How are "standard" voltages determined?

[MikH]

Hey,
   Just curious, does anyone know how the standard voltages (5V, 3.3V, etc) were originally decided upon?  Is there some cabal that decides this kind of thing?

[ftransform]

If there was a cabal then it would be 6.6 volts!

[Simon]

I'm guessing roughly randomly but bearing in mind what the components needed and could withstand on a physical level.

You will find that 3.3V is 2/3 of 5, the first attempt at lowering processor consumption, and after that 1.6V which is about 1/2 and from then it was about how low can we go without crashing the processor, because the lower the voltage the less heat and the more stable, but there will be a sweetspot below which the voltage is not enough to keep the speed as more voltage means faster speeds but you much watch the heat because more heat means slower switching - Ah the fie art of overclocking.......

[ejeffrey]

Most of the values are from the E12 series of preferred values.  I would guess that is why 3.3 V instead of say 3.1.  The notable exception is 5V, presumably chosen for being 5.

In specialized cases with strong power/performance constraints like PC DRAM banks, finer steps are common.

[Simon]

Well now that we have stopped wowing over processors that can work faster than we can see an LED flash at "standards" have been abandoned and fine tuning to squeeze the most possible has taken over. New processors work on 1.2V and the steps are like 0.05V

[saturation]

I think its a mix between practical design noise immunity [ buses, cables etc.,], fastest logic switching speed and power dissipation. 

When TTL became the workhorse logic, 5V became the defacto standard, as different logic families came to be and excelled over TTL, like CMOS, they maintained the 5V to maintain compatibility; there are were hybrid designs using both technologies as logic evolved over time, and now that TTL is mostly obsolete, designers can lower rail voltages to those of the modern chipsets.

http://en.wikipedia.org/wiki/Logic_family#Monolithic_integrated_circuit_logic_families_compared