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Power on sequencing I understand. But why do power off sequencing? I could understand if you had to do something before power died, or maybe you just wanted to shutdown I/O before core so that no crazy pins are toggled while you're on your down.

Is there a situation where you could risk damaging the part? Only thing I think of there is maybe ESD diodes.

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The same problems as during power on apply to powering off.

A common problem is that in ICs, transistors are placed so that their junctions are reverse biased by the supply rails. If a higher voltage supply turns on slower or shuts of faster than a lower voltage one, that assumption may no longer be true, large currents can flow through the (now) forward biased junctions, destroying them.

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