Updated April 13, 2023

## Introduction to Digital Circuit

The digital circuit is embedded with many logic circuits programmed to perform logical operations. Such logic circuits are NOT, OR, and AND which are combined to carry out any specific operations. The logic circuits are implemented using circuit symbols and logic expressions by JIS symbols and MIL symbols or any notifications can also be applied. They have a truth table that represents any combination of inputs to get the desired output. A digital logic circuit is the one where the voltages are consumed with finite voltages and distinct values. These logic circuits are classified as sequential logic and combinational logic circuits. It is implied mostly on digital electronics like calculators, mobile phones, and computers.

### How does Digital Circuit work?

The central processing unit occupies the hardware circuits that include microchips, electronic circuits, integrated circuits, and programming technologies. But both hardware and software must communicate to perform the basic operations. The computers perform all the fundamental operations in binary digit format and in some cases, decimal values are also implied. The logic operations are performed by circuits using digital logic gates. The digital gates are physical chunks of integrated circuits used to execute the logical task by using Boolean logic. The logic gates are primarily embedded in electronic semiconducting switches like transistors, diodes, etc. But the pneumatic molecules, logics, fluid logic, electromagnetic relays, and mechanical components are also used to imply the logic gates. These practical logic gates are developed by CMOS technology, MOSFET, FET. Varied types of logic gates are NOT gates, AND gates, OR gates, XNOR, NOR, and EXOR, EXNOR are the most of the used logic gates to execute a logical task. All the common logic gates work on the Boolean function to give the output based on the truth table and obtain the output in a single binary digit. Hence these logical gates are known as Boolean logic gates or binary logic gates. The logic circuits are built of fundamental building blocks in the digital electronic system.

### Types of Logic Circuits

The digital circuits are also called switching circuits as the level of voltage is changed from one value to another value simultaneously. And so it is termed logic circuit as they obey a set of logic rules according to the truth table. The logic circuit is broadly divided into combinational and sequential logic circuits.

#### 1. Combinational Digital Logic Circuit

- The combinational logic circuits are based on logic gates like OR gate, AND gate, NOT gate, NOR gate, and NAND gate. The NOR and NAND come under universal gates. These gates are merged to form a complex switching circuit. The combination logic gate is comprised of building blocks and the output can be derived instantly which is based on the given input. The combinational circuits don’t hold any separate memory components.
- The best example of combinational circuits is decoder and encoder. The decoder changes the binary code data and the input to a different number and different output lines. A few examples of combinational switching devices are code converters, full adder, half adder, decoder, encoder, de-multiplexer, multiplexer, etc. The combinational circuits can be implemented in microcontrollers and microprocessors to design the hardware and software devices of a computer.

#### 2. Sequential Digital Logic Circuits

The sequential digital circuit is not similar to combinational circuits. Here the output of the device is not based on the given input and also based on the inputs given at the past time. In other terms, the sequential logic circuit’s output is based on the given input and also the current state of the circuit. Here it holds a separate memory component to save the past input and output. In simple, the combination of digital circuits along with memory components is called sequential digital circuits. It is implemented using finite state machines. The best example of sequential logic devices is flip flops, counters, with memory components and digital logic circuits. The two given inputs are used by the combinational circuits to provide different outputs. The output derived from memory components is given to the combinational circuits. The internal output and input is the section of secondary components. They have a variable state developed by storage components and the secondary output devices are used for excitations of the allotted storage components. The sequential logic circuits are divided into clock-driven, pulse drive, and event-driven

#### 3. Circuits with Clock-Driven Components

They are synchronous where the transition of output happens when the input is in the form of clock pulse input. It is called synchronous sequential circuits dependent on the clock and pulsed inputs.

#### 4. Circuits with Event-Driven Components

- They are asynchronous where the transition of output happens when the input is not in the form of clock pulses. It is not dependent on the clock signal but these asynchronous sequential circuits rely on input pulses.
- The sequential circuit has the output in the form of level and pulse output. The pulse output sustains according to concerning input and can be minimized in a few cases. But for the clocking sequential circuits, the pulse output is obtained at the same duration as the generated clocked pulse. The output level marks the output which fluctuates according to the given input pulse and sustains the same state until it receives the next clock pulse.
- The counter is implemented in a different form of logic circuits. They operate in pulse trains and based on the division ratio, the pulse output is set only the number of the pulse is reached.

### Conclusion

Finally, the combinational circuits react to changes in input. The sequential circuits remember the past and present state and merge with changes in input to develop a new state. The synchronous sequential devices are implemented to complex sequential circuits which are based on input and are also decided by an input clock pulse. The result is delivered with few glitches and can be predicted. It is implemented in hardware circuits. The asynchronous sequential circuits are decided to produce a rapid and response according to the speed of input decided by the devices instead of a clock. Here the process is hard to design and similar to the operation of the brain.

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