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If the propagation delay of a gate is said to be 10ns, what will happen if a pulse of width less than 10ns is applied ? Say a buffer has prop. delay = 10ns. Will it swallow the pulse like in Z2 ? Or propagate it with a delay 10 ns like in Z1 ? Propagation delay is actually inertial or transport delay ?

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The detail depends on how the gate is built internally. Some have more elements in series than others, which will reduce the length of pulse they will transmit.

In practice, a pulse of 2nS will likely get swallowed by a gate with a 10nS delay. However, as the pulse length is increased, it will likely be transmitted, at least in part, before it's as long as 10nS.

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The propagation delay is a conservative estimate for the analog settling time after modification of an input on the worst case path. You cannot draw any more conclusions about the part from that number alone.

Important points:

  • it's a conservative estimate, so it has been adjusted by engineers so that only a negligible number of parts will even come close to it and parts that do not perform to this specification can be discarded without incurring heavy costs.
  • it describes the behavior of an analog circuit. After the settling time, interpreting the analog signal as a digital signal is safe, but before that, this needn't be the case.
  • there are multiple signal propagation paths of different lengths. For gates with multiple inputs, that is obvious, but even a NOT gate uses two transistors, which might not switch at the exact same time.

Especially the bit about seeing gates as analog devices on the lowest level is important for power consumption estimates when cascading gates. During the settling time, the output is essentially undefined and might also be at an intermediate level. A gate using this as an input could then have transistors in half-conductive states, drawing more power than normally, and in turn outputting an invalid signal that causes the same down the gate tree.

So, both answers are allowed as well as anything in between, or a voltage that corresponds to neither 0 nor 1.

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