excitation table

Guri Bee

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20-10-2024

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Understanding Excitation Tables: A Guide for Sequential Circuit Design

Excitation tables are essential tools in the design of sequential circuits, allowing you to map the desired behavior of a circuit to its physical implementation. This guide delves into the concept of excitation tables, exploring their structure, creation, and practical application.

What are Excitation Tables?

An excitation table is a tabular representation of the next state of a flip-flop based on its current state and the input signals. It essentially outlines the "rules" governing the transitions between states in a sequential circuit.

Key Components of an Excitation Table:

• Present State (Q): Represents the current state of the flip-flop.
• Next State (Q+): Represents the desired state of the flip-flop after the clock cycle.
• Inputs (I): The input signals affecting the flip-flop's behavior.
• Excitation (X): The required input to the flip-flop to achieve the desired transition from the present state to the next state.

Example:

Consider a JK flip-flop. Its excitation table looks like this:

This table shows that to transition from a present state of 0 to a next state of 0 (i.e., stay in the same state), the J input needs to be 0, while the K input can be either 0 or 1.

Creating an Excitation Table

1. Define the Circuit's States: List all possible states of the circuit.
2. Determine the Inputs: Identify all input signals that influence the state transitions.
3. Define Desired State Transitions: For each state, define the desired next state based on the input signals.
4. Fill the Excitation Table: For each possible combination of present state and inputs, determine the required excitation inputs (J, K, etc.) to achieve the desired next state.

Using Excitation Tables in Circuit Design

1. State Assignment: Assign binary codes to each state of the sequential circuit.
2. State Transition Diagram: Create a state transition diagram based on the desired behavior of the circuit. This visual representation helps understand the circuit's operation.
3. Excitation Table Creation: Use the state transition diagram to fill the excitation table for each flip-flop used in the circuit.
4. Logic Implementation: From the excitation table, derive the logic equations for each flip-flop's input (J, K, etc.). These equations are then implemented using logic gates.

Example: Implementing a Simple Counter

Let's design a simple 2-bit counter using JK flip-flops. The counter should cycle through states 00, 01, 10, 11, and repeat.

1. States: 00, 01, 10, 11

2. Inputs: None, as this is a free-running counter.

3. Transitions:

   • 00 -> 01
   • 01 -> 10
   • 10 -> 11
   • 11 -> 00

4. Excitation Tables:

   • Flip-flop Q1: | Q1 | Q2 | Q1+ | J1 | K1 | |---|---|---|---|---| | 0 | 0 | 0 | 0 | X | | 0 | 1 | 1 | 1 | X | | 1 | 0 | 1 | 0 | X | | 1 | 1 | 0 | X | 1 |
   • Flip-flop Q2: | Q1 | Q2 | Q2+ | J2 | K2 | |---|---|---|---|---| | 0 | 0 | 1 | 1 | X | | 0 | 1 | 0 | X | 1 | | 1 | 0 | 1 | 1 | X | | 1 | 1 | 0 | X | 1 |

5. Logic Implementation:

   • J1 = Q2
   • K1 = Q2
   • J2 = 1
   • K2 = 1

Using this implementation, the 2-bit counter will cycle through the desired states.

Conclusion

Excitation tables are indispensable tools in the design of sequential circuits. By systematically mapping state transitions to flip-flop inputs, they simplify the process of designing and implementing complex circuits. Understanding excitation tables is crucial for anyone working with sequential logic, whether it's in academic settings or industry applications.

Note: The information presented in this article is based on common concepts in digital logic design and utilizes examples found in various resources, including GitHub repositories. For specific applications and advanced design techniques, consult relevant textbooks and online resources.