Rich,
?
That can work IF the fail rate is slow enough from initial 5V to trigger
level to assure enough work (save state).? Very iffy and depends on
precision in the supervisory device.? Also those NV-rams have their
own power fail so writing to them could be locked out.? ?The
general assumption with most supervisory chips is assuring
power good before releasing reset.? ?Less so for power fail (unsafe).
?
The big boys (DEC, DG, IBM, and others) did it by first detecting
when?main power fails (AC power) or has decayed below acceptable
limits.? That is used to provide POWER-FAIL or some similar named?
control voltage.? The also provide a set of signal to say DC-GOOD.
That is then used to trigger high priority interrupt, or in the case
of Z80 NMI (non maskable interrupt).
?
This is all assembly language work.
?
I've built systems that do the latter as most (many) supervisory
circuits have boundary cases that may fail.? Harder than it looks.
as then the power supply must have enough "hold" time before
it completely fails to regulate.? The system I still have uses the
raw DC side that is typically 6-8 volts higher than DC out (5V bus).
That gives early warning of potential power fail and? assert
NMI.? ?That was an analog supply with some larger caps to
store enough for 80-100 milliseconds of known good
regulated output.? Typically a NMI routine that preserves
state is under a few milliseconds then it self halts before
power fail.
?
Start up is a Debug monitor system with option of
restore-resume in an orderly way or do a cold start.
?
An alternate way is to have the CPU turn itself off.
Simple push button to NMI makes the system? go to
a quiescent state (stop the machine or finish disk write)
then store its state and under program control
VIA IO port (relay or electronics switch) turn off
the power.
?
The latter is used for many cases as power fail is a?
random event that may have impacts (in robotics?
you need all stop, in an orderly way!) where power
off can be a user command (or event) with
orderly shut down.
?
Startup can be restore and run or cold start depending
on system and potentially establishing? external states
for external hardware or mechanics before resuming
operation but that is a system level consideration like
a machine tool or robotics application.
?
Its a system level application or use situation programming
with hardware to support it.
?
?
Allison
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