User`s manual

Chapter 3 Software Configuration
Page:3-10
PMB-1751LF USER
′ S MANUAL
3-8. WATCHDOG TIMER CONFIGURATION
This board has watchdog timer function for monitoring whether the system
is still work or not after a period of time. The user can select watchdog
timer to system reset or NMI (Non Maskable interrupt) depending on the
jumper set in “Reset/NMI/Clear Watchdog Selection” as described in
chapter 2. This is defined at I/O port 443H. When you want to enable the
watchdog timer, please write I/O port 443H, then the system will either
reset itself or perform the NMI function. Likewise, when you want to
disable the function, write I/O port 441H, the system will run the command
to stop the Watchdog function.
In PMB-1751LF watchdog function, You must write your program so when
it writes I/O port address 443 for enable watchdog and write I/O port
address 441 for disable watchdog. The timer's intervals have a tolerance of
25% so you should program an instruction that will refresh the timer about
every second.
The following program shows you how to program the watch timer in your
program.
Watchdog enable program:
MOV AX, 000FH (choose the values you need; start from 0)
MOV DX, 0443H
OUT DX, AX
Watchdog disable program:
MOV AX, 000FH (this value can be ignored)
MOV DX, 0441H
OUT DX, AX
The Watchdog Timer control table is as follows:
Level Value Time/sec Level Value Time/sec
1 F 0 9 7 64
2 E 8 10 6 72
3 D 16 11 5 80
4 C 24 12 4 88
5 B 32 13 3 96
6 A 40 14 2 104
7 9 48 15 1 112
8 8 56 16 0 120