combinational logic - basavaraj talawarbinary adder–subtractor 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1,...
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Combinational Logic
Ch. 4. Mano & Ciletti.
Outline● Combinational Circuit Analysis● Design Procedure
– BCD to Excess-3 Converter
● Binary Adder – Subtractor– HA, FA, Carry Lookahead Generator
● BCD Adder● Binary Multiplier● Magnitude Comparator● Decoder, Boolean Functions using Decoders● Encoders● Multiplexers, Boolean Function Implementation● Three State Gates
Analysis
Analysis
Truth Table
Truth Table
Design Procedure● Code Conversion
– BCD to Excess-3 code
BCD toExcess-3BCD to
Excess-34
BCD_A E-34
Design Procedure● Code Conversion
– BCD to Excess-3 codeBCD to
Excess-3BCD to
Excess-34
BCD_A E-3
4
BCD to Excess-3 Code
BCD to Excess-3 Code
BCD to Excess-3● K-Maps for w, x, y, and z.
BCD to Excess-3
BCD to Excess-3
Binary Adder–Subtractor● Bit-wise Addtion:
– 0 + 0 = 0– 0 + 1 = 1– 1 + 0 = 1– 1 + 1 = 10
●
Binary Adder–Subtractor● 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1, and
1 + 1 = 10● Outputs: Sum and Carry bits
Binary Adder–Subtractor● 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1, and
1 + 1 = 10● Outputs: Sum and Carry bits● Half Adder: Combinational circuit to
add two bits
Binary Adder–Subtractor● 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1, and 1
+ 1 = 10● Outputs: Sum and Carry bits● Half Adder: Combinational circuit to
add two bits● Full Adder: Adds three bits (two
significant bits and a previous carry)
Binary Adder–Subtractor● 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1, and 1 + 1 = 10● Outputs: Sum and Carry bits● Half Adder: Combinational circuit to add two
bits● Full Adder: Adds three bits (two significant
bits and a previous carry)● Binary Adder for two n-bit numbers: n full
adders in cascade .
Half Adder
Half Adder
HalfAdderHalf
Adder
a
bsum
carry
Half Adder - TT
Half Adder
Half Adder
Full Adder● Inputs: Addend, Augend, Carry bits● Outputs: sum, carry
Full Adder● 3 Inputs: Addend, Augend, Carry bit● Outputs: sum, carry
HalfAdderHalf
Adder
a
bsum
carry_out
carry_in
Full Adder – TT
Full Adder – TT
Full Adder
Full Adder
Full Adder
S
Full Adder using Half Adders
● Half Adder Outputs
● Full Adder Outputs
Full Adder using Half Adders
C=x ' yz+xy ' z+xyz '+xyzC=x ' yz+xy ' z+xyz '+xyz
C=x ' yz+xy ' z+xyC=x ' yz+xy ' z+xy
C=z (x ' y+xy ' )+xyC=z (x ' y+xy ')+xy
Full Adder using Half Adders
Full Adder using Half Adders
Full Adder using Half Adders
Binary Adder● 1011 + 0011
Binary Adder● 1011 + 0011
4-bit Adder
4-bit Adder
Ripple Carry AdderRipple Carry Adder
4-bit Adder
Ripple Carry AdderRipple Carry Adder
Which is the critical path?Which is the critical path?
