTCLR
cx, 2
;ES:SI points to reset value for
;WDTCLR
;I/O address of WDTCLR
;clear direction flag (autoincrement)
;2 bytes will be written
;LOCKed reload sequence
;The WDT down counter
;has been reloaded.