Sequential Circuits

Combinatorial circuits

Circuits where each output depends only on the present values of the inputs.

Sequential Circuits

Circuits where outputs depends only on both the current inputs and the past history of the circuit (stored in memory elements).

State

The stored contents of the memory elements in a sequential circuit, representing its current condition.

State Transitions

When input value change, the circuit may either remain in the same state or move into a new state. Over time, the circuit follows a sequence of states.

Storage Elements

Circuit components that store logic signal values, enabling the memory behavior.

Alarm System

• Input Set: Activates the alarm (triggers state change to “ON”).

• Input Reset: Turns off the alarm (returns to “OFF” state).

• Alarm must stay ON even if SET returns to 0 → requires memory.

Rudimentary Memory Element

A feedback loop made of two inverters:

• If A = 0 → B = 1, circuit remains stable

• If A = 1 → B = 0, circuit remains stable

• The circuit has two possible states.

Limitations of Inverter Loop

Although it demonstrates memory, it is not practical because it lacks a controlled way to change states.

Practical Memory Circuits

Useful memory elements can be with logic gates (e.g., latches and flip-flops)