Computer Engineering 1 (ECE290)
Chapter 2 – Combinational Logic CircuitsLogic Circuits
Part 1 – Gate Circuits and Boolean Equations
HOANG TrangReference: © 2008 Pearson Education, Inc.
Overview Part 1 – Gate Circuits and Boolean Equations
• Binary Logic and GatesBinary Logic and Gates• Boolean Algebra• Standard Forms
Part 2 Circuit Optimization Part 2 – Circuit Optimization• Two-Level Optimization• Map Manipulation
P ti l O ti i ti (E )• Practical Optimization (Espresso)• Multi-Level Circuit Optimization
Part 3 – Additional Gates and Circuits• Other Gate Types• Exclusive-OR Operator and Gates• High-Impedance Outputs
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 2
Binary Logic and GatesBinary variables take on one of two values.(0,1)L i l t t bi l dLogical operators operate on binary values and
binary variables.Basic logical operators are the logic functionsg p g
AND, OR and NOT. (any function: from 3 basic function)Logic gates (electronic circuit ) implement logicLogic gates (electronic circuit ) implement logic
functions.Boolean Algebra: a useful mathematical system
f if i d f i l i f ifor specifying and transforming logic functions.We study Boolean algebra as a foundation for
designing and analyzing digital systems!
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 3
designing and analyzing digital systems!
Binary Variablesy Recall that the two binary values have
different names:different names:• True/False• On/Off• On/Off• Yes/No• 1/0• 1/0
We use 1 and 0 to denote the two values. Variable identifier examples: Variable identifier examples:
• A, B, y, z, or X1 for now• RESET START IT or ADD1 later
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 4
• RESET, START_IT, or ADD1 later
Logical Operationsg p The three basic logical operations are:
• AND • OR• NOT
AND is denoted by a dot (·), (^). y ( ), ( ) OR is denoted by a plus (+), (v). NOT is denoted by an overbar ( ¯ ) a NOT is denoted by an overbar ( ), a
single quote mark (') after, or (~) before the variable
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 5
the variable.
Notation Examples
Examples:• is read “Y is equal to A AND B.”Or Y=AB• is read “z is equal to x OR y ”
BAY yxz
• is read z is equal to x OR y.• is read “X is equal to NOT A.”
Note: The statement:AX
Note: The statement: 1 + 1 = 2 (read “one plus one equals two”)1 + 1 = 10B (2) ??? Binary (this is: binary1 + 1 = 10B (2) ??? Binary (this is: binary operation
is not the same as
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 6
is not the same as1 + 1 = 1 (read “1 or 1 equals 1”). (logic)
Operator Definitions
Operations are defined on the values "0" and "1" for each operator:
AND OR NOT AND0 · 0 = 0
OR
0 + 0 = 0NOT
10 0 · 1 = 01 · 0 = 0
0 + 1 = 11 + 0 = 1
01
1 · 1 = 1 1 + 1 = 1
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 7
Truth Tables
Truth table a tabular listing of the values of a function for all possible combinations of values on itsfunction for all possible combinations of values on its arguments
Example: Truth tables for the basic logic operations:
XNOT
XZ
p g p
Z = X·YYXAND OR
X Y Z = X+Y
0110
X XZ
010000
Z = X·YYX X Y Z = X+Y0 0 00 1 1 01
111001010 0 1 1
1 0 11 1 1
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 8
111 1 1 1
Logic Function Implementation
Using SwitchesF i t
Switches in parallel => OR
• For inputs: logic 1 is switch closed logic 0 is switch openlogic 0 is switch open
• For outputs: logic 1 is light on
Switches in series => ANDg g
logic 0 is light off.• NOT uses a switch such
N ll l d it h NOTthat: logic 1 is switch open logic 0 is switch closed
CNormally-closed switch => NOT
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 9
logic 0 is switch closedFor IC
implementation
Logic Function Implementation (Continued)
Example: Logic Using SwitchesB C
AC
Light is on (L = 1) for
D
Light is on (L = 1) for L(A, B, C, D) =
and off (L = 0) otherwiseand off (L = 0), otherwise. Useful model for relay circuits and for CMOS
gate circuits, the foundation of current digital
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 10
g , glogic technology
Logic Gates
In the earliest computers, switches were opened and closed by magnetic fields produced byand closed by magnetic fields produced by energizing coils in relays. The switches in turn opened and closed the current paths.opened and closed the current paths.
