Question

What is the difference between combinational and sequential logic?

Answer 1

Combinational and sequential logic are two fundamental types of digital circuits, and they differ primarily in how they process inputs and manage state.

### Combinational Logic
- **Definition**: In combinational logic, the output is determined solely by the current inputs. There is no memory or state involved.
- **Characteristics**:
  - **No Feedback**: Outputs do not depend on previous inputs.
  - **Examples**: Basic logic gates (AND, OR, NOT), multiplexers, demultiplexers, adders, and encoders.
  - **Timing**: Outputs change immediately in response to changes in inputs.

### Sequential Logic
- **Definition**: In sequential logic, the output depends on both the current inputs and the previous states (history) of the circuit. It incorporates memory elements.
- **Characteristics**:
  - **State**: Contains memory elements (like flip-flops) to store past inputs.
  - **Feedback**: Outputs can affect future inputs and states.
  - **Examples**: Flip-flops, registers, counters, and state machines.
  - **Timing**: Outputs can change based on clock signals, allowing for synchronization.

### Key Differences
- **Memory**: Combinational logic has no memory; sequential logic has memory.
- **Output Dependence**: Combinational outputs depend only on current inputs; sequential outputs depend on past and current inputs.
- **Behavior**: Combinational circuits respond instantly to inputs, while sequential circuits may have delayed responses due to clock cycles and state changes.

In summary, combinational logic is about instantaneous input-output relationships, while sequential logic involves history and state management.

Answer 2

The primary difference between combinational and sequential logic lies in how they process and store information:

1. **Combinational Logic:**
   - **Definition:** Combinational logic circuits produce outputs based solely on the current inputs, with no regard to past inputs or states.
   - **Characteristics:**
     - **Memoryless:** They do not store any state or history.
     - **Output Determination:** The output is a function of the present input values only.
     - **Examples:** Basic logic gates (AND, OR, NOT), multiplexers, adders, and encoders.

2. **Sequential Logic:**
   - **Definition:** Sequential logic circuits produce outputs based on both current inputs and the history of past inputs (i.e., they have memory).
   - **Characteristics:**
     - **Memory:** They store state information using elements like flip-flops or latches.
     - **Output Determination:** The output depends on the sequence of inputs, including past input values.
     - **Examples:** Counters, shift registers, and state machines.

In summary, combinational logic is about real-time computation with no memory, while sequential logic involves memory and state to track and respond to input sequences.