1007ICT / 1807ICT / 7611ICT Computer Systems & Networks
3A. Digital Logic and Digital Circuits
Last Section: Data Representation
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Topics Covered:
Representing binary integers
Conversion from binary to decimal
Hexadecimal and octal representations
Binary number operations
One’s complement and two’s complement
Representing characters, images and audio
Lecture Content
Learningobjectives
Digitallogic,Basiclogicgates,Booleanalgebra Combinatoriallogicgates
© . Revised and updated by , , and Wee Lum 3
Learning Objectives
At the end of this lecture you will have:
Gained an understanding of basic logic gates
Learnt the truth tables associated with the basic logic gates
Gained an understanding of combinatorial logic gates
Learnt the truth tables associated with combinatorial logic gates
© . Revised and updated by , , and Wee Lum 4
Digital Logic (Section 2.2)
All digital computers are built from a set of low
Logic Gates.
level digital logic switches or
Gates operate on binary signals that only have one of two values:
Signalsfrom0to2voltsisusedtorepresentabinary0(OFF) Signalsfrom3to5voltsisusedtorepresentabinary1(ON) Signals between 2 and 3 volts represent an invalid state
Three basic logic functions that can be applied to binary signals:
More complex functions can be built from these three basic gates
AND: OR: NOT:
outputtrueifALLinputsaretrue outputtrueifANYinputistrue outputistheinverseoftheinput
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Basic Logic Gates (Section 2.4)
Boolean expression
Truth Table
x = a AND b
x = a OR b
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Boolean Algebra
There is a basic set of rules about combining simple binary functions.
x OR 0 = x x OR 1 = 1 x OR x = x x OR x = 1 (x)=x
xAND0 = 0 xAND1 = x xANDx = x xANDx = 0
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Combinatorial Logic Gates
Next Slide
Symbol Equivalent
Boolean expression
Truth Table
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x = a AND b x = a OR b x = a XOR b
Boolean Algebra – 2
This second set of rules are more powerful. OR – form AND – form
(xORy) = xANDy
(xANDy) = xORy
OR – form AND – form
NAND = Theorem
DeMorgan’s
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The eXclusive-OR Gate (XOR)
Looking at the truth table we see that the XOR function can be described as:
x = (aANDb)OR(aANDb) x=aXORb
This function can be built in 3 ways: Demorgan’s Theorem
aaa bbb aaa bbb
x = (aANDb)OR(aANDb) x = (aANDb)OR (aANDb) x = (aANDb)AND(aANDb)
© . Revised and updated by , , and Wee Lum 10
© . Revised and updated by , , and Wee Lum 11
Logic Unit
Let’s try to create a “programmable” logic unit that permits us to apply a predefined logic function to a given set of inputs.
Output Select
We need a function that lets us select what operation to perform
AND OR XOR
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Have considered:
Operation of basic logic gates
Combinatorial logic gates, Truth tables
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Logic unit, Selection logic, Decoder logic
Multiplexing and demultiplexing
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