Discussion
The propagation delay of a logic gate is
Answer: B
Correct answer: the time between an input change and the corresponding output change
Propagation delay is the time taken for a change at the input to appear at the output. It is measured between the 50 per cent points of the two transitions, and it is usually quoted separately for a low-to-high and a high-to-low transition, since the two are rarely equal. It is the fundamental limit on clock speed. The longest chain of gates between two flip-flops - the critical path - must settle within one clock period, so the maximum frequency is the reciprocal of that total delay plus the flip-flop’s own setup time. It is also why a ripple counter is slower than a synchronous one: each stage waits for the one below it, and the delays accumulate. The fourth option describes fan-out, the number of gate inputs a single output can drive, which is a different specification and is itself limited by current and by the loading capacitance that increases the delay.
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