digital logic: max clock frequency

Question

Answer 1

Twisted Ring Counter = Jhonson Counter 
There are 4 d flip flop means there will be total 8 total states 
Frequency = $\frac{f}{2n}$
                = $\frac{1}{8Tc}$ =  $\frac{1}{8*40}$ =  3.125Mhz   

Comments

As per my understanding..Twisted ring counter is a synchronous counter. and tile delay should be:

Total delay>=Propagation delay of one flip flop+propagation delay of gates

T=40ns

so

f= 1/40

f=25 MHZ

 

correct me if i am wrong!!!!

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According to me
in normal Jhonson counter output of each flip flop is after f/2n
and i have calculated of one flip flop frequency not the all.

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 Frequency(output) = fin/2nf2n

In question ,it is asking about maximum usable clock frequency.

in

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both are same i think!!
bcoz both means after how much time we get the output?