Combinational vs Sequential digital VHDL circuits

what is the difference? Combinational vs Sequential digital VHDL circuits

The difference between combinational vs sequential digital circuits lies in whether the output(s) depends on past output(s) or not. A combinational digital circuit is one in which the output(s) depends only on the current input(s).  Whereas, a sequential digital circuit is one in which the output(s) depends not only on the current input(s) but also on the past output(s). In terms of circuit design, a sequential circuit contains a feedback loop whereas a combinational circuit does not.

Example of combinational digital circuits are multiplexers, encoder, decoder etc and example of sequential circuits are latches, flip-flops, counters and registers. That is a sequential circuit has memory where past processed data are stored and the current output depends on previous output as well as the current inputs. Any logic digital circuit as a whole is composed of combinational and sequential digital circuits. For example, a register input might be connected to a multiplexer. Whichever the circuits both combinational and sequential circuits or components are made up the basic logic gates.

Below is combinational 2 to 4 decoder using VHDL code written in Active-HDL VHDL software.

And below is an example of sequential circuit of shift register using VHDL language.

 

 

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