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Boolean Algebra
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Introduction
• 1854: Logical algebra was published by George Boole known today as “Boolean Algebra”
– It’s a convenient way and systematic way of expressing and analyzing the operation of logic circuits.
• 1938: Claude Shannon was the first to apply Boole’s work to the analysis and design of logic circuits.
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Boolean Operations & Expressions
• Variable – a symbol used to represent a logical quantity.
• Complement – the inverse of a variable and is indicated by a bar over the variable.
• Literal – a variable or the complement of a variable.
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Boolean Addition
• Boolean addition is equivalent to the OR operation
• A sum term is produced by an OR operation with no AND ops involved. – i.e. A B, A B, A B C , A B C D
– A sum term is equal to 1 when one or more of the literals in the term are 1.
– A sum term is equal to 0 only if each of the literals is 0.
0+0 = 0 0+1 = 1 1+0 = 1 1+1 = 1
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0·0 = 0 0·1 = 0 1·0 = 0 1·1 = 1
Boolean Multiplication
• Boolean multiplication is equivalent to the AND operation
• A product term is produced by an AND operation with
no OR ops involved. – i.e. AB, AB, ABC , ABCD – A product term is equal to 1 only if each of the literals in
the term is 1. – A product term is equal to 0 when one or more of the
literals are 0.
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Commutative Laws
• The commutative law of addition for two variables is written as: A+B = B+A
A A+B
B
B B+A
A
• The commutative law of multiplication for two variables is written as: AB = BA
A AB
B
B B+A
A
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Associative Laws
B+C
A+B
BC
AB
• The associative law of addition for 3 variables is written as: A+(B+C) = (A+B)+C
A A+(B+C)
B
C
A
B (A+B)+C
C
• The associative law of multiplication for 3 variables is written as: A(BC) = (AB)C
A A(BC)
B
C
A
B (AB)C
C
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Distributive Laws
B+C AB
AC
• The distributive law is written for 3 variables as follows: A(B+C) = AB + AC
B A
C B X
A X
A
C
X=A(B+C) X=AB+AC
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Rules of Boolean Algebra
1.A 0 A 7.A A A
2.A 1 1 8.A A 0
3.A 0 0
9.A A
4.A 1 A 10.A AB A
5.A A A 11.A AB A B
6.A A 1 12.( A B)( A C) A BC
___________________________________________________________
A, B, and C can represent a single variable or a combination of variables.
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DeMorgan’s Theorems
• DeMorgan’s theorems provide mathematical verification of:
– the equivalency of the NAND and negative-OR gates
– the equivalency of the NOR and negative-AND gates.
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DeMorgan’s Theorems
• The complement of two or more ANDed variables is equivalent to the OR of the complements of the individual variables.
• The complement of two or
more ORed variables is equivalent to the AND of the complements of the individual variables.
NOR
X Y
NAND
X Y
Negative-OR
X Y
Negative-AND
X Y
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DeMorgan’s Theorems (Exercises)
• Apply DeMorgan’s theorems to the expressions:
X Y Z
X Y Z
X Y Z
W X Y Z
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DeMorgan’s Theorems (Exercises)
• Apply DeMorgan’s theorems to the expressions:
( A B C)D
ABC DEF
AB CD EF
A BC D(E F )
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Boolean Analysis of Logic Circuits
• Boolean algebra provides a concise way to express the operation of a logic circuit formed by a combination of logic gates
– so that the output can be determined for various combinations of input values.
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CD
B+CD
Boolean Expression for a Logic Circuit
• To derive the Boolean expression for a given logic circuit, begin at the left-most inputs and work toward the final output, writing the expression for each gate.
C
D
B
A A(B+CD)
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Constructing a Truth Table for a Logic Circuit
• Once the Boolean expression for a given logic circuit has been determined, a truth table that shows the output for all possible values of the input variables can be developed.
– Let’s take the previous circuit as the example:
A(B+CD)
– There are four variables, hence 16 (24) combinations of values are possible.
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Constructing a Truth Table for a Logic Circuit
• Evaluating the expression
– To evaluate the expression A(B+CD), first find the values of the variables that make the expression equal to 1 (using the rules for Boolean add & mult).
– In this case, the expression equals 1 only if A=1 and B+CD=1 because
A(B+CD) = 1·1 = 1
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Constructing a Truth Table for a Logic Circuit
• Evaluating the expression (cont’)
– Now, determine when B+CD term equals 1.
