Ampro Corporation 486E - Manual (Page 75)

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 proper start-up after any interruption.
The Little Board/486e ROM BIOS supports the board’s watchdog timer function in two ways:
! There is an initial watchdog timer setting, specified using SETUP, which determines whether
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 seconds, and 90 seconds.
! There is a standard ROM-BIOS function that can be used by application software to start, stop,
and retrigger the watchdog timer function.
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 retrigger the timer so that a watchdog timer time-out does not
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 Little Board/486e CPU you can also jumper the alarm
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 (NMI). RESET is a hard reset signal, the same as pressing
the Reset button. Watchdog timer responses are summarized 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 unaffected
W4 Open Disabled No action
Selects -IOCHK Selects RESET W4