Usefu for B.Sc Electronics students

1. Combinational logic circuits
Dr. C. Saritha
Lecturer in Electronics
SSBN Degree and PG College
Anantapur
December 22, 2012 1

2. overview
 What is combinational logic circuit ?
 Examples of combinational logic
circuits
 Binary-adder
 Binary-subtractor
 Binary-multiplier
 Decoders
 Multiplexers
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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 .
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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 .
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5. Circuit & truthtable of Half-Adder
Circuit diagram Truthtable
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6. Full-Adder :
 A combinational circuit that performs
the addition of three bits at a time is
called “ Full-Adder ’’
 Block diagram & the truth table of Full-
Adder .
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7. Circuit diagrams of full-adder
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8. Binary-Adder
 A Binary-Adder is 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 .
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9. For adding 4-bit binary numbers we
need four full-adders as shown below :
consider two binary numbers
A=1011 & B =0011
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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 .
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11. Circuit & truthtable of half-
subtractor
 circuit diagram  Truthtable
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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 .
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13. Circuit & truthtable of full subtractor
 circuit diagram  Truthtable
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14. Binary-subtractor
 The subtraction can 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 .
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15. Four-bit adder-subtractor
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16. Continues…..
 In this circuit 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 .
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17. Binary-multiplier
 The multiplication of 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 .
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18. 2-bit by 2-bit multiplier circuit
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19. Decoders
 A decoder is 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 .
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20. Three - to - eight line decoder
 This three - to - eight line decoder is
used to binary to octal conversions .
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21. Multiplexer
 multiplexer is a 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 ’
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22. Four - to - one line multiplexer
circuit & truth table :
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23. THANK YOU
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