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Topic: Why don't CPUs and microcontrollers operate on 24v? (Read 2 times) previous topic - next topic


There is another consideration for using higher voltage, ( as in CMOS at 12v that I designed around for 40 years until I saw the Arduino light )

With  12v CMOS logic there is a typical 0.45vcc noise immunity ,  so 4 volts of noise on an input can be tolerated without it corrupting an input.

I havn't yet built an embedded Arduino project for a car ( although I do have one in the pipeline ) but I would be wary about using 3v3 logic without a buffer and filter for that notoriously ( electrical ) environment.
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Look at it this way:  The physical limits of materials are based on the strength of the electrical fields that they're exposed to.  (Air arcs over about about 20,000V per cm, for instance.)  Electrical field strength is measured in "Volts per Meter."  You get stronger electric field strength by increasing the voltage or by making your distances smaller.  The reason that modern electronics are so cheap is that the size of individual transistors keeps getting smaller; to keep the electric field strength similar, the operating voltage has to go down.
A vacuum tube operates at hundred of volts and has distances of a few mm; about 20000V/m field strength.  The transistors in a modern CPU have layers than are about 5 nano-meters thick (that's "not very many" atoms thick, BTW.)   Even at 1V operating voltage, that's something like 200000000V/m field strength!


Second, if you look at the specs for a transistor, you'll notice that many of the ratings are specified as CURRENTS. Although we often use voltages as a reference in digital circuits, deep down inside they're really ANALOG circuits, which require a certain amount of current to operate. If you design for a higher operating voltage, you still need to run a minimum amount of current through the transistors to get them to switch.

Do the math:

3.3 V x 0.010 A = 0.033 W

24 V x 0.010 A = 0.24 W

Multiply those Watts times thousands of transistors in an IC  and you can see where the power requirements (and heat) could quickly get out of hand.

Nicely! laid out
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Why don't CPUs and microcontrollers operate on 24v?

Slew rates?
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Every time you switch a transistor in a CPU, heat is released. The higher voltage you run, the more heat you generate. How fast can you run a 12V CPU without special cooling?

You want to run real fast highly complex CPU's then you gotta run the smallest transistors at low voltage, and still need heat sinks and fan for the hotrods.
I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

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