SparkFun Forums

[winston]

Something's got me curious.

ICs like the 4116 memory and the Intel 8080 CPU are not single supply - not only do they require +5v, but -5v and +12v - and must be powered up and down in a very specific order (-5v must be powered first, and +12v last, and the reverse order when powering off).

I've only seen vague "bias the substrate" explanations for this assortment of voltages needed for these ICs - does anyone here have a good explanation of the purpose of these voltages - why they were needed and how they became eliminated from later designs, such as the Z80 or 4164 memory?

[leon_heller]

I think those chips used a different process from the later NMOS devices.

The newer 5V only NMOS chips had the generator on the chip. They used an oscillator and charge-pump to generate the negative voltage.

Leon

[wiml]

I don't think on-chip oscillators and charge pumps became common until much later than that. I could be wrong.

OTOH, I don't know why (eg) the Z80 had simpler power supply requirements than earlier NMOS stuff. Interesting question. Wikipedia says that although the 8080 supposedly required four rails, some clones would operate with the -5v at 0v and the +12v at +5v. My guess is that the extra rails were needed to keep circuits biased properly but that better control over gate threshold voltages or something like that allowed the extra voltages to be eliminated. I'm still trying to google up a definitive answer, though...