Later, vacuum tubes that open and close current paths electronically replaced relays.p y p y
Today, transistors are used as electronic switches that open and close current paths.p p
Optional: Chapter 6 – Part 1: The Design Space
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 11
Logic Gate Symbols and Behavior
XZ 5 X 1 Y
XZ 5 X · Y X Z 5 X
Logic gates have special symbols:
(a) Graphic symbols
OR gateY
Z 5 X 1 YY
Z 5 X Y
AND gate
X Z 5 X
NOT gate orinverter
X 0 0 1 1
(a) Graphic symbols And waveform behavior in time as follows:
Y 0 1 0 1
X · Y(AND) 0 0 0 1X Y(AND) 0 0 0 1
X 1 Y(OR) 0 1 1 1
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 12(b) Timing diagram
(NOT) X 1 1 0 0
Gate Delay
In actual physical gates, if one or more input h th t t t h th t tchanges causes the output to change, the output
change does not occur instantaneously.Th d l b t i t h ( ) d th The delay between an input change(s) and the resulting output change is the gate delaydenoted by tG:denoted by tG:
tInput 0
1
t 0 3tG tGOutput
0
01
tG = 0.3 ns
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 13Time (ns)0 0.5 1 1.5
Logic Diagrams and ExpressionsEquationTruth Table
X Y Z ZYX F
Logic Diagram
ZYX F
00 1 010 0 100 0 0
X
Y F
g g
11 0 111 0 000 1 100 1 0
Y
Z11 1 111 1 011 0 1
Boolean equations, truth tables and logic diagrams describe the same function!: 3 methods and same function
Truth tables are unique; expressions and logic diagrams are
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 14
q ; p g gnot. This gives flexibility in implementing functions.
Boolean Algebrag An algebraic structure defined on a set of at least two elements, B,
together with three binary operators (denoted +, · and ) that
1. 2. X . 1 X=
g y psatisfies the following basic identities:
X + 0 X=
3.5.7.
4.6.8.
X . 0 0=X . X X=
0=X . X
+X 1 1=X + X X=
1=X + X7.9.
11. Commutative
8. 0X X
10. X + Y Y + X= XY YX=
1X XX = X
13.15.17.
AssociativeDistributiveDeMorgan’s
12.14.16.
(X + Y) Z+ X + (Y Z)+=X(Y + Z) XY XZ+=X + Y X . Y=
(XY) Z X(YZ)=X + YZ (X + Y) (X + Z)=X . Y X + Y=
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 15
17. DeMorgans16. X + Y X Y X Y X + YAND -> OR, OR-> AND)
Some Properties of Identities & the Algebra
i i i i i i i
If the meaning is unambiguous, we leave out the symbol “·”
The identities above are organized into pairs. These pairs have names as follows:
1-4 Existence of 0 and 1 5-6 Idempotence1 4 Existence of 0 and 1 5 6 Idempotence7-8 Existence of complement 9 Involution
10-11 Commutative Laws 12-13 Associative Laws14-15 Distributive Laws 16-17 DeMorgan’s Laws
The dual of an algebraic expression is obtained by interchanging + and · and interchanging 0’s and 1’s.
The identities appear in dual pairs. When there is only one identity on a line the identity is self-dual i e the
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 16
one identity on a line the identity is self dual, i. e., the dual expression = the original expression.
Some Properties of Identities & the Algebra (Continued)
Unless it happens to be self dual the dual of an
(Continued)
Unless it happens to be self-dual, the dual of an expression does not equal the expression itself.
Example: F = (A + C) · B + 0 Example: F = (A + C) · B + 0dual F = (A · C + B) · 1 = A · C + B
Example: G = X Y + (W + Z) Example: G = X · Y + (W + Z)dual G =
E l H A B + A C + B C Example: H = A · B + A · C + B · Cdual H =
A f h f i lf d l?
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 17
Are any of these functions self-dual?
Some Properties of Identities & the Algebra(Continued) There can be more that 2 elements in B, i. e.,
elements other than 1 and 0 What are some
(Continued)
elements other than 1 and 0. What are some common useful Boolean algebras with more than 2 elements?
