Digital Logic Design
ENGG1015
1st Semester, 2011
Dr. Kenneth Wong
Department of Electrical and Electronic Engineering
Lowered AbstractionLowered AbstractionApplicationsHigh • Image & Video Processing
Systems
gLevel
• Computer & Embedded Systems• Computer Networky
Digital
Computer Network• Mobile Network
• Combinational LogicgLogic
Combinational Logic• Boolean Algebra
Last Circuits • Basic Circuit Theoryweek
ThisElectricalSignals
LowLevel
• Voltage, Current• Power & Energy
This week
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 2
MotivationMotivationHow do you build a computer system?P ibl “ l t i
CPUC t l +Possible answer: “use electronic
circuits”P ti ll t b t t
Control + Datapath
Partially correct… but way too complicated for human (or even computers) to handle
ALUp )
• Too many things to consider: voltage, current, resistance, loading effect… +
The study of digital logic helps to design large digital systems with a easier and mathematically soundeasier and mathematically sound abstraction
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 3
Digital Logic DesignDigital Logic DesignIn a digital system, all signals take on discrete values.• Also referred as states
Most modern digital systems operate on 2 discrete states• binary logic system
We represent the two states as• True and false• 1 and 0• High and Low
Remember: They are not binary numbers -- although you can represent binary numbers using logical statescan represent binary numbers using logical statesThey are not voltage value – although they are usually represented using voltage
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 4
Logic FunctionLogic FunctionA logic function takes 1 or more logic input to produce 1 single logic outputproduce 1 single logic output
S ti “d fi ” l i f ti1 2( , , , )ny f x x x= L
Sometimes one may “define” logic functions that produce more than 1 output
( ) ( )fBut note that a multi-output function can always be implemented as a grouping of multiple
1 2 1 2( , , , ) ( , , , )m ny y y f x x x=L L
be implemented as a grouping of multiple single-output functions• i.e. just a short handj
Mathematically, it a function produce only 1 output
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 5
Representing Logic OperationsRepresenting Logic OperationsEach function can be represented equivalently in 3 ways:equivalently in 3 ways:• Truth table• Boolean logic expression• Boolean logic expression• Schematics Truth Table
B lBoolean ExpressionSchematics
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 6
Truth TablesTruth TablesDescribe how a logic circuit’s output depends on the logic levels present at the inputs.p p
All the possible combinations of inputs are listed
If the truth table is known, we completely know how the circuit behave!!
7
3 Basic Logic Functions3 Basic Logic FunctionsAlso called a logic gate
NOTORAND
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 8
OR GateOR Gate
Boolean expression
X = A + B
Truth table0 = LOW1 = HIGH
The output of an OR gate is HIGH iff one orgate is HIGH iff one or more inputs are HIGH
time
Timing Diagram9
• OR gate can have more than 2 inputs:
44--Input OR GateInput OR Gate
Summary of OR operation:• Produce a result of 1 whenever any input is 1. Otherwise 0.• The expression x=A+B is read as “x equals A OR B”
10
• Example of the use of an OR gate in an alarm system
Alarm is activated whenever the temperature exceeds aAlarm is activated whenever the temperature exceeds a maximum value VTR or whenever the pressure goes above a certain limit VPR
11
More examples
12
Quick Quiz (1)What is the only set of input conditions that will produce a LOW output for anythat will produce a LOW output for any OR gate?• Any one of the input is LOW• Any one of the input is HIGHy p• All inputs are LOW• All inputs are HIGH• All inputs are HIGH
13
Quick Quiz (2)W it th B l i f iWrite the Boolean expression for a six-input OR gate• X=A+B+C+D+E+F
If th A i t i i l iIf the A input in previous example is permanently kept at HIGH, what will the resultant output waveform be?• Always LOWAlways LOW• Always HIGH
S ti LOW ti HIGH• Sometimes LOW, sometimes HIGH• None of the above
14
AND GateAND Gate
X ABBoolean expressionTruth table
0 = LOW
X = AB
1 = HIGH
The output of an AND Timing Diagram
e ou pu o agate is HIGH only when all inputs are HIGHall inputs are HIGH.
