Max Clock frequency with a Flip-Flop

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Discussion Overview

The discussion revolves around determining the maximum clock frequency of two interconnected J-K flip-flops sharing a common clock. Participants explore the implications of propagation delays, setup times, and hold times on the clock frequency, considering both theoretical and practical aspects of flip-flop operation.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about the maximum clock frequency given specific propagation delays and setup times for the flip-flops.
  • Another participant emphasizes the importance of meeting setup and hold times when drawing timing diagrams for the flip-flops.
  • A participant summarizes that the output of the first flip-flop must remain stable for the hold time of the second flip-flop, leading to a consideration of propagation delays and setup times.
  • There is a suggestion that the maximum clock rate could be determined to be 2 ns based on the setup time, but this is contested.
  • A later post introduces an algebraic approach, concluding that the clock period must be at least 7 ns to satisfy the timing requirements.
  • Another participant agrees with the algebraic conclusion, affirming that it aligns with a straightforward connection of the two flip-flops.

Areas of Agreement / Disagreement

Participants express differing views on the maximum clock rate, with some suggesting it could be 2 ns based on setup time, while others argue for a longer period based on propagation delays and hold times. The discussion remains unresolved regarding the exact maximum clock frequency.

Contextual Notes

Participants reference specific timing parameters (propagation delay, setup time, hold time) but do not reach a consensus on how these parameters definitively determine the maximum clock frequency. The discussion includes assumptions about the timing relationships between the flip-flops.

Number2Pencil
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I need to know the max clock frequency of two identical J-K flip flops linked together (the first flip flop has HIGHs running into its inputs, and the second flip flop has the outputs (Q and Q bar) running to its inputs). They share a common clock.

The data sheets say the prop delay (low to high and high to low) is 5 nano seconds, and the set-up time is 2 nano seconds. the max frequency is usually just given, but this time it's not.

would it be 1/5nano seconds, 1/2 nano seconds, or 1/7 nano seconds?? I can't figure it out...
 
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There will be a setup and hold time specified for the FFs, as well as propagation delays from input to output. If you draw a timing diagram, you will need to ensure that the setup and hold times are met for both FFs. So if they are linked in series, the output of the 1st FF must not change before the hold time of the 2nd FF is met. So the minimum prop delay through the first FF must not be shorter than the hold time requirement of the 2nd FF. And the max clock rate is limited by the delay of the 2nd FF and the worst-case setup time of the 2nd FF.


EDIT -- fixed a couple typos
 
so combining what you said with what I have:

--Qa must not change faster than 2 ns
--the minimum prop delay through 1st must not be shorter than hold time. True: prop delay = 5 ns, hold time = 2 ns

--max clock rate = 5 ns...worst-case: max clock time = 2 ns


so the maximum clock rate would be 2 ns?
 
Number2Pencil said:
so the maximum clock rate would be 2 ns?
No. You need the data to get through the first FF and be stable for the setup time before you can provide the next clock edge that will clock the FFs (whichever it is that clocks the flop, rising or falling).
 
after making some timing diagrams, I tried a little algebraic approach:

clock peroid - 5ns >= 2ns
clock peroid >= 7ns
 
Looks right to me, at least for a straghtforward hookup of two FFs. Good job.
 

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