Overview of Digital Circuit Design

Digital circuits only have two output states, they are ON (1) and OFF (0). Different from analog circuits that are able to generate output with continuous values. Learning digital circuits will also make us learn about Boolean algebra, digital numeration, and of course logic gates.

What is Digital Circuit

An easy approach to understand what a digital circuit is to understand digital circuits vs analog circuits.

Digital circuit is a circuit built for digital electronics. This circuit processes and generates digital signals. In contrast to analog circuits which process and generate analog signals, digital circuits are mainly used to process and generate digital signals.

There are several differences between digital and analog input and output we need to be familiar with.

There is an interesting explanation for the difference between digital signal and analog signal. Digital signals are presented in discrete values which are limited to express all the values in the line. Analog signals are presented in real numbers which are able to give value at any point in the line, integer or decimal numbers.

We can observe the comparison below. The discrete signal depends on the time sampling delay. The shorter the delay or the higher the frequency, more values are captured.

Of course being a digital circuit does not mean it is not designed without any analog electronics. It is still important to us to consider designing a digital circuit with the help of analog electronics.

Digital circuits are usually built from logic gates that usually come in a packaged integrated circuit. This is what we know as digital integrated circuits. No matter what we use to build a digital circuit, we need to be familiar with Boolean logic functions.

Digital circuits have advantages over analog circuits. The most notable advantage is its ability to transmit the signals digitally so it has minimum degradation caused by noise. This noise is extra homework for us if we design an analog circuit. 

Digital circuits mainly use direct current (DC) circuits to do logic operation. This type of power is preferred since it provides steady voltage and current to the circuit. Not only that, the binary numbers are represented by positive DC voltage as 1 and zero volt as 0. Or in other words, full DC supply voltage as logic 1 and zero volt as logic 0.

Since the digital circuit generates outputs based on a combination of inputs, if we use alternating current (AC) circuits, the input can be disturbed by the changes of supply direction.

Digital Logic Circuit

Digital circuits have more precision when transmitting and presenting electronic signals. The use of binary digits (0 and 1) makes it easier to distinguish two states of signal. Unlike analog signals where distinguishing 0 and 1 can be more difficult since it is affected by noise, digital numbers are able to reconstruct signals without error.

Processing digital numbers such as binary digits forces us to deploy extra digital circuits. This hardship is also found in analog circuits where we need additional improvement on our circuit to minimize noise in each processing and transmitting signals.

Digital circuit can always be represented as a truth table for input and output relationship. When a set of logic gates are connected together, their final output can be calculated by creating their truth table. Whatever our inputs, the outputs are already determined.

Keep in mind the states below:

• Binary 1 = HIGH = TRUE
• Binary 0 = LOW = FALSE

Whether it is binary, decimal, hexadecimal, the 0 and 1 states are still used to design and to convert. Usually if a logic circuit receives and generates +3.5 V to +5V, it will count as state 1, and lower than that will count as state 0.

There are two types of digital circuit, they are:

• Sequential digital circuit is affected by the previous output states as its present input states.
• Combinational digital circuit only uses the present input states.

The summary of combinational and sequential logic is shown below. The left is the combinational while the right is sequential:

These states will be processed further by logic gates:

• AND
• OR
• NOT

These three can be further combined to be NAND, NOR, XOR, and many more.

Digital Circuit Design

Digital circuit design is the design process of building a circuit consisting of several semiconductors such as logic gates, transistors, logic gates and a few basic types of circuit components such as resistor, inductor, and capacitor. Digital circuit follows the Boolean algebra and the discrete signals, zero and one.

When doing a digital circuit design, we must aim to make our circuit as simple as possible. The logic redundancy is used often by engineers to reduce the complexity of the circuit.

The more complex our circuit, the more cost we should spend and more error we will get. Less complex means less component count, less potential errors, and less cost. In order to achieve this, we do not have to deploy advanced components right off the bat.

Like what we have learned about electric circuit analysis, a digital circuit has its own way to be analyzed and optimized. The well known techniques are:

• Karnaugh maps
• Binary diagram
• Boolean algebra
• Heuristic computer method
• Quine – McCluskey algorithm.

The first three are the basics and the rest can be performed easily with a computer-aided design system.

Microcontrollers and Programmable Logic Controllers as the examples of embedded systems, are very popular to build a complex system with minimum circuit design with their own strong features.

Just as stated above, the input and output relationship of digital circuits can be calculated accurately with the Truth Table. But keep in mind, there are two types of digital systems, combinational systems and sequential systems.

A combinational system is a digital circuit whose output is determined by its present output.

Opposed to that, a sequential system has its output fed back to its inputs. This way, the output depends on its past inputs to generate a “sequence” of operations.

Furthermore, the combinational systems are divided into synchronous sequential systems and asynchronous sequential systems.

The difference is the synchronous system changes its output state when the clock signal changes state. The asynchronous system propagates changes whenever inputs change. Learning this digital circuit design we will learn about the well-known JK Flip Flop, SR Flip Flop, D Flip Flop, T Flip Flop, and Master Slave Flip Flop.

Synchronous digital circuit is easy to make with a basic flip-flop such as D, T, RS, or JK. Of course only combining flip-flops will get us nowhere. We need one circuit element for a state register. This is also one type of flip-flops, generating a binary number, acting as a Clock (CLK).

In order to build a synchronous system, we will make a combinational logic and a set of flip-flops as a state register. This combinational logic generates the binary number for the next state.

Every time the CLK finishes one cycle (1 – 0 – 1), the output will be shifted to right and the state register also captures the feedback from the output from its previous state. This is a disadvantage of a synchronous system since it is controlled by the CLK speed, it is unable to operate at its full speed. But since it can be controlled with the CLK, it is easy to verify the synchronous system whether it follows our design or not.

Opposed to that, the asynchronous system does not depend on CLK thus it can utilize its maximum speed of its logic states. The examples of this are switch debouncer, synchronizer flip-flop, and arbiter.

Asynchronous system is basically harder to predict and design its output since we need to evaluate all the possible states in all possible timings. We need to be careful to design this type of digital circuit because we can accidentally produce unexpected output at unexpected times.

This is the reason an asynchronous digital circuit is unstable.

Digital Integrated Circuits

Just as explained above, digital circuits are usually built from a set of logic gates. The construction of logic gates can be connected to each other to make a combinational logic.

Basic or complex combinational logic can be solved, analyzed, and designed by mapping the Boolean logic. Logic gates are usually made by electrical switches such as transistors. The output of logic gates can be used as inputs for other logic gates.

Digital integrated circuit is a device that combines several digital electrical components in a small semiconductor chip. Observe the integrated circuit SN7400N below which has three AND gates in a single chip.

Of course there are a lot of chip varieties to serve as logic gates for our needs.

As stated in the beginning, digital circuit and digital integrated circuit are excellent at processing discrete values (0 and 1). With this characteristic, digital IC is also used as a switching device since it represents the ON and OFF conditions.

Digital integrated circuits can be built from the combination of flip-flops, logic gates, and multiplexers. Whether it is easier to build than an analog circuit, it depends on our calculation of Boolean algebra to simplify the circuit.

There are several application of digital integrated circuit, the examples are:

• Logic integrated circuit
• Memory chip
• Power management integrated circuit
• Programmable integrated circuit
• Interface integrated circuit

Keep in mind that digital circuits and analog circuits often complement each other, that is why there are digital to analog converter circuits (DAC) and analog to digital circuits (ADC).

Frequently Asked Questions