1533--Input AND GateInput AND Gate
Summary of the AND operation• The AND operation is performed the same as ordinary• The AND operation is performed the same as ordinary
multiplication of 1s and 0s.• An AND gate output will be 1 only for the case when all inputs
are 1; for all other cases the output will be 0are 1; for all other cases the output will be 0.• The expression x=A•B is read as “x equals A AND B.”
Review Questions
16
Quick Quiz (3)What is the only input combination that will produce a HIGH at the output of awill produce a HIGH at the output of a five-input AND gate?• Any one of the input is LOW• Any one of the input is HIGHy p• All inputs are LOW• All inputs are HIGH• All inputs are HIGH
17
Quick Quiz (4)Wh t l i l l h ld b li d tWhat logic level should be applied to the second input of a two-input AND gate if the logic signal at the first input is to be inhibited (prevented) fromto be inhibited (prevented) from reaching the output?
A LOW input will keep the output LOW• A LOW input will keep the output LOW• A LOW input will keep the output HIGH• A HIGH input will keep the output LOW• A HIGH input will keep the output HIGHp p p
True or false: An AND gate output will ff f O f
18
always differ from an OR gate output for the same input conditions.
The NOT Operation & InverterThe NOT Operation & Inverter
X A Boolean expression
Truth table
X = A
The output of a NOT gate is always the
0 = LOW1 = HIGH
The output of a NOT gate is always the complement (opposite) of the input.
A NOT gate is sometimes referred as an inverter especially in circuit designsan inverter, especially in circuit designs
19
Other Simple GatesOther Simple GatesALL logic functions, no matter how complex, can be completely expressed using the 3 basic operations AND, OR, g p , ,NOT.
However, many systems utilizes more than just the 3 basic logic gates j g gbecause it makes the design cleaner and easier to understand (for human)and easier to understand (for human).
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 20
NAND GateNAND Gate
X = ABX = AB
bubble
Output 0 iff ALL inputs are 1sOutput 0 iff ALL inputs are 1sComplement of an AND gateNote the “bubble” at the output of the symbol, and the bar over the expression y , pAB. Both of them signifies the complement nature to the AND gate
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 21
NOR GateNOR Gate
X = A + BX = A + B
Output 1 iff ALL inputs are 0sOutput 1 iff ALL inputs are 0s
Complement of an OR gatep g
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 22
XOR GateXOR Gate
X A ⊕ B
A B X0 0 0
X = A ⊕ B0 1 11 0 1
Exclusive OR gate
1 1 0
Exclusive-OR gate
Output 1 iff exactly one input is 1p y p
Similar to an OR gate, except that when b th i t 1 th t t i 0both inputs are 1, the output is 0
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 23
3 Representations of Logic Functions3 ep ese o s o og c u c o sRecall that any complex logic function can be expressed in 3 ways: Truth Table, Boolean Expression, Schematics, p ,
Only Truth Table representation is unique
We can convert representation from oneWe can convert representation from one form to the other
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 24
Schematics to Boolean ExpressionSchematics to Boolean ExpressionExample: logic circuit with its Boolean expression
Question: how to interpret A·B+C?• Is it A·B ORed with C ? Is it A ANDed with B+C ?
O d f d f B l l b AND b f OR P thOrder of precedence for Boolean algebra: AND before OR. Parentheses make the expression clearer, but they are not needed for the case on the preceding slide.
Therefore the case below is different:
25
Whenever an INVERTER is present in a logic-circuit diagram, its output expression is simply equal to the input expression with a baroutput expression is simply equal to the input expression with a bar over it.
More examples
26
Quick Quiz (5)What is the output expression of the following logic circuit diagram?following logic-circuit diagram?
• ( )x ABC A D= +
•
•
( )x ABCD=
( )x ABC A D= +•
•( )x ABC A D= +
x ABCD=
27
Quick Quiz (6)What is the output expression of the following logic circuit diagram?following logic-circuit diagram?
