Gate 2001_
Duration: 3 min
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The video is a lecture on digital logic design, specifically analyzing a sequential circuit problem from the GATE 2001 exam. The instructor solves a problem involving a 3-bit counter using D flip-flops. He derives excitation equations from the circuit diagram, constructs a state transition table, and traces the state sequence starting from an initial condition to identify the correct option among multiple choices.
Chapters
0:00 – 2:00 00:00-02:00
The instructor introduces a GATE 2001 problem involving a circuit with three D flip-flops labeled $Q_0$ (LSB), $Q_1$, and $Q_2$ (MSB). He derives the excitation equations by tracing the connections: $D_0$ is the output of an XOR gate with inputs $Q_2$ and $Q_1$, so $D_{0N} = Q_{2P} \oplus Q_{1P}$. $D_1$ is connected to $Q_0$, so $D_{1N} = Q_{0P}$. $D_2$ is connected to $Q_1$, so $D_{2N} = Q_{1P}$. He then sets up a state transition table with columns for Present State ($Q_{2P}, Q_{1P}, Q_{0P}$) and Next State ($Q_{2N}, Q_{1N}, Q_{0N}$) and begins filling it systematically for all 8 states from 000 to 111.
2:00 – 3:11 02:00-03:11
The instructor completes the state table and traces the sequence starting from the initial state $Q_0=1, Q_1=0, Q_2=0$ (decimal 1). He follows the transitions: 1 (001) goes to 2 (010), then to 5 (101), then to 3 (011), then to 7 (111), then to 6 (110), then to 4 (100), and finally back to 1 (001). He draws this cycle on the board to visualize the flow: 1 -> 2 -> 5 -> 3 -> 7 -> 6 -> 4 -> 1. By comparing this derived sequence with the given options, he identifies option (B) '1, 2, 5, 3, 7, 6, 4' as the correct answer.
The lecture demonstrates a systematic approach to analyzing sequential circuits. First, the excitation equations for the flip-flops are derived from the combinational logic feeding their inputs. Second, a state transition table is constructed to map every present state to its corresponding next state. Finally, the sequence is traced starting from the specified initial state to determine the circuit's behavior over time. This method allows for the verification of the circuit's counting sequence against the provided multiple-choice options.