power down

The reason that I said that expecting anything to work during power down
is wishful thinking is this: once the voltage to a digital chip goes
below the minimum specification of the chip, the behavior of the chip
becomes indeterminate.

For example: the old Western Digital 1791 double density disk controller
chip would sometimes glitch in such a way during power down that it
would write to the floppy - you could see the floppy light blink when
this happened.

Unless chips are specifically designed to handle power down conditions
this sort of thing happens.  For example - any competently designed
Flash memory has to refuse to write if the voltage is below spec.

As to flushing the buffers and doing a shutdown when a power fail
condition occurs - I believe that Linux already has code to handle a
power down such as I described. What I have described is very similar to
a UPS signaling the kernel that power is going down. Linux can do an
ordered shutdown when it receives the signal.

Qualifying digital circuitry with a POWER GOOD signal is very similar to
protecting the circuitry with a typical 'SCR over voltage crowbar
circuit': it makes the engineer feel good - but it doesn't actually do
much of anything.

Why doesn't the crowbar work? After all, it is a text book circuit. The
answer is that the SCR is a power device which takes on the order of 10
microseconds to turn on while the delicate chips are destroyed by a few
nanoseconds of over voltage. The result is that the SCR never turns on -
the fuse blows because the weakest digital chip  shorts the power supply