Carry Propagation● Rename HA outputs
Carry Propagation● Rename HA outputs
● FA outputs:
Carry Propagation● Rename HA outputs
● FA outputs: (Ci is the third input)
Carry Propagation
Carry Propagation
Carry Generation
Carry Generation
Carry Lookahead Generator
4-bit Carry Lookahead Adder
Binary Subtractor
Binary Subtractor
BCD Adder● Inputs: Two digits (0 – 9), Carry (0 –
1)● Max sum = 19 (9 + 9 + 1)● Use the 4-b Binary Adder● Convert the Binary output to BCD
BCD Adder
BCD Adder
+6 =
BCDAdder
Binary Multiplier● 2 bit Multiply: B1B0 x A1A0
●
Binary Multiplier● 2 bit Multiply: B1B0 x A1A0
●
Binary Multiplier● 2 bit Multiply:
B1B0 x A1A0
Binary Multiplier● 4b x 3b. B3B2B1B0 x A2A1A0
Binary Multiplier
● 4b x 3b. B3B2B1B0x A2A1A0
Magnitude Comparator● Compares two numbers A and B● Outputs: A > B, A = B, or A < B
ComparatorComparator
nA A=B
nBA<BA>B
Magnitude Comparator● Consider A3A2A1A0, B3B2B1B0
● A = B
Magnitude Comparator● Consider A3A2A1A0, B3B2B1B0
● A = B
Magnitude Comparator● Consider A3A2A1A0, B3B2B1B0
● A > B
Magnitude Comparator● Consider A3A2A1A0, B3B2B1B0
● A > B
Magnitude Comparator● Consider A3A2A1A0, B3B2B1B0
● A > B
● A < B
Magnitude Comparator
Decoders● n-to-m-line Decoder:
– n-bit Input; 2n bit Output
2-to-4-lineDecoder
2-to-4-lineDecoder
2A
D3
D2
D1
D0
Decoder – TT
Decoder – TT
Decoder
Decoder with Enable
2-to-4-lineDecoder
2-to-4-lineDecoder
2AD
3
D2
D1
D0En
Decoder with Enable
2-to-4-lineDecoder
2-to-4-lineDecoder
2AD
3
D2
D1
D0En
Decoder with Enable
2-to-4-lineDecoder
2-to-4-lineDecoder
2AD
3
D2
D1
D0En
Decoder with Enable
2-to-4-lineDecoder
2-to-4-lineDecoder
2AD
3
D2
D1
D0En
Decoder with Enable
2-to-4-lineDecoder
2-to-4-lineDecoder
2AD
3
D2
D1
D0En
Decoder with Enable
Boolean using Decoder● Decoder provides 2n minterms of n
input variables● Any Boolean function can be
expressed in sum-of-minterms● Any n-input, m-output combinational
circuit can be implemented with an n-to-2n-line decoder and m OR gates.
Full Adder using Decoder
Full Adder using Decoder
Encoders● Inverse of a decoder.● Encoder has 2n (or fewer) input
lines and n output lines
23-to-3-line Encoder
23-to-3-line Encoder
23-to-3-line Encoder
23-to-3-line Encoder
Priority Encoder● If two or more inputs are equal to 1
at the same time, input having the highest priority will take precedence.
Priority Encoder
Priority Encoder
Priority Encoder
Priority Encoder – Maps
Priority Encoder – Maps
Priority Encoder
Priority Encoder
Multiplexers● Selects binary information from one of
many input lines and directs it to a single output line.
● Selection of the input line is controlled by a set of selection lines– 2n input lines ==> n selection lines
● Also called Data Selector
Multiplexer
MultiplexerS Y
0 I0
1 I1
MultiplexerS Y
0 I0
1 I1
Y= I 0×S+ I1×SY= I 0×S+ I1×S
MultiplexerS Y
0 I0
1 I1
Y= I 0×S+ I1×SY= I 0×S+ I1×S
4-to-1 Multiplexer
4-to-1 Multiplexer
I0
I1
I2
I3
Y
s2
4-to-1 Multiplexer
I0
I1
I2
I3
Y
s2
4-to-1 Multiplexer
I0
I1
I2
I3
Y
s2
Y=S1S0×I 0+S1S0×I 1+S1S0× I 2+S1S0×I 3Y=S1S0×I 0+S1S0×I 1+S1S0× I 2+S1S0×I 3
4-to-1 MultiplexerI0
I1
I2
I3
Y
s2
Y=S1S0×I 0+S1S0×I 1+S1S0× I 2+S1S0×I 3Y=S1S0×I 0+S1S0×I 1+S1S0× I 2+S1S0×I 3
Quadruple two-to-one-line Multiplexer
A
BY
s
4
4
4
E
Boolean Function Implementation
● Boolean function of n variables– F(x,y,z)
● n–1 variables of the function are connected to the selection inputs– x, and y; for the function F(x,y,z)
● Remaining single variable is used for data input– z, z, 0, 1
Boolean Function Implementation
x y z F
Boolean Function Implementation
Boolean Function Implementation
Boolean Function ImplementationA B C D F
Boolean Function Implementation
Boolean Function Implementation
Three State Gates
Three State Gates
2-to-1-line Mux2-to-1-line Mux
Summary● Combinational Circuit Analysis● Design Procedure
– BCD to Excess-3 Converter
● Binary Adder – Subtractor– HA, FA, Carry Lookahead Generator
● BCD Adder● Binary Multiplier● Magnitude Comparator● Decoder, Boolean Functions using Decoders● Encoders● Multiplexer, Boolean Function Implementation● Three State Gates
Extra
Three State Gates
4-to-1-line Mux4-to-1-line Mux
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