– The term B+CD=1 if either B=1 or CD=1 or if both B and CD equal 1 because
B+CD = 1+0 = 1
B+CD = 0+1 = 1
B+CD = 1+1 = 1
• The term CD=1 only if C=1 and D=1
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Constructing a Truth Table for a Logic Circuit
• Evaluating the expression (cont’)
– Summary:
– A(B+CD)=1
• When A=1 and B=1 regardless of the values of C and D
• When A=1 and C=1 and D=1 regardless of the value of B
– The expression A(B+CD)=0 for all other value combinations of the variables.
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Constructing a Truth Table for a Logic Circuit
INPUTS OUTPUT
A B C D A(B+CD)
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
INPUTS OUTPUT
A B C D A(B+CD)
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 1
INPUTS OUTPUT
A B C D A(B+CD)
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1 1
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 1
• Putting the results in truth table format
A(B+CD)=1
When A=1 and
B=1 regardless
of the values
of C and D When A=1 and C=1 and D=1 regardless of
the value of B
INPUTS OUTPUT
A B C D A(B+CD)
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
0 0 1 1 0
0 1 0 0 0
0 1 0 1 0
0 1 1 0 0
0 1 1 1 0
1 0 0 0 0
1 0 0 1 0
1 0 1 0 0
1 0 1 1 1
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 1
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Distinction between analog and digital signal
Analog signal
• Analog signal is a continuous signal
• Denoted by sine wave
• Stored in the form of wave signal
• Analog instrument draws large
power
• Low cost and portable
Digital signal
• Digital signals are discrete time
signals
• Denoted by square waves
• Stored in the form of binary bit
• Digital instrument draws only
negligible power
• Cost is high and not easily
portable
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Types Of Numbers
• Natural Numbers
– The number 0 and any number obtained by
repeatedly adding a count of 1 to 0
• Negative Numbers
– A value less than 0
• Integer
– A natural number, the negative of a natural number,
and 0.
– So an integer number system is a system for
‘counting’ things in a simple systematic way
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Exponent Review
• An exponent (power) tells you how many times to
multiply the base by itself:
– 21 = 2
– 22 = 2 x 2 =4
– 23 = 2 x 2 x 2 = 8
• 20 = 1 (ANY number raised to power 0 is 1)
• 1 / x2 = x -2
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Decimal Numbering System
• How is a positive integer represented in decimal?
• Let’s analyze the decimal number 375:
375 = (3 x 100) + (7 x 10) + (5 x 1)
= (3 x 102) + (7 x 101) + (5 x 100)
5 x100 = 5 +
7 x101 = 70 +
3 x 102 = 300
375
5 7 3
102 101 100 Position weights
Number digits
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Decimal System Principles
• A decimal number is a sequence of digits
• Decimal digits must be in the set:
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9} (Base 10)
• Each digit contributes to the value the number
represents
• The value contributed by a digit equals the product of
the digit times the weight of the position of the digit in
the number
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Decimal System Principles
• Position weights are powers of 10
• The weight of the rightmost (least significant digit)
is 100 (i.e.1)
• The weight of any position is 10x, where x is the
number of positions to the right of the least
significant digit
Position weights digits
104 103 102 101 100
3 7 5
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Bits
• In a computer, information is stored using digital
signals that translate to binary numbers
• A single binary digit (0 or 1) is called a bit
– A single bit can represent two possible states,
on (1) or off (0)
• Combinations of bits are used to store values
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Data Representation
• Data representation means encoding data into bits
– Typically, multiple bits are used to represent the
‘code’ of each value being represented
• Values being represented may be characters, numbers,
images, audio signals, and video signals.
• Although a different scheme is used to encode each type
of data, in the end the code is always a string of zeros
and ones
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Decimal to Binary
24 23 22 21 20 Position weights
digits
• So in a computer, the only possible digits we can use to
encode data are {0,1}
– The numbering system that uses this set of digits is
the base 2 system (also called the Binary Numbering
System)
• We can apply all the principles of the base 10 system to
the base 2 system
1 0 1 1
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Binary Numbering System
• How is a positive integer represented in binary?