1.2.
Algebra of SetsAlgebra of n-bit binary vectors
If B contains only 1 and 0, then B is called the switching algebra which is the algebra we use
t ftmost often.
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 18
Boolean Operator Precedence The order of evaluation in a Boolean
expression is:1. Parentheses2 NOT2. NOT3. AND4 OR4. OR
Consequence: Parentheses appeard OR iaround OR expressions
Example: F = A(B + C)(C + D)
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 19
Example 1: Boolean Algebraic Proof
A + A·B = A (Absorption Theorem)Proof Steps Justification (identity or theorem)Proof Steps Justification (identity or theorem)
A + A·B= A · 1 + A · B X = X · 1= A · ( 1 + B) X · Y + X · Z = X ·(Y + Z)(Distributive Law)
= A · 1 1 + X = 1= A X · 1 = X
Our primary reason for doing proofs is to learn:p y g p• Careful and efficient use of the identities and theorems of
Boolean algebra, and• How to choose the appropriate identity or theorem to apply
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 20
pp p y pp yto make forward progress, irrespective of the application.
Example 2: Boolean Algebraic Proofs
AB + AC + BC = AB + AC (Consensus Theorem)Proof Steps Justification (identity or theorem)Proof Steps Justification (identity or theorem)
AB + AC + BC= AB + AC + 1 · BC ?= AB + AC + 1 · BC ? = AB +AC + (A + A) · BC ?=
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 21
Example 3: Boolean Algebraic Proofs
Proof Steps Justification (identity or theorem)
)ZX(XZ)YX( Y YProof Steps Justification (identity or theorem)
=YXZ)YX(
=
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 22
Useful Theorems
ninimizatioM yyyxyyyx x x
tionSimplificayxyxyxyx
Absorption xyxxxyxx
x x
yyyyyy
tionSimplificayxyxyxyx
Consensuszyxzyzyx
x xx x
x yy
zyxzyzyx
LawssDeMorgan'xx
x x
y x yyy gy y
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 23
Proof of Simplification
yyyxyyyx x x yyyyyy
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 24
Proof of DeMorgan’s Laws
yx x y yx yx yx x y yx yx
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 25
Boolean Function Evaluation
x y z F1 F2 F3 F4 0 0 0 0 0xF2
xyF1 z
yz 0 0 0 0 00 0 1 0 1 0 1 0 0 0zxyxF4
xzyxzyxF3xF2
yzy
0 1 0 0 00 1 1 0 0 1 0 0 0 1
zxyxF4
1 0 0 0 11 0 1 0 1 1 1 0 1 11 1 0 1 11 1 1 0 1
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 26
Expression Simplification An application of Boolean algebra Simplify to contain the smallest number
of literals (complemented and uncomplemented variables):
DCBADCADBADCABA= AB + ABCD + A C D + A C D + A B D= AB + AB(CD) + A C (D + D) + A B D AB + AB(CD) + A C (D + D) + A B D= AB + A C + A B D = B(A + AD) +AC
(A ) A C iHOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 27
= B (A + D) + A C 5 literals
Complementing Functions
Use DeMorgan's Theorem to l t f ticomplement a function:
1. Interchange AND and OR operators2. Complement each constant value and
literal Example: Complement F =
F = (x + y + z)(x + y + z)x zyzyx
F = (x + y + z)(x + y + z) Example: Complement G = (a + bc)d + e
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 28
G =
Overview – Canonical Forms
What are Canonical Forms? Minterms and Maxterms Index Representation of Minterms and p
Maxterms Sum-of-Minterm (SOM) RepresentationsSum-of-Minterm (SOM) Representations Product-of-Maxterm (POM) Representations
R i f C l f F i Representation of Complements of Functions Conversions between Representations
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 29
Canonical Forms
It is useful to specify Boolean functions in p ya form that:• Allows comparison for equality.p q y• Has a correspondence to the truth tables
Canonical Forms in common usage:Canonical Forms in common usage:• Sum of Minterms (SOM)• Product of Maxterms (POM)• Product of Maxterms (POM)
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 30
Minterms
Minterms are AND terms with every variable present in either true or complemented formpresent in either true or complemented form.
