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Combinational circuits
This document discusses combinational logic circuits such as adders, subtractors, multipliers, decoders, and multiplexers. It provides circuit diagrams and truth tables for half adders, full adders, half subtractors, full subtractors, decoders, and multiplexers. It also describes how to build binary adders and subtractors using these basic components and how multiplication of binary numbers is performed.
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Combinational circuits
- 1. Combinational logic circuits Dr. C. Saritha Lecturer in Electronics SSBN Degree and PG College Anantapur December 22, 2012 1
- 2. overview What iscombinational logic circuit ? Examples of combinational logic circuits Binary-adder Binary-subtractor Binary-multiplier Decoders Multiplexers December 22, 2012 2
- 3. Combinational logic circuit Combinational circuits consists of logic gates whose outputs depends on the present inputs .They have no memory element . It consists of input variables , logic gates & output variables . December 22, 2012 3
- 4. Binary-Adder Half-Adder : A combinational circuit that performs the addition of two bits at a time is called “ Full-Adder ‘’ . The input variables designate the augend and addend bits ; the output variables produce the sum & carry . December 22, 2012 4
- 5. Circuit & truthtableof Half-Adder Circuit diagram Truthtable December 22, 2012 5
- 6. Full-Adder : Acombinational circuit that performs the addition of three bits at a time is called “ Full-Adder ’’ Block diagram & the truth table of Full- Adder . December 22, 2012 6
- 7. Circuit diagrams offull-adder December 22, 2012 7
- 8. Binary-Adder A Binary-Adderis a digital circuit that produces the arthimatic sum of two binary numbers at a time . It can also construct by cascading no of full-adders we get a N-bit binary- adder circuit . An N-bit adder requires n full-adders with each output carry connected to the input carry to the next full-adder . December 22, 2012 8
- 9. For adding 4-bitbinary numbers we need four full-adders as shown below : consider two binary numbers A=1011 & B =0011 December 22, 2012 9
- 10. Binary-subtractor Half-subtractor : A combinational circuit which performs the subtraction of two bits at a time is called “ Half-subtractor “ . It has two inputs & two outputs . The two inputs x & y form the minuend & subtrahend & D is the difference output & B is the borrow output . December 22, 2012 10
- 11. Circuit & truthtableof half- subtractor circuit diagram Truthtable December 22, 2012 11
- 12. Full subtractor A combinational circuit which performs the subtraction of three bits at a time is called “ Full subtractor “ . A full subtractor has three inputs x y Bin & two outputs D & Bout . December 22, 2012 12
- 13. Circuit & truthtableof full subtractor circuit diagram Truthtable December 22, 2012 13
- 14. Binary-subtractor The subtractioncan be done by taking the 2’s compliment of B and adding it to A . 2’s compliment can be obtained by taking the 1’s compliment and adding 1 to least significant bit . 1’s compliment can be obtained by changing 1’s into 0’s & 0’s into 1’s . December 22, 2012 14
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- 16. Continues….. In thiscircuit the mode input m controls the operation . When m=0 , the circuit is an adder circuit . When m=1, the circuit becomes a subtractor . The c & v are two outputs . C is for carry & v is for overflow . If v=0 after an addition or subtraction ,then no overflow occurred and the n-bit result is correct . If v=1 , then the result of the operation contains n+1 bits . December 22, 2012 16
- 17. Binary-multiplier The multiplicationof binary numbers is performed in the same way as multiplication of decimal numbers . If we take the multiplication of 2-bit numbers . The multiplicand bits are B1 & B0 , the multiplier bits are A1 & A0 and the product is C3,C2,C1,C0 . December 22, 2012 17
- 18. 2-bit by 2-bitmultiplier circuit December 22, 2012 18
- 19. Decoders A decoderis a combinational circuit that converts binary information from n input lines to a maximum of 2n unique output lines . Generally a decoder is called as n to m line decoder . A decoder is used to convert binary form to any other desired form . December 22, 2012 19
- 20. Three - to- eight line decoder This three - to - eight line decoder is used to binary to octal conversions . December 22, 2012 20
- 21. Multiplexer multiplexer isa combinational circuit that selects binary information from one of many input lines and directs it to a single output line . Normally there are 2n input lines and n selection lines whose bit combinations determines which input is selected . A multiplexer is also called ‘ Data selector ’ December 22, 2012 21
- 22. Four - to- one line multiplexer circuit & truth table : December 22, 2012 22
- 23. THANK YOU December 22,2012 23