15-05-2012, 11:03 AM
Combinational Circuits
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Designing Combinational Circuits
In general we have to do following steps:
Problem description
Input/output of the circuit
Define truth table
Simplification for each output
Draw the circuit
Binary adder
Binary adder that produces the arithmetic sum of binary numbers can be constructed with full adders connected in cascade, with the output carry from each full adder connected to the input carry of the next full adder in the chain
Note that the input carry C0 in the least significant position must be 0.
Binary Subtractor
The subtrcation A – B can be done by taking the 2’s complement of B and adding it to A because A- B = A + (-B)
It means if we use the inveters to make 1’s complement of B (connecting each Bi to an inverter) and then add 1 to the least significant bit (by setting carry C0 to 1) of binary adder, then we can make a binary subtractor.