Johnson Counter
Duration: 3 min
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AI Summary
An AI-generated summary of this video lecture.
The video lecture introduces the Johnson Counter, also known as a Twisted Ring Counter or Switch-tail Ring Counter. The instructor explains that while a standard k-bit ring counter provides k distinguishable states, a switch-tail configuration doubles this number. The defining characteristic is a circular shift register where the complemented output (Q-bar) of the last flip-flop is connected to the input (D) of the first flip-flop. The instructor uses a 4-bit D flip-flop diagram to illustrate this feedback loop. He then begins to construct a state table to demonstrate the counting sequence, starting with an initial state of 0000. Finally, he derives the formula for the total number of states in a Johnson counter, which is 2n, and calculates it for a 4-bit system.
Chapters
0:00 – 2:00 00:00-02:00
The instructor defines the Johnson Counter using on-screen text that describes it as a 'circular shift register with the complemented output of the last flip-flop connected to the input of the first flip-flop.' He points to a diagram of four D flip-flops connected in a shift register, highlighting the feedback path from the inverted output of the last stage to the input of the first. He then draws a state table with columns for CLK, Q0, Q1, Q2, and Q3. He populates the initial row (CLK 0) with zeros (0000) and draws arrows to visualize the shifting of bits from one flip-flop to the next, preparing to trace the sequence.
2:00 – 2:44 02:00-02:44
The instructor focuses on calculating parameters for this specific counter configuration. He writes the general formula '2^n - 2n' on the board. He substitutes n=4 into the equation, writing '2^4 - 2(4) = 16 - 8'. This calculation highlights the relationship between the total possible states (2^n) and the valid states (2n) for a Johnson counter. He emphasizes this formula as a key property of the switch-tail ring counter.
The lecture progresses from a theoretical definition to a practical calculation. It starts by distinguishing the Johnson Counter from a standard ring counter based on its feedback mechanism (complemented output). The instructor visually demonstrates the circuit structure and begins the process of analyzing its state sequence using a table. The lesson culminates in a mathematical derivation of the counter's modulus (number of states), establishing the formula 2n as a critical takeaway for exam preparation.