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nuclear123

curious question on logical gates

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I thought all gates was equally fast. One clock cycle each. But this is not really my area so I could be wrong.

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It very much depends on the technology, but TTL ought to be the same switching speed for NOT and NAND since there's only 1 transistor's switching time in the chain.

Why are you asking?

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because i thought a NAND was a combination of a not and and gate and would therefore be slower but i've been told otherwise and i was curious

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Transistors in the gate are arranged in such way that the gate exhibits instantaneous output response for a given input. IC designers often use optimised gate libraries and the actual difference between NAND and NOT is really not that huge, even with more than 2 inputs.

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" the gate exhibits instantaneous output response for a given input."

It's not instant -- there are all sorts of charge/time curves in there. It **looks** instant, but it's very definitely not. The minute you get non-trivial circuitry, you have to run clock lines along with signals to things exactly to allow the switching to settle properly.

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Yeah I was over simplifying here. I mean we could start talking about clock skew, slew rate, and parasitic elements in gates, but what I was meant to say the signal propagation cost is roughly the same for a NOR gate and a NAND gate. So the state change is a "one step" process; a NAND gate does not introduce an extra overheads in propagation chain.

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