Given that each binary variable may appear normal (e.g., x) or complemented (e.g., ), there x( g , ) p ( g , ),are 2n minterms for n variables.
Example: Two variables (X and Y)produce2 2 4 bi ti2 x 2 = 4 combinations:
(both normal)(X normal, Y complemented) (minterm)YX
XY(X normal, Y complemented) (minterm)(X complemented, Y normal)(both complemented)
YXYXYX
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 31
Thus there are four minterms of two variables.
Maxterms
Maxterms are OR terms with every variable in true or complemented formtrue or complemented form.
Given that each binary variable may appear normal (e.g., x) or complemented (e.g., x), there ( g , ) p ( g , ),are 2n maxterms for n variables.
Example: Two variables (X and Y) produce2 x 2 = 4 combinations:
(both normal)( l l d)
YX (x normal, y complemented)(x complemented, y normal)(b th l t d)
YX YX
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 32
(both complemented)YX
Maxterms and Minterms
Examples: Two variable minterms and maxterms.
Index Minterm Maxterm0 x y ( x + y1 x y x + y2 x y x + y3 x y x + y
The index above is important for describing which variables in the terms are true and
3 x y x + y
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 33
which are complemented.
Standard Order Minterms and maxterms are designated with a subscript The subscript is a number, corresponding to a binary
pattern The bits in the pattern represent the complemented or
normal state of each variable listed in a standard order. All variables will be present in a minterm or maxterm and
will be listed in the same order (usually alphabetically) E ample: For ariables a b c: Example: For variables a, b, c:
• Maxterms: (a + b + c), (a + b + c)• Terms: (b + a + c), a c b, and (c + b + a) are NOT in ( ), , ( )
standard order.• Minterms: a b c, a b c, a b c• Terms: (a + c) b c and (a + b) do not contain all
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 34
• Terms: (a + c), b c, and (a + b) do not contain all variables
Purpose of the Index
The index for the minterm or maxterm, d bi b i d texpressed as a binary number, is used to
determine whether the variable is shown in the true form or complemented form.true form or complemented form.
For Minterms:• “1” means the variable is “Not Complemented” and1 means the variable is Not Complemented and • “0” means the variable is “Complemented”.
For Maxterms:• “0” means the variable is “Not Complemented” and • “1” means the variable is “Complemented”.
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 35
Index Example in Three Variables
Example: (for three variables) Assume the variables are called X, Y, and Z. The standard order is X, then Y, then Z. The Index 0 (base 10) = 000 (base 2) for three
variables). All three variables are complemented f i t 0 ( ) d i blZYXfor minterm 0 ( ) and no variables are complemented for Maxterm 0 (X,Y,Z).
• Minterm 0 called m is
Z,Y,X
ZYX• Minterm 0, called m0 is .• Maxterm 0, called M0 is (X + Y + Z).• Minterm 6 ?
ZYX
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 36
• Maxterm 6 ?
Index Examples – Four VariablesIndex Binary Minterm Maxterm
i Pattern m Mi Pattern mi Mi
0 0000 1 0001
dcba dcba dcba ?1 0001
3 00115 0101
dcbadcba
dcba dcba
??
5 0101 7 0111
10 1010
dcba dcba dcba
dcba dcba ?
0 0 013 110115 1111
dcba dcbadbadcba dcba
?c
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 37
Minterm and Maxterm Relationship
Review: DeMorgan's Theoremandyxy·x yxyx and
Two-variable example: and
yxy·x yxyx
yxM2 yx·m2 Thus M2 is the complement of m2 and vice-versa.
Since DeMorgan's Theorem holds for n variables, the above holds for terms of n variables
y2 y2
the above holds for terms of n variables giving:
andi mM i ii Mm and Thus Mi is the complement of mi.
i mM i ii Mm
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 38
Function Tables for Both
Minterms of Maxterms of2 variables 2 variablesx y m0 m1 m2 m3 x y M0 M1 M2 M3
0 0 1 0 0 00 1 0 1 0 0
0 0 0 1 1 10 1 1 0 1 1
1 0 0 0 1 01 1 0 0 0 1
1 0 1 1 0 11 1 1 1 1 0
Each column in the maxterm function table is the complement of the column in the minterm function
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 39
ptable since Mi is the complement of mi.
