# What is the difference between excitation table and truth table?

In flip-flops what is truth and excitation table iam getting confused between them ,earlier I learnt a truth table of flip-flops but now its called excitation table so I wanna know whole thing behind this in detail ,what's the difference between truth table and excitation table and how are truth table and excitation table for a flip-flops are constructed.Detailed answer will be appreciated a lot.

Truth table shows the corresponding outputs for every combination of inputs. Consider the truth table of jk flip flop below

Q(t+1) is new output
Q(t) is present output
Q(t) bar is compliment of present o/p

Now look at excitation table below

Excitation table shows what should be the values of j and k so as to change the present output to the desired value (next output)

Recall Q(t+1) is new output
Q(t) is present output

1st combination: present state is 0 and we have to maintain same value to be next state meaning Q(t) and Q(t+1) is 0. Look at truth table. We can "reset" the o/p to 0 or maintain "unchanged" state. That is j and k should have either 1st or 2nd combinations of truth table. (J,k) is (0,0) or (0,1). Means j "must" be 0 and k is dont care (maybe 0 or 1).

2nd combination: present state is 0 and we have to change it to 1. Meaning Q(t) is 0 and Q(t+1) is 1. Look at truth table. We can "set" the o/p to 1 or "toggle" the state from 0 to 1. That is j and k should have either 3rd or 4th combinations of truth table. (J,k) is (1,0) or (1,1). Means j "must" be 1 and k is dont care (maybe 0 or 1).

3rd combination: present state is 1 and we have to change it to 0. Meaning Q(t) is 1 and Q(t+1) is 0. Look at truth table. We can "reset" the o/p to 0 or "toggle" the state from 1 to 0. That is j and k should have either 2nd or 4th combinations of truth table. (J,k) is (0,1) or (1,1). Means j is dont care (maybe 0 or 1) and k "must" be 1.

4th combination: present state is 1 and we have to maintain same value to be next state. Meaning Q(t) and Q(t+1) is 1. Look at truth table. We can "set" the o/p to 1 or maintain "unchanged" state. That is j and k should have either 1st or 3rd combinations of truth table. (J,k) is (0,0) or (1,0). Means j is dont care (maybe 0 or 1) and k "must" be 0.

Hence excitatation table is reverse of the truth table and is applicable only in seqential ciruits where the system remembers what the earlier output is.

• Lol i didn't look the date u asked the question. U would hav figured it out by now Mar 25, 2017 at 18:10

The truth table maps current states and inputs to outputs (on a circuit level).

An excitation table is used when a particular gates needs a particular output in order to implement the truth table. This is usually done for sequential elements (latches and flip flops), since the relationships between their inputs and their next state is not always direct.

A D-flip flop has the simplest excitation table (Q = D at the rising edge). For other sequential elements it is not so straightforward, and the current output state may be needed to properly compute the next state.

For example a T flip flop needs to know the current state to know what the proper input is to cause a '0' at the output. Also JK and SR latches have slightly more complex excitation tables.