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For synchronous series counter of modulus 256, the propagation delay for each flip flop is 25 nsec and propagation delay of each two input AND gate is 5 nsec. What is the maximum frequency of the MOD256 counter ? (in MHz)
in Digital Logic
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2 Answers

17 votes
 
Best answer

This is a 4-bit Synchronous Binary Up Counter

Here, each Flip flop get clock pulse at the same time. So, when the clock is applied, in 25 nsec all 4 flip-flop will give output + we need inputs for the next stage of flip-flops so additional 5 nsec + 5 nsec (two AND gates) will be needed. So, minimum time period needed for clock is 25 + 5 + 5 = 35 nsec.

According to ques. for MOD-256 counter we need 8 flip-flops
$\therefore$ we need (8-2) AND gates in series connection (as mentioned two input AND gates only) 

$F\_{clk}$ $= \frac{1}{T_{FF} + (8-2) T_{gate}}\\ = \frac{1}{25 + (8 -2) 5 }\\ = 0.01818 \times 10^{9}\\ = 18.18\ \ MHZ$


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0
explain the formula how?
5

@lokesh here i think ur problem how to get time period so i try to explain.. here propagation delay of FF  is taken  and also in question given that  2 - input AND gate so here if we design that in initial first two FF doesn't required any AND gate but AND gate is required from third FF because it takes a 2-input FF from FF1 and FF2  so that here initial FF1 and FF2 not reqiured any FF other than all FF required FF so here total  n-FF so i substract two FF.   so its final (n-2) * Tpd(AND)..i hope it help u

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ty Anirudh! I was considering AND gate for output
0

@Hradesh patel Any valid reference for the above formula, other than Made Easy ?

1

@Kapil see the first binary up counter in this link

https://www.doc.ic.ac.uk/~nd/surprise_96/journal/vol4/cwl3/report.html#design

assume initial state 0000 now when a clock is applied we need 25ns to get DCBA rt?

then we have to give 5ns + 5ns for generating inputs for FFs for next state rt?

that means 25+ 5 + 5 = 35 ns rt?

0
caz both AND gates are connected in series so we need to add each AND gate delay
0

Right !! I missed the word series 

and took it as parallel counter . In case of parallel counter, it will be $33.33$ MHZ.

0
what about the other ques??
1
You have commented there, Arjun Sir will check that. That one is also is also a case of series counter.
0
@Hradesh Patel, how it can be 4 bit counter, as it is given that it should ne mod256, How it can be 4 bit, please explain
4 votes

Propagation Delay for Synchronous counter does not depend on the no of flip flops because in sychronous counter clock is applied to each flip flop at the same time .

so Propagation delay of Synchronous counter = Propagation delay of Flip flop + delay due to combination circuit

                                                                  = 25 nsec + 5 nsec =30 nsec

So frequency = 1/ Time period = 1/30ns =33.33 MHZ


Not exact duplicate question ;- https://gateoverflow.in/26442/gate1991_5-c


edited by
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ans to ques given in made easy is 18.18
0

How ?
 Anirudh  kindly have a look 

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I didnt understood the given ans...they considered (n-2) AND gates and I dont know how and which??
0

with one single gate you cannot count till 256 ryt?

http://www.allaboutcircuits.com/textbook/digital/chpt-11/synchronous-counters/

0
Amit in case of asynchronous counter we will add the delay of all ffs right? Suppose the same question was replaced with asynchronous  then wat would be the answer?
1

this answer contradicts https://gateoverflow.in/26442/gate1991_5-c

both are series connection.In the link i shared only one gate delay considered but in the current question we are taking all?

0
@rahul sharma 5 , I think that answer will be 1/30 only here because that is the way synchronous counters work.
If it would have been asynchronous then answer will be 1/(6*5 + 8*25). Did you find any information regarding this ?
0
How are you getting 1/30?

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