• Let’s analyze the binary number 110:
110 = (1 x 22) + (1 x 21) + (0 x 20)
= (1 x 4) + (1 x 2) + (0 x 1)
So a count of SIX is represented in binary as 110
0 +
2 +
4
6
0 x20 =
1 x21 =
1 x 22 =
20
0
22 21
1 1
Position weights
Number digits
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Binary to Decimal Conversion
To convert a base 2 (binary) number to base
10 (decimal):
Add all the values (positional weights)
where a one digit occurs
Positions where a zero digit occurs do
NOT add to the value, and can be
ignored
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Binary to Decimal Conversion
Example: Convert binary 100101 to decimal
(written 1 0 0 1 0 12 ) =
1*20 +
0*21 +
1*22 +
0*23 +
0*24 +
1*25
1 +
4 +
32
3710
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Binary to Decimal Conversion
Example #2: 101112
positional powers of 2: 24 23 22 21 20
decimal positional value: 16 8 4 2 1
binary number: 1 0 1 1 1
16 + 4 + 2 + 1 = 2310
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Binary to Decimal Conversion
Example #3: 1100102
positional powers of 2: 25 24 23 22 21 20
decimal positional value: 32 16 8 4 2 1
binary number: 1 1 0 0 1 0
32 + 16 + 2 = 5010
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Decimal to Binary Conversion
The Division Method:
1) Start with your number (call it N) in base 10
2) Divide N by 2 and record the remainder
3) If (quotient = 0) then stop
else make the quotient your new N, and go back to step 2
The remainders comprise your answer, starting with the last
remainder as your first (leftmost) digit.
In other words, divide the decimal number by 2 until you reach
zero, and then collect the remainders in reverse.
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Decimal to Binary Conversion
Using the Division Method: Divide decimal number by 2 until you reach zero, and then
collect the remainders in reverse.
E 101102
xample 1: 2210 =
2 ) 22 Rem:
2 ) 11 0
2 ) 5 1
2 ) 2 1
2 ) 1 0
0 1
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Decimal to Binary Conversion
Using the Division Method
E
1110002
xample 2: 5610 =
2 ) 56 Rem:
2 ) 28 0
2 ) 14 0
2 ) 7 0
2 ) 3 1
2 ) 1 1
0 1
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Decimal to Binary Conversion
The Subtraction Method:
Subtract out largest power of 2 possible (without going below zero), repeating until you reach 0.
Place a 1 in each position where you COULD subtract the value
Place a 0 in each position that you could NOT subtract out the value without going below zero.
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Decimal to Binary Conversion
Example 1: 2110
21
- 16
5 1 0 1 0 1 - 4
1
- 1 Answer: 2110 = 101012
0
26 25 24 23 22 21 20
64 32 16 8 4 2 1
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Decimal to Binary Conversion
Example 2: 5610
56 26 | 25 24 23 22 21 20
- 32 64| 32 16 8 4 2 1
24 | 1 1 1 0 0 0
- 16
8
- 8 Answer: 5610 = 1110002
0
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Octal Numbering System
• Base: 8 • Digits: 0, 1, 2, 3, 4, 5, 6, 7
Octal number: 3578
= (3 x 82 ) + (5 x 81) + (7 x 80)
To convert to base 10, beginning with the
rightmost digit, multiply each nth digit by 8(n-1), and add all of the results together.
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Octal to Decimal Conversion
Example 1: 3578
positional powers of 8: 82 81 80
decimal positional value: 64 8 1
Octal number: 3 5 7
(3 x 64) + (5 x 8) + (7 x 1)
= 192 + 40 + 7 = 23910
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Octal to Decimal Conversion
• Example 2: 12468
positional powers of 8: 83 82 81 80
decimal positional value: 512 64 8 1
Octal number: 1 2 4 6
(1 x 512) + (2 x 64) + (4 x 8) + (6 x 1)
= 512 + 128 + 32 + 6 = 67810
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Decimal to Octal Conversion
The Division Method:
1) Start with your number (call it N) in base 10
2) Divide N by 8 and record the remainder
3) If (quotient = 0) then stop
else make the quotient your new N, and go back to step 2
The remainders comprise your answer, starting with the last
remainder as your first (leftmost) digit.
In other words, divide the decimal number by 8 until you reach
zero, and then collect the remainders in reverse.
