Sequential Circuits

Counter other than MOD-2n

Q-Can we design a ripple counter other than MOD-2n?

Ans: Yes we can. For this we’ll first design the counter with value which is multiple of 2 but greater than the count required. Then we use a combinational circuit to reset the counter after the required value of count is achieved. Let’s take an example:

Design a MOD-14 counter.

First we design a counter of 2’s multiple greater than 14 which is 16. So we first design a MOD-16 counter as:

Now we need to design a combinational circuit which would take care that counter is reset when count value reaches 13. For this we first draw the waveforms as:

As we have to count till 13 and reset again. We see that when-ever Q4=1, Q3=1 & Q1=1, when have to reset the value of all the flip-flops so that we get the value of count as 0. Hence we take NAND of these 3 variables due to which we get a zero when all 3 variables are 1 and output of NAND gate is connected to all the ACTIVE LOW CLEAR lines to reset all flip-flops as follow. We also have to make sure that the output of this NAND gate is zero only after 13.

And now we the output waveforms as:

And we can clearly observe that we have achieved MOD-14 counter as all count values are reset after 13 but in this method we have to observe the output waveforms and then decide the combinational circuit to reset value after certain count.

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