Digital circuits can be classified into two types:
→ Sequential digital circuits.
Combinational Digital Circuits:
• In these circuits “the outputs at any instant of time depends on the inputs present at that instant only.” are made up from basic gates (AND, OR, NOT) or universal gates (NAND, NOR) gates that are "combined" or connected together to produce more complicated switching circuits. These logic gates are the building blocks of combinational logic circuits. An example of a combinational circuit is a decoder, which converts the binary code data present at its input into a number of different output lines, one at a time producing an equivalent decimal code at its output.
• For the design of Combinational digital circuits Basic gates (AND, OR, NOT) or universal gates (NAND, NOR) are used. Examples for combinational digital circuits are Half adder, Full adder, Half subtractor, Full subtractor, Code converter, Decoder, Multiplexer, Demultiplexer, Encoder, ROM, etc.
Sequential Digital Circuits
• Sequential logic differs from combinational logic in that the output of the logic device is dependent not only on the present inputs to the device, but also on past inputs; i.e., the output of a sequential logic device depends on its present internal state and the present inputs. This implies that a sequential logic device has some kind of memory of at least part of its ``history'' (i.e.
• A simple memory device can be constructed from combinational devices with which we are already familiar. By a memory device, we mean a device which can remember if a signal of logic level 0 or 1 has been connected to one of its inputs, and can make this fact available at an output. A very simple, but still useful, memory device can be constructed from a simple OR gate, as shown in Figure below:
Combinational and Sequential Logic Circuits:
