Why would a Intel 8080 chip be destroyed if +12 V is connected before −5 V?
I don't have a complete answer for you, but the 8080 was one of Intel's first chips to use an NMOS process rather that the PMOS process of the 4004, 4040, and 8008 chips. In NMOS, the substrate must be the most negative point in the entire circuit, in order to make sure that the isolating junctions of other circuit elements are properly reverse-biased.

So, I suspect that the -5V supply, among other things, is tied directly to the substrate, and if the other voltages are supplied without this bias present, there are all kinds of unintended conduction paths through the chip, many of which could lead to latch-up and self-destruction.

To answer your last question, if your power supply doesn't have the correct sequencing by design, then you need a separate sequencer &mdash; a circuit that itself requires the -5V supply to be present before it allows the other voltages to reach the chip.



To echo some of the comments on your question, I don't recall any special care being taken in the actual 8080-based systems of the day.

However, such systems were usually built with four power supplies &mdash; or more precisely, two pairs of power supplies: &plusmn;5V and &plusmn;12V (-12V would have been used in any serial interfaces), each driven from a transformer winding and a bridge rectifier. It would have been natural for the 5V supplies to come up before the 12V supplies &mdash; and of those two, -5V would be quicker than +5V, being far less heavily loaded.

So (again I'm guessing), the power supplies either "just worked" in terms of sequencing, or the danger was not really as severe as the datasheet writers would have you believe.