Observations In the function tables:
• Each minterm has one and only one 1 present in the 2n terms ( i i f 1 ) All h i 0(a minimum of 1s). All other entries are 0.
• Each maxterm has one and only one 0 present in the 2n terms All other entries are 1 (a maximum of 1s).
W i l t f ti b "ORi " th We can implement any function by "ORing" the minterms corresponding to "1" entries in the function table. These are called the minterms of the function.
We can implement any function by "ANDing" the maxterms corresponding to "0" entries in the function table. These are called the maxterms of the function.
This gives us two canonical forms:• Sum of Minterms (SOM) (Sum of Product SOP)• Product of Maxterms (POM) (Product of Sum POS)
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 40
Product of Maxterms (POM) (Product of Sum POS)
for stating any Boolean function.
Minterm Function Example
Example: Find F1 = m1 + m4 + m7
x y z index m1 + m4 + m7 = F1
F1 = x y z + x y z + x y z
0 0 0 0 0 + 0 + 0 = 00 0 1 1 1 + 0 + 0 = 10 1 0 2 0 + 0 + 0 = 00 1 1 3 0 + 0 + 0 = 01 0 0 4 0 + 1 + 0 11 0 0 4 0 + 1 + 0 = 11 0 1 5 0 + 0 + 0 = 01 1 0 6 0 + 0 + 0 = 0
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 41
1 1 0 6 0 + 0 + 0 = 01 1 1 7 0 + 0 + 1 = 1
Minterm Function Example
F(A, B, C, D, E) = m2 + m9 + m17 + m23( , , , , ) 2 9 17 23
F(A, B, C, D, E) =
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 42
Maxterm Function Example Example: Implement F1 in maxterms:
F1 = M0 · M2 · M3 · M5 · M6F1 = M0 M2 M3 M5 M6
)zyz)·(xy·(xz)y(xF1 z)yx)·(zyx·( z)yx)·(zyx·(
x y z i M0 M2 M3 M5 M6 = F10 0 0 0 0 1 1 1 = 0 1 0 0 1 1 1 1 1 1 1 = 10 1 0 2 1 0 1 1 1 = 00 1 1 3 1 1 0 1 1 = 0
1 1 0 1 1
1 0 0 4 1 1 1 1 1 = 11 0 1 5 1 1 1 0 1 = 01 1 0 6 1 1 1 1 0 0
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 43
1 1 0 6 1 1 1 1 0 = 01 1 1 7 1
1 1 1 1 = 1
Maxterm Function Example
141183 MMMM)D,C,B,A(F
F(A, B,C,D) =141183)(
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 44
Canonical Sum of Minterms Any Boolean function can be expressed as a
Sum of MintermsSum of Minterms.• For the function table, the minterms used are the
terms corresponding to the 1's• For expressions, expand all terms first to explicitly
list all minterms. Do this by “ANDing” any term missing a variable v with a term ( ).v v
Example: Implement as a sum of minterms.
Fi t d t
yxxf
)(f First expand terms:Then distribute terms: Express as sum of minterms: f = m3 + m2 + m0
yx)yy(xf yxyxxyf
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 45
Express as sum of minterms: f m3 m2 m0
Another SOM Example
Example: Th th i bl A B d C hi h
CBAF There are three variables, A, B, and C which
we take to be the standard order. Expanding the terms with missing variables:Expanding the terms with missing variables:
Collect terms (removing all but one of duplicate terms):
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 46
Express as SOM:
Shorthand SOM Form
From the previous example, we started with:
We ended up with:CBAF
F = m1+m4+m5+m6+m7
This can be denoted in the formal shorthand:
Note that we explicitly show the standard )7,6,5,4,1()C,B,A(F m
variables in order and drop the “m” designators.
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 47
Canonical Product of Maxterms Any Boolean Function can be expressed as a Product of
Maxterms (POM).• For the function table, the maxterms used are the terms
corresponding to the 0's.• For an expression, expand all terms first to explicitly list all
maxterms Do this by first applying the second distributivemaxterms. Do this by first applying the second distributive law , “ORing” terms missing variable v with a term equal to and then applying the distributive law again.
