Product Reference
2–59
Watchdog Timer
The purpose of a watchdog timer is to restart the system should some mishap occur. Possible
problems include:
! A failure to boot properly
! Application software losing control
! Temporary power supply problems including spikes, surges, or interference
! Failure of an interface device
! Unexpected conditions on the bus
! Other hardware or software malfunctions
The watchdog timer helps assure prope r st art-u p af ter an y in terr up tion.
The Little Board/486e ROM BIOS supports the board’s watchdog timer function in two ways:
! There is an initial watchdog timer setting, spe cifie d us ing S ETU P, wh ich dete rmine s whe ther
the watchdog timer is used to monitor the system boot, and if so, the length of the timeout. The
options are Disable, 30 seconds, 60 se cond s, an d 90 sec onds .
! There is a standard ROM-BIOS function that can be u sed by app licatio n so ftwa re to start, sto p,
and retrigger the watchdog timer functio n.
The initial time-out should be set using SETUP to be long enough to guarantee that the system can
boot and pass control to the application. Once the system is booted and the application is running,
the application must periodically retrig ger the t imer so th at a watc hdog timer t ime-o ut do es no t
occur. If the time-out does occur, the system responds in a manner determined by how the
watchdog timer jumper, W4, is set. See Figure 2–11.

Figure 2–11. Watchdog Timer Response Jumper (W4)

The watchdog timer uses the standard alarm feature of the real-time clock. In a standard AT, the
alarm output is connected to IRQ8. On the Litt le Boar d/486e C PU you can al so jump er the alar m
output to I/O Channel Check (-IOCHCK) or RESET with W4. I/O Channel Check is the bus signal
that triggers a non-maskable interrupt (N MI). RESE T is a hard rese t sig nal, the same as p res sing
the Reset button. Watchdog timer responses are summ arized in Table 2–45.
Table 2–45. Watchdog Timer Setup

Jumper W4 SETUP WDT Response

W4-1/2 Shorted Enabled I/O Channel Check (NMI)
W4-2/3 Shorted Enabled Hardware Reset
W4 Open Enabled IRQ8 turns off interrupt. System continues unaf f ected
W4 Open Disabled No action

Selects -IOCHK Selects RESET

W4