A combinational logic circuit implement logical functions where its outputs depend only on its current combination of input values. On the other hand sequential circuits, unlike combinational logic, have state or memory.
The main difference between sequential circuits and combinational circuits is that sequential circuits compute their output based on input and state, and that the state is updated based on a clock. Combinational logic circuits implement Boolean functions and are functions only of their inputs.
There are 3 ways to represent combinational logic functions
To obtain the boolean expressions and truth tables from the combinational logic circuit, we need to analyse the circuit. First ensure that the circuit is combinational - that is there is no feedback of an output to an input that the output depends on.
For each output functions, write it in terms of its input variables and intermediate signals, and then expand intermediate signals until the outputs are expressed only in terms of the inputs.
The truth table can be derived from the Boolean expressions, or by directly working out from the circuit, the outputs for each possible combination of inputs.
If there are n input variables
From the examples below, change the inputs to observe the outputs.
You have learnt how to obtain the boolean expressions and truth tables from the logic circuits. Next you will learn, using Guided Worked Examples, how to design combinational logic circuits in minutes.
Combinational logic circuits design comprises the following steps
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