Example: Convert to product of maxterms:
vv
Example: Convert to product of maxterms:
Apply the distributive law:yxx)z,y,x(f
)(1))(( Add missing variable z:
yx)y(x1)y)(xx(xyxx
zyx)zyx(zzyx
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 48
Express as POM: f = M2 · M3
zyx)zyx(zzyx
Another POM Example Convert to Product of Maxterms:
BACBCAC)Bf(A Use x + y z = (x+y)·(x+z) with ,
and to get:
BACBCAC)B,f(A,
Bz AyC),B(Ax C
d o ge :
Then use to get:
Bz)BCBC)(AACBC(Af
yxyxx g
and a second time to get:
yy)BCC)(AABCC(f
Rearrange to standard order,)BC)(AABC(f
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 49
to give f = M5 · M2C)B)(ACBA(f
Function Complements
The complement of a function expressed as a f i t i t t d b l ti thsum of minterms is constructed by selecting the
minterms missing in the sum-of-minterms canonical forms.canonical forms.
Alternatively, the complement of a function expressed by a Sum of Minterms form is simplyexpressed by a Sum of Minterms form is simply the Product of Maxterms with the same indices.
Example: Given )7,5,3,1()z,y,x(F mp )7,5,3,1(),y,( m
)6,4,2,0()z,y,x(F m)7,5,3,1()z,y,x(F M
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 50
)7,5,3,1()z,y,x(F
Conversion Between Forms
To convert between sum-of-minterms and product-of maxterms form (or vice versa) we follow theseof-maxterms form (or vice-versa) we follow these steps:• Find the function complement by swapping terms in theFind the function complement by swapping terms in the
list with terms not in the list.• Change from products to sums, or vice versa.
Example:Given F as before: Form the Complement:
)7,5,3,1()z,y,x(F m)6,4,2,0()z,y,x(F m
Then use the other form with the same indices – this forms the complement again, giving the other form
),,,(
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 51
of the original function: )6,4,2,0()z,y,x(F M
Standard Forms
Standard Sum-of-Products (SOP) form:ti itt OR f AND tequations are written as an OR of AND terms
Standard Product-of-Sums (POS) form:equations are written as an AND of OR termsequations are written as an AND of OR terms
Examples:• SOP: BCBACBA • SOP:• POS:
These “mixed” forms are neither SOP nor POS
BCBACBA C·)CB(A·B)(A
These mixed forms are neither SOP nor POS••
C)(AC)B(A B)(ACACBA
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 52
)(
Standard Sum-of-Products (SOP)
A sum of minterms form for n variables can be written down directly from a truthcan be written down directly from a truth table.• Implementation of this form is a two level• Implementation of this form is a two-level
network of gates such that:• The first level consists of n-input AND gates, e st eve co s sts o n put N gates,
and• The second level is a single OR gate (with
fewer than 2n inputs). This form often can be simplified so that
th di i it i i l
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 53
the corresponding circuit is simpler.
Standard Sum-of-Products (SOP) A Simplification Example:
( )
)76541(m)CBA(F Writing the minterm expression:
F = A B C + A B C + A B C + ABC + ABC
)7,6,5,4,1(m)C,B,A(F
F = A B C + A B C + A B C + ABC + ABC Simplifying:
F =F =
Simplified F contains 3 literals compared to 15 in
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 54
Simplified F contains 3 literals compared to 15 in minterm F
AND/OR Two-level Implementation of SOP Expressionof SOP Expression The two implementations for F are shown
b l it i it t hi h i i l !below – it is quite apparent which is simpler!ABC FA
ABC
C F
BC
F
A
ABC
A
ABC
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 55
BC
SOP and POS Observations
The previous examples show that:• Canonical Forms (Sum of minterms Product of• Canonical Forms (Sum-of-minterms, Product-of-
Maxterms), or other standard forms (SOP, POS) differ in complexityB l l b b d i l• Boolean algebra can be used to manipulate equations into simpler forms.
• Simpler equations lead to simpler two-level p q pimplementations
Questions:H tt i “ i l t” i ?• How can we attain a “simplest” expression?
• Is there only one minimum cost circuit?• The next part will deal with these issues.
HOANG Trang, Ref: Pearson Education, Inc. Chapter 2 - Part 1 56
e e p w de w ese ssues.