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Decimal to Octal Conversion
Using the Division Method:
Example 1: 21410 = 3268
8 ) 214 Rem:
8 ) 26 6
8 ) 3 2
0 3
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Decimal to Octal Conversion
Example 2: 433010 = 103528
8 ) 4330 Rem:
8 ) 541 2
8 ) 67 5
8 ) 8 3
8 ) 1 0
0 1
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Decimal to Octal Conversion
The Subtraction Method:
Subtract out multiples of the largest power of 8 possible (without going below zero) each time until you reach 0.
Place the multiple value in each position where you COULD subtract the value.
Place a 0 in each position that you could NOT subtract out the value without going below zero.
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Decimal to Octal Conversion
Example 1: 31510
315
- 256 (4 x 64)
59
- 56 (7 x 8)
3
82 81 80
64 8 1
4 7 3
- 3 (3 x 1)
0 Answer: 31510 = 4738
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Decimal to Octal Conversion
Example 2: 201810
2018 84 83 82 81 80
-1536 (3 x 512) 4096 512 64 8 1
482 3 7 4 2
- 448 (7 x 64)
34
- 32 (4 x 8)
2
- 2 (2 x 1) Answer: 201810 = 37428
0
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Hexadecimal (Hex) Numbering System
• Base: 16 • Digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
Hexadecimal number: 1F416
= (1 x 162 ) + (F x 161) + (4 x 160)
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Hexadecimal (Hex) Extra Digits
Decimal Value Hexadecimal Digit
10 A
11 B
12 C
13 D
14 E
15 F
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Hex to Decimal Conversion
• To convert to base 10:
– Begin with the rightmost digit
– Multiply each nth digit by 16(n-1)
– Add all of the results together
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Hex to Decimal Conversion
• Example 1: 1F416
positional powers of 16: 163 162 161 160
decimal positional value: 4096 256 16 1
Hexadecimal number: 1 F 4
(1 x 256) + (F x 16) + (4 x 1)
= (1 x 256) + (15 x 16) + (4 x 1)
= 256 + 240 + 4 = 50010
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Hex to Decimal Conversion
• Example 2: 25AC16
positional powers of 16: 163 162 161 160
decimal positional value: 4096 256 16 1
Hexadecimal number: 2 5 A C
(2 x 4096) + (5 x 256) + (A x 16) + (C x 1)
= (2 x 4096) + (5 x 256) + (10 x 16) + (12 x 1)
= 8192 + 1280 + 160 + 12 = 964410
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Decimal to Hex Conversion
The Division Method:
1) Start with your number (call it N) in base 10
2) Divide N by 16 and record the remainder
3) If (quotient = 0) then stop
else make the quotient your new N, and go back to step 2
The remainders comprise your answer, starting with the last
remainder as your first (leftmost) digit.
In other words, divide the decimal number by 16 until you
reach zero, and then collect the remainders in reverse.
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Decimal to Hex Conversion
Using The Division Method:
Example 1: 12610 =
16) 126 Rem:
16) 7 14=E
0 7
7E16
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Decimal to Hex Conversion
Example 2: 60310 =
16) 603 Rem:
16) 37 11=B
16) 2 5
0 2
25B16
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Decimal to Hex Conversion
The Subtraction Method:
Subtract out multiples of the largest power of 16 possible (without going below zero) each time until you reach 0.
Place the multiple value in each position where you COULD to subtract the value.
Place a 0 in each position that you could NOT subtract out the value without going below zero.
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Decimal to Hex Conversion
Example 1: 81010
810 162 161 160
- 768 (3 x 256) 256 16 1
42
- 32 (2 x 16)
10
3 2 A
- 10 (10 x 1)
0 Answer: 81010 = 32A16
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Decimal to Hex Conversion
Example 2: 15610
156
- 144 (9 x 16)
12
- 12 (12 x 1)
0
162 161 160
256 16 1
9 C
Answer: 15610 = 9C16
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Binary to Octal Conversion
The maximum value represented in 3 bit is: 23 – 1 = 7
So using 3 bits we can represent values from
0 to 7 which are the digits of the Octal numbering system.
Thus, three binary digits can be converted to one
octal digit.
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Binary to Octal Conversion
Three-bit Group Decimal Digit Octal Digit
000 0 0
001 1 1
010 2 2
011 3 3
100 4 4
101 5 5
110 6 6 111 7 7
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Octal to Binary Conversion
Ex : Convert 7428 to binary
Convert each octal digit to 3 bits:
7 = 111
4 = 100
2 = 010
111 100 010
7428 = 1111000102
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Binary to Octal Conversion
Ex : Convert 101001102 to octal
Starting at the right end, split into groups of 3:
10 100 110
110 = 6
100 = 4
010 = 2 (pad empty digits with 0)
101001102 = 2468
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Binary to Hex Conversion
The maximum value represented in 4 bit is: 24 – 1 = 15
So using 4 bits we can represent values from
0 to 15 which are the digits of the Hexadecimal numbering system.