• ( )x D A B C E= + + ⋅
•
• x D A B C E⎡ ⎤= + + ⋅ ⋅⎣ ⎦
x D A BC E⎡ ⎤= + + ⋅⎣ ⎦
( )
•
• ( )x D A B C E⎡ ⎤= + + ⋅⎣ ⎦
x D A B C E⎡ ⎤= + + ⋅ ⋅⎣ ⎦
28
PrecedencePrecedenceGiven an Boolean expression• First, perform all inversions of single terms• Perform all operations with parenthesesPerform all operations with parentheses• Perform an AND operation before an OR
operation unless parentheses indicateoperation unless parentheses indicate otherwiseIf i h b it f• If an expression has a bar over it, perform the operations inside the expression first
d th i t th ltand then invert the result
29
Determining output level from a diagramg p g
30
Example: Draw the circuit diagram to implement the expressionExample: Draw the circuit diagram to implement the expressionX = (A + B)(B + C)
Example: Draw the circuit diagram that implements the expression
using gates having no more than three inputs)( DABCAx += using gates having no more than three inputs. )( DABCAx +=
31
Boolean Expressions to SchematicsBoolean Expressions to SchematicsWhen the operation of a circuit is defined by a Boolean expression, we can draw a logic-circuitBoolean expression, we can draw a logic circuit diagram directly from that expression.Example: draw the circuit for
Done in two stepsy = AC + BC + ABC
p
32
Switch Logic (1)Switch Logic (1)We can construct a logic-circuit diagram using switches and relaysswitches and relays.Normally-open (NO)/normally-close switches:
To implement logical AND and logical OR functions on two human inputs A and B as follows:follows:
The logical value of input is reversed by using a NC switch instead of NO switch 33
Switch Logic (2)Switch Logic (2)Instead of relying human input, a relay should be usedbe used.In other word, a relay allows its switching activity be controlled by another electricalactivity be controlled by another electrical signal.For example:p
OA OR B
34
Quick Quiz (7)What is the output expression of the following switch logic circuit?following switch-logic circuit?
• ( )x A B C= + ⋅
•
•
( )( )x A B C= + ⋅
( )x A B C= + ⋅•
•
( )x A B C= +
( )x A B C= + ⋅
35
In conclusion…In conclusion…AND, OR, NOT are 3 basic logic gates that can implement all logic functions
All logic functions can be representedAll logic functions can be represented as (1) truth table (2) schematics (3) Boolean expressions
The same logic functions can beThe same logic functions can be converted between the 3
t ti ilrepresentations easily.
Only truth table representation is uniqueOnly truth table representation is unique
1st semester, 2011 Digital Logic - ENGG1015 - K. Wong 36
1st semester, 2011 37
HomeworkHomeworkHomework 1 is out• Due: 5pm on 14 Oct, 2011• Turn in physical copy of your answer• Homework boxes near Room 712, Chow Yei
Ching Building
Homework 2 will be out next week• Due: 5pm on 4 Nov, 2011
Individual homeworkG d t t d f fi lGood way to study for finalZero tolerance on plagiarismp g
1st semester, 2011 38ENGG1015 – E. Lam/K. Wong
Project GroupsProject GroupsNeed to form a group of 4 by the end of this week (Sat 9am)Email the following group information to Leon: culei@eee hku hkEmail the following group information to Leon: [email protected] the following subject in your email:• “ENGG1015 group information”
I l d i th b d f il th f ll i i f tiInclude in the body of your email the following information:• Group Name• Group Members
4 i f + UID• 4 pairs of name + UID• Group leader
The group leader’s role:R ibl f th ti f ll th b• Responsible for the actions of all the members
• Make sure the members can work safely in the lab• Act as main correspondence for the group
R t + t t i l• Request + return materials• NO extra credit
1st semester, 2011 39ENGG1015 – E. Lam/K. Wong
Rough ScheduleRough ScheduleWeek 7/8 (10-21 Oct)• Lab bench “move-in”• Get initial equipments (basic sensors, tools)• Brainstorm on ideas• Put in your 1st equipment request (if and only if you havePut in your 1 equipment request (if and only if you have
some ideas on what you need. See bullet above)Week 8 (17-21 Oct)
G t t k t d f 1st t• Get stock requested from 1st request• Brainstorming session• 2-4pm
Week 9 (24-28 Oct)• Mid-term check-point presentation• 2-4pm2 4pm
Week 10-13• Work on project
1st semester, 2011 40ENGG1015 – E. Lam/K. Wong
Working on the ProjectWorking on the Project
The lab will be opened from 9am toThe lab will be opened from 9am to 11pm every weekday
You may work on the project any time you want during that periodyou want during that period
Safety goes first
NO water/fire/chemical !!!
1st semester, 2011 41ENGG1015 – E. Lam/K. Wong