What are Digital Logic Circuits with their Differences?

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Combinational and Sequential circuits are the most essential concepts to be understood in digital electronics. Combinational logic sometimes also referred to as time-independent logic is a type of digital logic which is implemented by Boolean circuits, where the output is a pure function of the present input only.

Sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs. Hey Gautam, To design sequential circuit also we use gates. But in case of sequential circuit we use feed back gates memory elements along with combinational logic. So that it can work like a latch or flip-flop. If you still have doubt, let us know. We would explain it in a detailed way.

In sequential circuit block diagram ,it is said that some outputs are again feedbacked to inputs,after storing them in memory. But,where is that memory in sr latch. In sr latch ,it is seen that the output is directly taken to the inputs,and their is no memory. Memory in the block diagram shows a delay for which the previous output can be hold and combinational logic vs sequential logic be used as the feedback input in the present clock cycle.

So, there is no explicit memory block in the SR latch. Your email address will not be published. Notify me of follow-up comments by email. Notify me of new posts by email. Output is a function of clock, present inputs and the previous states of the system. Do not have the ability combinational logic vs sequential logic store combinational logic vs sequential logic state.

Have memory to store the present states that is sent as control input enable for the next operation. It does not require any feedback. It simply outputs the input according to the logic designed. It involves feedback from output to input that is stored in the memory for the next operation. Used mainly for Arithmetic and Boolean combinational logic vs sequential logic. Used for storing data and hence used in RAM. Logic gates are the elementary building blocks. Flip flops binary storage device are the elementary building unit.

Independent of clock and hence does not require triggering to operate. Clocked Triggered for operation with electronic pulses. Dear Sagnik, Memory in the block diagram shows a delay for which the previous output can be hold and can be used as the feedback input in the present clock cycle.

Leave a Reply Combinational logic vs sequential logic reply Your email address will not be published. Sorry, your blog cannot share posts by email.

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In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history. That is, sequential logic has state memory while combinational logic does not. Sequential logic is used to construct finite state machines , a basic building block in all digital circuitry.

Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic. A familiar example of a device with sequential logic is a television set with "channel up" and "channel down" buttons. If the television is on channel 5, pressing "up" switches it to receive channel 6.

However, if the television is on channel 8, pressing "up" switches it to channel "9". In order for the channel selection to operate correctly, the television must be aware of which channel it is currently receiving, which was determined by past channel selections.

When a "channel up" or "channel down" input is given to it, the sequential logic of the channel selection circuitry calculates the new channel from the input and the current channel.

Digital sequential logic circuits are divided into synchronous and asynchronous types. In synchronous sequential circuits, the state of the device changes only at discrete times in response to a clock signal. In asynchronous circuits the state of the device can change at any time in response to changing inputs. Nearly all sequential logic today is clocked or synchronous logic.

In a synchronous circuit, an electronic oscillator called a clock or clock generator generates a sequence of repetitive pulses called the clock signal which is distributed to all the memory elements in the circuit. The basic memory element in sequential logic is the flip-flop. The output of each flip-flop only changes when triggered by the clock pulse, so changes to the logic signals throughout the circuit all begin at the same time, at regular intervals, synchronized by the clock.

The output of all the storage elements flip-flops in the circuit at any given time, the binary data they contain, is called the state of the circuit. The state of a synchronous circuit only changes on clock pulses.

At each cycle, the next state is determined by the current state and the value of the input signals when the clock pulse occurs. The main advantage of synchronous logic is its simplicity.

The logic gates which perform the operations on the data require a finite amount of time to respond to changes to their inputs. This is called propagation delay. The interval between clock pulses must be long enough so that all the logic gates have time to respond to the changes and their outputs "settle" to stable logic values, before the next clock pulse occurs. As long as this condition is met ignoring certain other details the circuit is guaranteed to be stable and reliable.

This determines the maximum operating speed of a synchronous circuit. Asynchronous sequential logic is not synchronized by a clock signal; the outputs of the circuit change directly in response to changes in inputs. The advantage of asynchronous logic is that it can be faster than synchronous logic, because the circuit doesn't have to wait for a clock signal to process inputs. The speed of the device is potentially limited only by the propagation delays of the logic gates used.

However, asynchronous logic is more difficult to design and is subject to problems not encountered in synchronous designs. The main problem is that digital memory elements are sensitive to the order that their input signals arrive; if two signals arrive at a flip-flop or latch at almost the same time, which state the circuit goes into can depend on which signal gets to the gate first.

Therefore, the circuit can go into the wrong state, depending on small differences in the propagation delays of the logic gates. This is called a race condition.

This problem is not as severe in synchronous circuits because the outputs of the memory elements only change at each clock pulse. The interval between clock signals is designed to be long enough to allow the outputs of the memory elements to "settle" so they are not changing when the next clock comes.

Therefore, the only timing problems are due to "asynchronous inputs"; inputs to the circuit from other systems which are not synchronized to the clock signal. Asynchronous sequential circuits are typically used only in a few critical parts of otherwise synchronous systems where speed is at a premium, such as parts of microprocessors and digital signal processing circuits.

The design of asynchronous logic uses different mathematical models and techniques from synchronous logic, and is an active area of research. From Wikipedia, the free encyclopedia. For the synthesizer company, see Sequential Circuits. John Wiley and Sons. Retrieved from " https: Digital electronics Automata computation Logic in computer science. Views Read Edit View history. This page was last edited on 16 January , at By using this site, you agree to the Terms of Use and Privacy Policy.