Sequential Logic Circuits

Sıralı Mantık Devreleri
Sıralı Mantık DevreleriShift RegisterT-tipi Flip Flop
JK Flip FlopJohnson Ring SayıcıD-tipi Flip Flop
MultivibratörlerFlip-Flop Dönüşümleri

Unlike combinational logic circuits, which change the state depending on the signals applied to their inputs, sequential logic circuits have a built-in type of "memory". These sequential logic circuits operate not only in the current state, but also on the digital signal inputs of the past. Sequential logic circuits recall past conditions. The next time signal then remains constant in its current state until it changes one of the conditions, giving "memory" to sequential logic circuits.

Sequential logic circuits are often referred to as Bistable devices, which can set the output state to one of two basic levels (Logic level "1" or Logic level "0"). this will remain indefinitely until another input trigger pulse or signal is applied.

Sequential Logic Representation

Sequential Logic Representation

The word "sequential" means that the process takes place one after the other, in a certain order. The actual time signal determines when the next change will take place.

Simple sequential logic circuits can consist of standard bistable (DOUBLE-DETERMINED) circuits. To give an example:

  1. Flip-flops
  2. Counters

In order to produce the sequential circuit we have learned, we are able to connect universal NAND doors and/or NOR doors in a certain combination.

Classification of Sequential Logic

Since standard logic gates are the building blocks of combinational circuits, flip-flops are the basic building blocks of sequential logic circuits. Sequential logic circuits can be used to produce more complex sequential circuits, such as flip-flops with simple edge triggers, memory devices, or counters.

  1. Event-Oriented
  2. Clock-Oriented
  3. Impact Oriented

In addition to the two logical conditions mentioned above, a third element is introduced that distinguishes sequential logic circuits from their combinational logic counterparts, that is, time. After the sequential logic circuits are reset, they return to their original stable state. Sequential circuits with loops or feedback paths are said to be "cyclical" in nature.

Sequential logic circuits are based on feedback to maintain their current state. This occurs if part of the output is fed back into the entrance. This is how it is shown:

Sequential Feedback Loop

Sequential Feedback Loop

The two inverters or doors are connected in series with Q output, which is fed back towards the entrance. Unfortunately this configuration never changes the situation, that is, the output will always be the same. This means that the log "1" or "0" is permanently adjusted in the circuit. However, by examining the most basic sequential logic components called SR flip-flops, we can see how the feedback works.

SR Flip-Flop

The SR flip-flop, also known as the SR latch, can be considered one of the most basic sequential logic circuits possible. All flip-flops can be created with additions to this basic storage element. A basic NAND door SR flip-flop circuit provides feedback from both outlets to their opposite inputs. It is widely used in memory circuits to store a single data bit.

The SR flip-flop actually has three inputs: Set, Reset, and current Q output, which is related to its current status or history.

SR Flip-Flop

Accuracy Table for Set-Reset Function

SR accuracy table

When both inputs are s = "1" and r = "1", it is easily understood that output Q and Q can be at the logic level "1" or "0", depending on the status of the s or R inputs before this input condition. Therefore, the S = R = "1" condition does not change the status of outputs Q' and Q. However, as a last word, input status S = "0" and R = "0" is undesirable or invalid.

S-R Flip Flop Switching Scheme

SR switching Schema

This imbalance can cause one of the outputs to switch faster than the other, which can cause the flip-flop to move from one state to another,

The basic NAND gate SR flip-flop requires logic "0" entries to change or change the status from Q to Q, and vice versa. We can change the status of positive outgoing input signals by adding this basic flip-flop circuit with the addition of two extra NAND doors that connect as inverters to the S and R inputs as shown.

Positive NAND Door SR Flip Flop

Positive NAND Door SR Flip-flop

NOR Gate SR Flip Flop

NOR Gate SR Flip-flop

Now that we know more about the subject, we can slowly come to the end of this article. Before we finish, I'd like to give you an example of an SR Latch integration. Underneath, you'll see an image of its internal structure, which I got from its datasheet.

  • Quad SR Bistable LATCH 74LS279

Then let's take a brief look at its internal structure.

74LS279 Internal Structure