Combinational and Sequential circuits are the most essential concepts to be understood in digital electronics.
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.
| Combinational Logic Circuits | Sequential Logic Circuits |
| Output is a function of the present inputs (Time Independent Logic). | Output is a function of clock, present inputs and the previous states of the system. |
| Do not have the ability to store data (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 operations. | 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). |
| Example: Adder [1+0=1; Dependency only on present inputs i.e., 1 and 0]. | Example: Counter [Previous O/P +1=Current O/P; Dependency on present input as well as previous state]. |
thank you for given valuable information
in combinational ckt we are using gate but in sequential ckt what will we use to designe
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.
what are the similarities between combinational circuit and sequential circuits
May be flipflops or latches
Thankyou! 🙂
Welcome Ruchita 🙂
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.
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. In the SR latch the gate delay in the NAND/NOR gates works like the memory block in the block diagram. So, there is no explicit memory block in the SR latch.
thank you,,,,,,but i wanna to ask u examples of each circuits
thank you for helping the difference between the two circuit
thank you for giving the information.
thanks
Most welcome Raj.
what if sequential circuits are combined
Hi Tharun,
Eventually Sequential circuits are also combinational circuits but with a memory element and a feedback path.
thank you for given valuable information
Thank you Ravindra for your appreciation.
CAN you please explain with the help of an example that the flip-flop work as a memory element. I have confusion in it.
Hi Amit,
You take an example of shift-register or a counter. Basic elements of both of these are flip-flops.
Let’s consider a 3-bit counter. It counts 000,001,010,011,100,101,110 and 111.
Let a is the counter. Then it can be written as:
a(t+1) = a(t) + 1
so, ‘a’ at time t+1 can be calculated only when ‘a’ holds a value at time ‘t’. Thus it behaves like a memory.
We can say registers also an memory element
Hi Harish,
Yes you can say registers also memory elements
Thanku so much😊
Most Welcome Ravin