# Electronics and Communication Engineering - Digital Electronics - Discussion

23.

A JK flip flop has tpd= 12 ns. The largest modulus of a ripple counter using these flip flops and operating at 10 MHz is

 [A]. 16 [B]. 64 [C]. 128 [D]. 256

Answer: Option D

Explanation:

Number of flip-flops = = 8.333 say 8

Modulus = 28 = 256.

 Deepak said: (Jun 27, 2015) How they have calculated no of flip flops? Can anybody explain?

 Sujideepika said: (Jul 12, 2016) Can anyone explain what is tpd?

 Apurnak said: (Jul 23, 2016) @Sujideepika - TPD is propagation delay.

 Javed Rizvi said: (Aug 4, 2016) Let's have an example. I bought some eggs for 30 rupees, and I know the cost of 1 egg which is 5 rupee. Then you can calculate the number of eggs I bought. By dividing i.e. 30/5 = 6 eggs. Ok. In the same ways in the given question, propogation delay time for 1 JK flip-flop is given and the combinational resulting operating frequency is also given, Then we can change the combinational frequency into time by the simple conversion I. E (time = 1 / frequency). And after that, we can calculate the number of flip-flop used by dividing I. E (total time taken / propagation time delay of one flip-flop). After that. If n flip-flop is given then the total modulus can be solved by the formula (modulus^n). Hope I can explain clearly.

 Javed Rizvi said: (Aug 4, 2016) At the last two lines. The formula used for the modulus will be (2^n).

 Akshay Damre said: (Sep 21, 2016) Exact calculation formula for the number of flip-flops used is, F = 1/(N * tpd). Where, N-> No of flip flops. F-> Frequency. Tpd-> prop. Delay time.

 Santosh said: (Dec 30, 2016) Nice explanation @Akshay.

 Karun Gupat said: (Feb 22, 2017) Thanks to all of you for explaining the question.

 Sridevi said: (May 7, 2020) Fmax-max operating freq. tpd- total propagation delay. n - no.of Flipflops. Fmax= 1/ n*tpd. n= 1/ (10MHz*12ns). n~ 8. For mod counter its is 2^n. so 2^8 =256.

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