Thus, four binary digits can be converted to one
hexadecimal digit.
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Binary to Hex Conversion
Four-bit Group Decimal Digit Hexadecimal Digit 0000 0 0
0001 1 1
0010 2 2
0011 3 3
0100 4 4
0101 5 5
0110 6 6
0111 7 7
1000 8 8
1001 9 9
1010 10 A
1011 11 B
1100 12 C
1101 13 D
1110 14 E
1111 15 F
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Binary to Hex Conversion
Ex : Convert 1101001102 to hex
Starting at the right end, split into groups of 4:
(pad empty digits with 0)
1101001102 = 1A616
1 1010 0110 0110 = 6 1010 = A 0001 = 1
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Hex to Binary Conversion
Ex : Convert 3D916 to binary
Convert each hex digit to 4 bits:
3 = 0011
D = 1101
9 = 1001
0011 1101 1001
3D916 = 11110110012 (can remove leading zeros)
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Conversion between Binary and Hex - Try It Yourself
• Convert the following numbers:
–10101111012 to Hex
–82F16 to Binary
• (Answers on NEXT slide)
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Answers
• 10101111012 10 1011 1101
= 2BD16
• 82F16 = 0100 0010 1111
100001011112
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Octal to Hex Conversion
To convert between the Octal and Hexadecimal numbering systems
Convert from one system to binary first
Then convert from binary to the new numbering system
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Hex to Octal Conversion
First convert the hex to binary:
Ex : Convert E8A16 to octal
1110 1000 10102
111 010 001 010 and re-group by 3 bits
(starting on the right)
Then convert the binary to octal: 7 2 1 2
So E8A16 = 72128
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Octal to Hex Conversion
Ex : Convert 7528 to hex
First convert the octal to binary:
111 101 0102
re-group by 4 bits
0001 1110 1010 (add leading zeros)
Then convert the binary to hex: 1 E A
So 7528 = 1EA16
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Basic Logic Gates and Basic Digital Design
• NOT, AND, and OR Gates
• NAND and NOR Gates
• DeMorgan’s Theorem
• Exclusive-OR (XOR) Gate
• Multiple-input Gates
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NOT Gate -- Inverter
NOT
Y = ~X
X Y
X
0
1
Y
1
0
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NOT
• Y = ~X (Verilog)
• Y = !X (ABEL)
• Y = not X (VHDL)
• Y = X’
• Y = X
• Y = X (textook)
• not(Y,X) (Verilog)
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NOT
X ~X ~~X = X
X ~X ~~X
0 1 0
1 0 1
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AND Gate
AND
X
Z
Y
Z = X & Y
X
0
0
1
1
Y
0
1
0
1
Z
0
0
0
1
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AND
• X & Y (Verilog and ABEL)
• X and Y (VHDL)
• X Y
• X Y
• X * Y
• XY (textbook)
• and(Z,X,Y) (Verilog)
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OR Gate
OR
X
Z
Y
Z = X | Y
X Y Z
0 0 0
0 1 1
1 0 1
1 1 1
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OR
• X | Y (Verilog)
• X # Y (ABEL)
• X or Y (VHDL)
• X + Y (textbook)
• X V Y
• X U Y
• or(Z,X,Y) (Verilog)
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Basic Logic Gates and Basic Digital Design
• NOT, AND, and OR Gates
• NAND and NOR Gates
• DeMorgan’s Theorem
• Exclusive-OR (XOR) Gate
• Multiple-input Gates
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NAND Gate
NAND
X
Z
Y
Z = ~(X & Y)
nand(Z,X,Y)
X
0
0
1
1
Y
0
1
0
1
Z
1
1
1
0
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NAND Gate
NOT-AND
X
Y
Z
1 0 0 1
1 1 1 0
W = X & Y
Z = ~W = ~(X & Y)
W
X Y W Z
0 0 0 1
0 1 0 1
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NOR Gate
NOR
X
Z
Y
Z = ~(X | Y)
nor(Z,X,Y)
X Y Z
0 0 1
0 1 0
1 0 0
1 1 0
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NOR Gate
