Multiple choice

In asynchronous counter, the total propagation delay is n * tpd, while in synchronous counter, it is tpd. Why?

  1. The propagation delay occurs due to late arrival of the clock in the flip-flop and its own delay.

  2. Total propagation delay is tpd only.

  3. No propagation delay since the clock is available at the same time to all the flip-flops.

  4. Propagation delay is tpd + (n - 1).

  5. Propagation delay does not depend on the clock.
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A Correct answer
Explanation

Propagation delay is the time required by one flip-flop to respond and produce output, and is given as tpd. In asynchronous counter, the clock is provided only at the first flip-flop. Other flip-flop gets the clock from the Q output of the previous flip-flop. So, time required when the last flip-flop will get the clock is number of flip-flops * response delay of one flip-flop. This total time delay is called propagation delay of the counter. In synchronous counter, all flip-flops get the clock simultaneously.

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