Hi, I'm Duck and I'm learning electronic circuits from scratch every day.
The other day, one of my seniors told me something like this during training.
Senior -"You understand combinational and sequential circuits, don't you?"
Duck - "Do you have flip-flops?"
Senior -"Well, yes, but it's important whether it retains data or not."
Duck: -"(Retain???)"
So we have considered the difference between combinational and sequential circuits.
This time we took a 2-bit addition circuit and a D-flip-flop as examples of each and checked the difference in operation.
Digital Circuit Example
The timing chart below shows the output of each circuit in relation to the input.
Timing Chart
| Input data (A, B) is immediately reflected by outputs (S, C) |
Input data (DATA) is output on the rising edge of CLOCK |
As shown above, the D-flip-flop output does not change except at the rising edge of the clock. After that, it continues to output the past data at the rising edge of the clock.
I see, it is a holding circuit because it can reflect the past state in the current output.
Conclusion
Combinational circuit・・・The output changes as soon as the input data changes.
→Output depends only on the current input
Sequential circuit ・・・・・・ input data does not change output until the rising edge of the clock
→Past input can be reflected in output
Appendix
Sequential circuits can also include combinational circuits, as in the circuit below.
You cannot make a synchronous circuit without a sequential circuit. In FPGA design, it is fundamental to build synchronous circuits, so these sequential circuits are important.
For more information on synchronous and asynchronous circuits, please refer to the related article.
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