NOT-OR
X W
Z
Y
W = X | Y
Z = ~W = ~(X | Y)
X Y W Z
0 0 0 1
0 1 1 0
1 0 1 0
1 1 1 0
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Basic Logic Gates and Basic Digital Design
• NOT, AND, and OR Gates
• NAND and NOR Gates
• DeMorgan’s Theorem
• Exclusive-OR (XOR) Gate
• Multiple-input Gates
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NAND Gate
X Z =
X Z
Y Y
Z = ~(X & Y) Z = ~X | ~Y
X Y W Z X Y ~X ~Y Z
0 0 0 1 0 0 1 1 1
0 1 0 1 0 1 1 0 1
1 0 0 1 1 0 0 1 1
1 1 1 0 1 1 0 0 0
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De Morgan’s Theorem-1
~(X & Y) = ~X | ~Y
• NOT all variables
• Change & to | and | to &
• NOT the result
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NOR Gate
X X Z Z
Y Y
Z = ~(X | Y) Z = ~X & ~Y
X Y Z X Y ~X ~Y Z
0 0 1 0 0 1 1 1
0 1 0 0 1 1 0 0
1 0 0 1 0 0 1 0
1 1 0 1 1 0 0 0
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De Morgan’s Theorem-2
~(X | Y) = ~X & ~Y
• NOT all variables
• Change & to | and | to &
• NOT the result
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De Morgan’s Theorem
• NOT all variables
• Change & to | and | to &
• NOT the result
• --------------------------------------------
• ~X | ~Y = ~(~~X & ~~Y) = ~(X & Y)
• ~(X & Y) = ~~(~X | ~Y) = ~X | ~Y
• ~X & !Y = ~(~~X | ~~Y) = ~(X | Y)
• ~(X | Y) = ~~(~X & ~Y) = ~X & ~Y
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Basic Logic Gates and Basic Digital Design
• NOT, AND, and OR Gates
• NAND and NOR Gates
• DeMorgan’s Theorem
• Exclusive-OR (XOR) Gate
• Multiple-input Gates
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Exclusive-OR Gate
XOR
X
Y Z
Z = X ^ Y
xor(Z,X,Y)
X Y Z
0 0 0
0 1 1
1 0 1
1 1 0
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XOR
• X
• X
• X
^
$
@
Y
Y
Y
(Verilog)
(ABEL)
X Y (textbook)
• xor(Z,X,Y) (Verilog)
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Exclusive-NOR Gate
XNOR
X
Y Z
Z = ~(X ^ Y)
Z = X ~^ Y
xnor(Z,X,Y)
X Y Z
0 0 1
0 1 0
1 0 0
1 1 1
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XNOR
• X ~^ Y (Verilog)
• !(X $ Y) (ABEL)
• X @ Y
X Y
• xnor(Z,X,Y) (Verilog)
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Basic Logic Gates and Basic Digital Design
• NOT, AND, and OR Gates
• NAND and NOR Gates
• DeMorgan’s Theorem
• Exclusive-OR (XOR) Gate
• Multiple-input Gates
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Multiple-input Gates
Z 1 Z 2
Z 3 Z 4
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Multiple-input AND Gate
Z 1
Output Z 1 is HIGH only if all inputs are HIGH
An open input will float HIGH
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Multiple-input OR Gate
Z 2
Output Z 2 is LOW only if all inputs are LOW
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Multiple-input NAND Gate
Z 3
Output Z 3 is LOW only if all inputs are HIGH
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Multiple-input NOR Gate
Z 4
Output Z 4 is HIGH only if all inputs are LOW
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Binarycodes:- BCD, GRAY,EBCDIC, ASCII
• It is the symbolic representation of discrete information which
may be represented in the form of numeric, alphabets &
special characters.
• In Digital Electronics, the binary digits 0 & 1 are used to
represent these symbols & are arranged according to the
rules of specific code.
• Infect, a binary code is a group of n-bits that can represent
distinct symbols.
• The interpretation of thebinary information is possible only if
the code in which this information is available is known.
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Binary codes:- BCD, GRAY,EBCDIC, ASCII
(A) Straight Binary Code
Used to represent numbers using natural (or straight) binary form.
