Lesson 07: Latches and Flip-Flops


Truth Table

  • A truth table provides a complete listing of all possible input combinations and their corresponding output values for a combinational logic circuit.
  • It covers all possible input states and shows the resulting output based on the logic function implemented by the circuit.
  • Truth tables are commonly used for analyzing and designing combinational circuits, such as logic gates.
  • Shows the relationship between inputs and outputs for all possible input combinations.
  • It's the most fundamental table, like a dictionary lookup for a circuit's output based on the inputs.

Function Table

  • A function table is essentially the same as a truth table.
  • Applies to combinational circuits only.
  • It can be used interchangeably with a truth table in many cases.
  • May use different terminology for inputs and outputs (e.g., Input1, Output) compared to a truth table (A, B, F).

Characteristic Table

  • A characteristic table (also known as a state table) describes the behavior of sequential circuits (e.g., flip-flops, registers).
  • Shows the relationship between the circuit's current state, inputs, and next state.
  • It may additionally include the output produced in the current state.
  • It describes how the circuit's memory (state) changes based on inputs and how it affects the next output.

Excitation Table

  • The excitation table is derived from the characteristic table.
  • Applies to sequential circuits only (circuits with memory).
  • Shows the input values required to transition the circuit from its current state to the next state.
  • It helps design the control logic for flip-flops (circuits that store state) to achieve the desired behavior.

State Diagram

  • Graphical representation of state table: these diagrams show states as nodes and transitions as directed arcs. They help visualize the behavior of the circuit over time.

State Table

  • These tables list all possible states of the circuit and show the transitions between these states based on input values.
  • Also called the transition table.

Here's a table summarizing the key differences:

FeatureTruth Table / Function TableExcitation TableCharacteristic TableState DiagramState table
Circuit Type Combinational & Sequential (Truth Table) Sequential Only Sequential Only Sequential Only Sequential Only
Focus Inputs & Outputs Inputs for Next State Current State, Inputs, Next State (and possibly Output) Visual representation of state transitions Tabular representation of state transitions
Information Provided Output for each input combination Inputs to cause a state transition State transitions and possibly output Graphical flow of states based on inputs Tabular listing of next state and output for each state-input combination


  • Think of a truth table/function table as a simple instruction manual - if you give it these inputs, you get this output.
  • An excitation table is like a recipe for changing state in a sequential circuit - use these inputs with the current state to get the desired next state.
  • A characteristic table is like a detailed record of how a sequential circuit behaves - it shows how the state changes based on inputs and what the output is at each step.