Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Theory of Operation
The advantage of AC power-fail detection is that it provides early
warning of impending DC power failure. When jumper W1 is
installed, a non-maskable interrupt (NMI) is sent to the processor
when AC power below 95 VAC is detected. Alternatively, a maskable
interrupt on level 5 (IR5) may indicate the AC power failure when
jumper W1 is removed and W9A is installed. However, this indicator
may not be seen if a higher priority interrupt is being serviced or
interrupts are disabled.
Proper response to the interrupt depends on the application running,
but in general the response should be to save important system
parameters in battery-backed memory and then place the processor in
an idle or read-only loop. This guarantees the processor will not be
writing at the time of DC power failure, and thus fully protects the
system.
STD DOS assumes the application will supply the
appropriate non-maskable or maskable interrupt service routine for
power-fail.
If more than one source of non-maskable interrupt is in the system, it
is important to poll the system to determine the proper source. By
installing both W1 and W9A, and masking the interrupt level 5 at the
interrupt controller, only the non-maskable interrupt occurs; the fact
that it occurs is latched at the interrupt controller. The non-maskable
interrupt service routine then reads the interrupt request register (IRR)
at the interrupt controller to determine whether the interrupt was for
power-fail and responds properly. This requires that the interrupt
controller be reinitialized after each system reset in order to reset the
masked power-fail interrupt, but this is a standard requirement upon
power-up of any STD bus system using interrupts. Refer to Chapter 4,
"Application Examples," for a complete example of power-fail
detection with proper system startup after power-fail NMI.
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