Sequential Circuits

Excitation table of Flip flops

Excitation of a flip-flop is actually exact opposite of what a truth table is. The truth table for the flip-flop gives us the output for the given combination of inputs and present output while an excitation table gives the input condition for the given output change.

E.g. As in truth table we say for T flip-flop if input T is 1 and previous Q is 0 then we have output as 1 while in excitation table we are given that present output Q is 0 and new Q is 1 then input T is 1.

Next we write truth table of various flip-flops and then we write their excitation flip-flops.

RS Flip-flop:

The truth table of the RS flip-flops is as:

Now to write the excitation table of this flip-flop we first write the various output changes possible as:

Now we can see from that truth table that to change output from 0 to 0, we can keep inputs S, R as 0, 0 or 0, 1 and we can write both the combinations as 0, X which means we just need to keep S=0 and R can have either of two possible values.

Similarly we can note that for output change from 0 to 1, we keep inputs at S=1, R=0.Similarly we can find the other cases and we get the table as:

Similarly we can find out the excitation tables for other kind of flip-flops as shown next:

D Flip-flop:

The excitation table of D flip-flop is as:

JK Flip-flop:

The excitation table of JK flip-flop is as:

For output change from 0 to 1 we can either keep inputs J, K as 1, 0 or we can make use of toggle input combination J=1, K=1 to get compliment of the output.

Similarly the other case 

T Flip-flop:

The excitation table of T flip-flop is as:

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