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Combinational Circuits ENEL 111

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Common Combinationals Circuits NAND gates and Duality Adders Multiplexers

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De Morgan again A NAND gate: Y = A.B = A + B is the same as an OR gate with two NOT gates Similarly a NOR gate is the same as an AND gate with two inverters Y = A + B = A.B not the individual terms change the sign not the lot

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Dual gates not the individual inputs change the gate not the output

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Truth Tables and Boolean Notation NAND Gate Representation It is possible to implement any boolean expression using only NAND gates XX NOT AND A B A.B OR A A+B B

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Truth Tables and Boolean Notation NAND Gate representation Implement the following circuit using only NAND gates x3 x2 x4 De Morgan can also be represented visually:

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Solution Dual the gates, remember two nots together can be removed. x3 x2 x4 A B A.B A A+B B AND feeding OR

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Exercise Implement NOT, AND and OR using NOR gates Example AND gate dual circuit:

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Solution Similar pattern to using NAND gates (not surprising) NOT AND OR XX A B A.B A A+B B X X A B A.B A+B A A.B B

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Truth Tables and Boolean Notation NOR Gate representation It is also possible to implement any boolean expression using only NOR gates Implement the following circuit using only NOR gates X4 X3 X2

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Solution Two NOR gates in sequence acting as NOT’s can be eliminated: X4 X3 X2

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Examples The half adder The half adder is a circuit for adding two single bit numbers Develop a truth table and Boolean expressions for the half adder S and C are the Sum and Carry ABSC

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Half adder The sum is XOR operation and the carry an AND: ABSC A B C S

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Examples The full adder Develop a truth table and Boolean expressions for the full adder, this circuit also includes a carry in. CinABSC full adder A B Cin Sum Cout

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Truth table for full adder CinABSCout Exercise: Complete the Karnaugh maps for the Sum and the Carry out columns

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K maps for sum and carry AB Cin AB Cin Sum – 1 when odd number of inputs is 1 = XOR gate Carry out - simplifies to 3 pairs Sum = Cin xor A xor B Cout = A.B + A.Cin + B.Cin

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Full adder circuit A B Cin Cout Sum Sum = Cin xor A xor BCout = A.B + A.Cin + B.Cin

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Examples The Multiplexer Selects one of 2 n inputs and copies it to a single output The selected line is determined from the bit combination (address) on the n selection lines e.g. 1 from 2 mutiplexer selabout sel ab out = a b sel out n = 1 0 1

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2:1 Multiplexer selabout selabout 00?0 01?1 1?00 1?11 if a is selected, don’t care about b. AB sel

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K map for 2:1 Multiplexer AB sel output = sel.a + sel.b Principal can be extended to 4:1 – 2 select lines and 4 data lines 8:1 – 3 select lines and 8 data lines and so on… data sel out

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What you should be able to do: Change circuits using one set of gates (eg AND, OR, NOT) to their equivalent using NAND or NOR gates only (and vice versa). Be familiar with half-, full- adders and multiplexer circuits. Be able to construct and interpret Karnaugh maps with up to 4 input variables. (if you need practice, come to the tutorial)

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