Datex-Ohmeda Aestiva 7900 SmartVent, Service Manual

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2/Theory of Operation
2-18 1503-0151-000 3/27/97
Watchdog Systems
68040 Software Watchdog Timer
The watchdog timer in the 68340 processor activates for a time period appropriate
for the application. If it times out due to a software execution fault (e.g. software
trapped in an infinite loop), a level 7 interrupt (non-maskable) is internally issued so
that the exception handler software can attempt to correct the error transparently.
Operating Mode Watchdog
This watchdog detects errant program execution using an address matching tech-
nique as well as providing a time-out function. The time base for the temporal mon-
itoring is independent from the system clock. Logical monitoring is accomplished
by assigning a unique watchdog toggle address to each defined mode of operation
and detecting a sequence error if, due to program flow interruption, an operating
mode is entered illegally. The output of this watchdog is connected to IRQ7 on the
68340 processor.
Error Response Sequence
Errors detected by either watchdog are handled in the following sequence:
• At the first detection of any error, a watchdog responds by issuing
an IRQ7 interrupt.
• If the exception handler software for IRQ7 cannot correct the error,
then the next detection of an error causes a soft system reset.
• If the error still cannot be corrected, then an audio alarm sounds (in-
dependent of processor interaction) and a cyclic soft reset contin-
ues until the error is corrected or the system is powered down.