[SOLVED] 程序代写代做代考 1007ICT / 1807ICT / 7611ICT Computer Systems & Networks

1007ICT / 1807ICT / 7611ICT Computer Systems & Networks
3B. Digital Logic and Digital Circuits
Dr. Sven Venema Dr. Vallipuram Muthukkumarasamy

Last Lecture:
Topics Covered:
• Digital logic, Basic logic gates, Boolean algebra
• Combinatorial logic gates

Lecture Content
 Learning objectives
 Logic unit, Selection logic, Decoder logic  Multiplexing and demultiplexing
 Half and Full adders
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 3

Learning Objectives
At the end of this lecture you will have gained an understanding of:
 Selection logic
 Decoder logic
 Multiplexors
 Demultiplexors
 Half and Full adders
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 4

Logic Unit
Lets try to create a “programmable” logic unit that permits us to apply a predefined logic function to a given set of inputs.
ab
Output Select
We need a function that lets us select what operation to perform

AND OR XOR
NOT

© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 5

Selection Logic
 Saywehaveanumberofinputs‘A,B,C,D’andwe want to select one of them to use in a logic function.
 Weneedaspecialfunctiontoswitchtheselected input into the output ‘X’ based on the selection.
 WecanuseinputsS0toS3toselectbetweenA-D
A B C D
Selector
X
S0 S1 S2 S3
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 6

Decoder Logic
 Justsaywewanttoselectbetweenoneof100’s of possible inputs – we need 100’s of selection inputs as well which gets too complicated.
 DecoderswithNinputsallowustoenableany one of 2N possible selection lines.
 Basicallyadecodertakesabinarycodednumber and enables the output representing the number
Inputs
Outputs
s1
s2
x0
x1
x2
x3
0
0
1
0
0
0
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
0
1
2 to 4 Decoder
x0 = s1ANDs2 x1 = s1ANDs2 x2 = s1ANDs2 x3 = s1ANDs2
X0 X1
X2 X3
2N outputs
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 7
S2 S1
N Inputs

Multiplexing
 Combiningtheselectoranddecoderwecan
create what is called a
 IngeneralifwehaveNinputswewanttoswitch between we need to have log2(N) selection lines.
Multiplexor
A B C D
X
s0
s1
X
0
0
A
0
1
B
1
0
C
1
1
D
Decoder
S0 S1 selection
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 8
2N inputs

Demultiplexing
We can also switch in the opposite direction to send one input ‘A’ into one of many different outputs (eg X1..Xn)

Decoder
A
X1 X2
Input X1 A X2
outputs X4
X3 X3
X4
(s0,s1) Select
S0 S1 selection
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 9

Half-Adders
In addition to logic functions we can also create maths functions.
The simplest math function is the half-adder which can add 2 digits (bits) to give a sum and a carry bit
0011A 0 1 0 1B+ 0 1 1 10
AB


A
B
Sum
C
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
1
Sum = A XOR B Carry = A AND B
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 10

Full-Adders
 Ifwewanttoaddmorethan1bitvaluestogether we need to deal with the carry.
 Full-addersacceptthetwoinputstobeaddedplus the carry from a previous stage.
 Thecircuitmustbeabletodealwithpropagation delays
00 00 01 01 00 10 11 11A 00 01 00 01 10 01 10 11B+ 00 01 01 10 10 11 101 110
A0 B0 A1 B1 A2 B2 A3 B3
Full Adder
Full Adder
Full Adder
Full Adder
Cin
Cout
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S0 S1 4 bit adder S2 S3 © Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan

Full-Adders
The logic to perform add with carry combines two half adders together.
Sum =AXORBXORCin
Cout = (AANDB)OR(CinAND(AXORB))

Cin
A
B
Sum
Cout
0
0
0
0
0
0
0
1
1
0
0
1
0
1
0
0
1
1
0
1
1
0
0
1
0
1
0
1
0
1
1
1
0
0
1
1
1
1
1
1
Cin AB
Sum Cout
Sum Carry
Sum A+B
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 12

Subtraction
 A–BisthesameasA+(-B)
 IfweconvertBtothenegativeequivalentofits value, we can use the basic adder as it is. We can use an XOR gate to do this.
 However,convertingBto–Busingtwo’s complement also requires adding 1
A B0 Sign bit 0
+0 if positive +1 if negative
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 13
Full Adder
S0

Summary
Have considered:  Selection logic  Decoder logic
 Multiplexors
 Demultiplexors
 Half and Full adders
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 14

Next….
 Arithmetic logic unit
 Binary multiplication and division
 Shifting
 Sequential Logic
 Data latches, S-R Latch
 Clocks and synchronisation
 Registers, Buses, Computer memory  Processors and Memory Organisation
© Ruben Gonzalez. Revised and updated by Sven Venema, Vallipuram Muthukkumarasamy, and Wee Lum Tan
Page 15