1. Natural BCD Code In this code, decimal 0 through 9 are represented by their natural binary
equivalents using four bits and eachdecimal digit is represented bythis four bit
codeindividually. Thiscodeisalsoknownas 8-4-2-1 codewhere 8, 4, 2,& 1 are
the weights of the four bits of the binary code of each decimal digit to the
straight binary system.
2. EXCESS-3 Codes This is another form of BCDcode, inwhich each decimal is coded into a 4-bit binary code.
Thecodefor eachdecimaldigit isobtained byaddingdecimal 3tothenatural BCDcodeof
the digit. For ex- decimal 2 is coded as 0010+0011=0101 in Excess-3 code.
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Binary codes:- BCD, GRAY, EBCDIC, ASCII
3. Gray Code
It is a very useful code in which a decimal number is represented in binary form in such a
way so that each gray code number differs from preceding & the succeeding number by a
single bit. For ex- the gray code for decimal number 5 is 0111 & for 6 is 0101. These two
codes differ by only one bit position (third from left). This code is used extensively for the
shaft encoders because of this property. These can be constructed by using the property
given below:-
• A 1-bit Gray code has two code words 0 & 1 representing decimal numbers 0 & 1.
• An n-bit (n≥2) Gray code will have first with the leading 0 appended.
• The last Graycodes with the leading 1 appended, will be equal to the Graycode
words of an (n-1)-bit Gray code, written in reverse order (assuming a mirror placed between first & last Graycodes) with a leading 1 appended.
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Classification ofCodes 1. Weighted & Non-weighted Codes
2. Self Complimentary & Sequential Codes
3. Alphanumeric codes
4. Error detection & Error correction Codes
1. Weighted Codes
The main characteristic of weighted code is that each binary digit is assigned a
specific weight. The common example of weighted code is BCD code or 8421
code in which the weights of different bits are 1, 2, 4 & 8.
Non-Weighted Code
These don’t follow the principle of positional weighting system i.e. each position
within the number doesn’t follow or have any fixed weight. For ex- Excess-3, Gray
code.
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2. Sequential Code
These are those codes in which each succeeding code is 1 binary
number greater than the preceding code. This property is used for
mathematical manipulation of data. For ex:- BCD
And Excess-3 Code.
3. Self-complementary Code
• A code is said to be self-complementary if the code for 9’s
complement of N i.e. 9-Ncanbeobtained by interchanging all 0s
and 1s.
• Decimal 9 is the complement of code for 0, 8 for 1, 7 for 2 and so on.
• For a code to be self complementing, the sum of all its weights
must be 9. digit.8421 and 5421 codes are not self complementing
codes whereas 5211,2421,3321, 4321 are self complementing.
• In general, a code is self-complementary if we produce a code by
taking the first complement of the digit which is same as 9’s
complement of the number.
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• Cyclic codes: Cyclic codes are those in which each successive
code word differs from the preceding one in only one bit
position.
• They are also called unit distance codes
• Example: gray code
Reflective Code:
Example : Graycode.
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Alphanumeric Codes • Apartfromnumericdata, acomputersystemmayprocess
somealphanumericdatajustliketheemployees’ names,
address as well as some special characters. An Alphanumeric
datagenerallyconsistofsequenceofcharacterswherea
character is any one of the following:-
• Letters or alphabets
• Digits 0-9
• Special characters(+,-,π)
• In thecomputersystem, eachcharacterisstoredinsome
code form depending upon the coding scheme. The
charactermaytake6, 7, or8 bits.Therearenumberofcodes
which are used for some specific application i.e. ASCII Code,
EBCDIC Code, UNIT Code, etc.
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VARIOUS DECIMAL CODES
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Importance ofCodes
1. The code refers to encryption system.
2.Computer and other digital circuits process data in
binary format.
3. Various binary codes are used to represent data.
4. The interpretation of data is only possible if the
code in which the information is available is known.
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Binary Coded Decimal (BCD)
• Wouldit be easyfor youifyoucan replace a decimal number with an individual binary code?
– Such as 0001 1001 = 1910
• The 8421 code is a type of BCD to do that.
• BCD code provides an excellent interface to binary systems:
– Keypad inputs
– Digital readouts
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Binary Coded Decimal (BCD)
– Used to represent the decimal digits 0 - 9.
– 4 bits areused.
– Eachbit positionhasaweight associated with it (weightedcode).
