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Digital Electronics
NUMBER SYSTEM
BINARY CODES
BOOLEAN ALGEBRA
K MAPS
COMBINATIONAL CKT
INTRODUCTION
ADDER
FULL ADDER(FA)
FA using HAs
BINARY ADDER
SERIAL ADDER
PARALLEL ADDER
CARRY LOOK AHEAD ADDER (CLA)
QUESTION (BCD to Excess-3 using ADDER)
SUBTRACTORS
FULL SUBTRACTOR
FS using HSs
MORE QUESTIONS
SEQUENTIAL CIRCUITS
TIMING CIRCUITS

4-bit ADDER and SUBTRACTOR in a SINGLE CIRCUIT

Q- Implement the 4-bit ADDER and SUBTRACTOR in a single circuit where we select one of the two using a select pin SEL.

Ans: The following circuit would work as required: In this circuit we use property of XOR gate by which XOR gate acts as a inverter when we have one input as 1.

When sel = 0 we get B1B2B3B4 un-complimented through XOR gate and also we have carry to circuit as 0 hence we get the result as sum of A and B

Y = A + B

When sel = 0 we get B1B2B3B4 complimented through XOR gate (as one input of XOR gate is 1) and also we have carry to circuit as 1 hence we get the result as difference of A and B

Y = A – B

 

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