14 votes

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)

19 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)

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

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

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

Right !! I missed the word series

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

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

0

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?