– Weights are: 8, 4, 2, and 1 from MSB to LSB (called 8-4-2-1 code).
– BCD Codes:
0: 0000 4: 0100 8: 1000
1: 0001 5: 0101 9: 1001
2: 0010 6: 0110
3: 0011 7: 0111
– Used to encode numbers for output to numerical displays
– Used in processors that perform decimal arithmetic.
– Example: (9750)10 = (1001 0111 0101 0000)BCD
9 7 5 0
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Binary CodedDecimal
Decimal
Digit 0 1 2 3 4 5 6 7 8 9
BCD 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
Note: 1010, 1011, 1100, 1101, 1110, and 1111 are INVALID CODE !
ex1: Decimal to BCD
(a) 35
(b) 98
(c) 170
(d) 2469
ex2: BCD-to-Decimal
(a) 10000110
(b) 001101010001
(c) 1001010001110000
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BCD Addition • BCD is a numerical code and can be used in
arithmetic operations. Here is how to add two BCD
numbers:
– Addthe two BCD numbers, using the rules for basic binary
addition.
– Ifa 4-bit sum is equalto or less than 9, it is avalid BCD
number.
– If a 4-bit sum > 9, or if a carry out of the 4 -bit group is
generatedit isaninvalidresult.Add6(0110)toa4-bitsum
in order to skip the sixinvalid states and return the code to
8421. If a carry results when 6 is added, simply add the
carry to the next 4 -bit group.
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BCD Subtraction
• Here is how to subtract two BCD numbers:
– Subtract the two BCD numbers, usingthe rulesfor basic
binary subtraction.
– If there is no borrowfrom the nexthigher group, no
correction is required.
– If there isborrowfromthe nextgroup,then (0110)is
subtracted from the difference term of this group.
– Example 38 –15
38 0011 1000 (38 in BCD)
15 0001 0101 (15 in BCD)
–
– 0010 0011 (No borrow, so it is a correct answer) 16
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The Gray Code
• The Gray code is unweighted and is not an
arithmetic code.
– There are no specific weights assigned to the bit
positions. 18
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The Gray Code
• Binary-to-Gray code conversion
– The MSB in the Graycode is the same as
corresponding MSB in the binary number.
– Going from left to right, add each adjacent pair of
binary code bits to get the next Gray code bit.
Discard carries.
ex: convert 101102 to Gray code
1 + 0 + 1 + 1 + 0 binary
1
1
1
0
1 Gray
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The Gray Code
• Gray-to-Binary Conversion
– The MSB in the binary code is the same as the
corresponding bit in the Gray code.
– Add each binary code bit generated to the Gray code bit in
the next adjacent position. Discard carries.
ex: convert the Gray code word 11011 to binary
Gray Binary
1 1 0 1 1
+ + + +
1 0 0 1 0
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The Gray Code
21
Decimal Binary Gray Code
8 1000 1100
9 1001 1101
10 1010 1111
11 1011 1110
12 1100 1010
13 1101 1011
14 1110 1001
15 1111 1000
Decima l
Binary Gray Code
0 0000 0000
1 0001 0001
2 0010 0011
3 0011 0010
4 0100 0110
5 0101 0111
6 0110 0101
7 0111 0100
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Excess 3 Code
• A BCD Code formed by adding 3 (0011)
to its true four bit binary value.
• Excess 3 is a self complementing code. If
the bits of the Excess-3 digit are inverted,
they yield the 9’s complement of the
decimal equivalent.
• Excess-3 code is useful for performing
decimal arithmetic digitally.
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Excess 3
Examples
• 3 = 0011 + 0011 = 0110 = 6 in E-3.
• 1 = 0001 + 0011 = 0100 = 4 in E-3
• If we complement 1’s = 1011 in E-3 this
is the code for an 8.
• 9s Complement of 1(0100) = (9 - 1) = 8
Self Complement
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Alphanumeric Codes
• Represent numbers and alphabetic characters.
– Also represent othercharacters such as symbols and various instructions necessary for conveying
information.
• TheASCIIisthe most common alphanumeric
code.
– ASCII = American Standard Code for Information
Interchange
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ASCII
• ASCII has 128 characters and symbols
represented bya 7-bit binary code.
– It can be considered an 8-bit code with the MSB
always 0. (00h-7Fh)
• 00h-1Fh (the first 32) – control characters
• 20h-7Fh – graphics symbols (can be printed or
displayed)
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