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Official PYQ · 2009

GATE CS 2009 Q27 — Sequential Circuits

Explain with AI

Given the following state table of an FSM with two states AA and BB,one input and one output.

PRESENTPRESENTNext State Next StateSTATE ASTATE BInputABOutput000001010100100010110100001010011001101011111001\small\begin{array}{|c|c|c|c|c|c|}\hline \textbf{PRESENT} & \textbf{PRESENT} & & \textbf{Next State } & \textbf{Next State} & \\ \textbf{STATE A} & \textbf{STATE B} & \bf {Input}& \bf {A} & \bf {B} & \bf{Output}\\ \hline \text{0} & \text{0} & \text{0} & \text{0} & \text{0} & \text{1} \\ \hline \text{0} & \text{1} & \text{0} & \text{1} & \text{0} & \text{0}\\ \hline \text{1} & \text{0} & \text{0} & \text{0} & \text{1} & \text{0}\\ \hline \text{1} & \text{1} & \text{0} & \text{1} & \text{0} & \text{0}\\ \hline \text{0} & \text{0} & \text{1} & \text{0} & \text{1} & \text{0} \\ \hline \text{0} & \text{1} & \text{1} & \text{0} & \text{0} & \text{1} \\ \hline \text{1} & \text{0} & \text{1} & \text{0} & \text{1} & \text{1} \\ \hline \text{1} & \text{1} & \text{1} & \text{0} & \text{0} & \text{1} \\ \hline \end{array}If the initial state is A=0,B=0A=0 ,B=0 what is the minimum length of  an input string which will take the machine to the state A=0,B=1A=0,B=1 with output=1output=1.

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Answer: A

To find the minimum length of the input string that takes the machine from the initial state (A,B)=(0,0)(A, B) = (0,0) to the state (0,1)(0,1) with an Output=1\text{Output} = 1, we can analyze the state transitions backward from the target condition.

Step 1: Identify the target transition
From the state table, look for transitions where the Next State is (0,1)(0,1) and the Output is 11:

  • Only the row with Present State =(1,0)= (1,0) and Input =1= 1 results in Next State =(0,1)= (0,1) with Output =1= 1.
Thus, the final transition must be from state (1,0)(1,0) to (0,1)(0,1) using input 11.

Step 2: Find the shortest path from (0,0)(0,0) to (1,0)(1,0)

  • From (0,0)(0,0), applying input 11 leads to state (0,1)(0,1) (Row 5).
  • From (0,1)(0,1), applying input 00 leads to state (1,0)(1,0) (Row 2).
So, the shortest input sequence to reach state (1,0)(1,0) from (0,0)(0,0) is 1010.

Step 3: Combine the sequences
Appending the final input 11 to the sequence 1010 gives the complete input string: Input String=101\text{Input String} = 101 Tracing this sequence: (0,0)→Input 1(0,1)→Input 0(1,0)→Input 1(0,1) with Output 1(0,0) \xrightarrow{\text{Input } 1} (0,1) \xrightarrow{\text{Input } 0} (1,0) \xrightarrow{\text{Input } 1} (0,1) \text{ with Output } 1 The minimum length of the input string is 33.

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