Chapter 3 Gate-Level
Minimization
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

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The Boolean functions also can be simplified by
map method as Karnaugh map or K-map.
The map is made up of squares, with each square
representing one minterm of the function.
This produces a circuit diagram with a minimum
number of gates and the minimum number of
inputs to the gate.
It is sometimes possible to find two or more
expressions that satisfy the minimization criteria.
1
Two-Variable map
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
Two-variable has four minterms, and consists of
four squares.
m1 + m2 + m3 = x’y + xy’ + xy = x + y
2
Three-Variable map
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Note that the minterms are not arranged in a binary
sequence, but similar to the Gray code.
For simplifying Boolean functions, we must recognize the
basic property possessed by adjacent squares.
m5+m7= xy’z + xyz = xz(y’ + y) = xz
cancel
y
3
Simplification of the number of
adjacent squares

A larger number of adjacent squares are
combined, we obtain a product term with fewer
literals.
1 square = 1 minterm = three literals.
2 adjacent squares = 1 term = two literals.
4 adjacent squares = 1 term = one literal.
8 adjacent squares encompass the entire map and
produce a function that is always equal to 1.

It is obviously to know the number of adjacent squares is
combined in a power of two such as 1,2,4, and 8.
4
Example
Ex. 3-3 F(x, y, z) = ∑(0, 2, 4, 5, 6)
F = z’ + xy’
5
3-2. Four-variable map
1 square = 1 minterm = 4 literals
2 adjacent squares = 1 term = 3 literals
4 adjacent squares = 1 term = 2 literals
8 adjacent squares = 1 term = 1 literal
16 adjacent squares = 1
6
Example
Ex. 3-6 F = A’B’C’ + B’CD’ + A’BCD’ + AB’C’
= B’D’ + B’C’ + A’CD’
7
Essential prime implicants
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If a minterm in a square is
covered by only one prime
implicant, that the prime
implicant is said to be
essential.
8
Prime implicant
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
A prime implicant is a product
term obtained by combining the
maximum possible number of
adjacent squares in the map.
This shows all possible ways that
the three minterms(m3,m9,m11)
can be covered with prime
implicants.
F = BD+B’D’+CD+AD
= BD+B’D’+CD+AB’
= BD+B’D’+B’C+AD
= BD+B’D’+B’C+AB’
9
3-3. Five-variable map
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Fig.3-12, the left-hand four-variable map represents the 16 squares
where A=0, and the other four-variable map represents the squares
where A=1.
In addition, each square in the A=0 map is adjacent to the
corresponding square in the A=1 map.
10
Five-variable map

It is possible to show that any 2k adjacent squares, for
k=(0,1,2,…,n) in an n-variable map, will represent an area
that gives a term of n−k literals(n>k). When n=k, it is
identity function.
11
example
Ex. 3-7 F(A, B, C, D, E) = (0, 2, 4, 6, 9, 13, 21, 23, 25, 29, 31)
Because of both parts of the map have the common term (A’BD’E+ABD’E)
so the sum of products is
F = A’B’E’ + BD’E + ACE
common
12
3-4. Product of sums
simplification

If we mark the empty squares by 0’s rather than
1’s and combine them into valid adjacent squares,
we obtain the complement of the function, F’. Use
the DeMorgan’s theorem, we can get the product
of sums.
Ex.3-8 Simplify the Boolean function in
(a) sum of products
(b) product of sums
F(A, B, C, D) = ∑(0, 1, 2, 5, 8, 9, 10)
13
Example
(a) SOPs
F= B’D’+ B’C’ + A’C’D
(b) POSs
F’= AB + CD + BD’
By DeMorgan’s thm
F= (A’+B’) .(C’+D’)
.(B’+D)
14
Gate implementation
15
Exchange minterm and maxterm
Consider the truth table
that defines the function F
in Table 3-2.
Sum of minterms
F(x, y, z) = ∑(1, 3, 4, 6)
Product of maxterms
F(x, y, z) = ∏(0, 2, 5, 7)
 In the other words, the 1’s
of the function represent
the minterms, and the 0’s
represent the maxterms.

16
3-5. Don’t care conditions
Ex.3-9 Simplify the F (w, x, y, z)= ∑(1, 3, 7, 11, 15) with
don’t-care conditions d(w, x, y, z) = ∑(0, 2, 5)
In part (a) with minterms 0 and 2 F = yz + w’x’
In part (b) with minterm 5
 F = yz + w’z
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3-6. NAND and NOR
implementation
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NAND gate is a universal gate because any digital system
can be implemented with it.
NAND gate can be used to express the basic gates, NOT,
AND, and OR.
18
Two graphic symbols for NAND
gate

In part (b), we can place a bubble (NOT) in each
input and apply the DeMorgan’s theorem, then get
a Boolean function in NAND type.
19
Two-level implementation
F = AB + CD
=
Double
complementation,
so can be removed
OR
gate,Fig.318
20
Multilevel NAND circuits
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1.
2.
3.
To convert a multilevel AND-OR diagram into an
all-NAND diagram using mixed notation is as
follows:
Convert all AND gates to NAND gates with AND-invert
graphic symbols.
Convert all OR gates to NAND gates with invert-OR
graphic symbols.
Check all the bubbles in the diagram. For every bubble
that is not compensated by another small circle along the
same line, insert an inverter or complement the input
literal.
21
Multilevel NAND circuits
,
,
22
NOR implementation
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The NOR operation is the dual of the NAND operation, all
procedures and rules for NOR logic are the dual of NAND
logic.
NOR gate is also a universal gate.
23
Two graphic symbols for NOR
gate

In part (b), we can place a bubble (NOT) in each
input and apply the DeMorgan’s theorem, then get
a Boolean function in NOR type.
24
Implementing F with NOR gates
F = (AB’ + A’B)(C + D’)
 To compensate for the bubbles in four inputs, it is
necessary to complement the corresponding input
literals.
25
3-7. Other two-level
implementations
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Some NAND or NOR
gates allow the
possibility of a wire
connection between the
outputs of two gates to
provide a wired logic.
Open-collector TTL
NAND gates, when tied
together, perform the
wired-AND logic (Fig.328).
The wired-AND gate is
not a physical gate.
Wired-And
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Nondegenerate forms
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We consider four types of gates: AND, OR, NAND,
and NOR. These will have 16 combinations of twolevel forms.
Eight of these combinations are said to be
degenerate forms, because they degenerate to a
single operation.
The other eight nondegenerate forms produce an
SOPs or POSs as follows:
AND-OR  3-4
NAND-NAND  3-6
NOR-OR
OR-NAND
OR-AND  3-4
NOR-NOR  3-6
NAND-AND
AND-NOR
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AND-OR-INVERT implementation

The two forms NAND-AND and AND-NOR are equivalent
forms and can be treated together.
F = (AB + CD + E)’
Shift back
28
OR-AND-INVERT implementation
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The OR-NAND form resembles the OR-AND form, except for
the inversion done by the bubble in the NAND gate.
F = [(A + B)(C + D)E]’
Shift back
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Tabular summary and example

Because of the INVERT part in each case, it is
convenient to use the simplification of F’ of the
function.
30
Example
Ex.3-11 Implement the function of Fig.3-31(a) with the four
two-level forms listed in Table 3-3.
The complement of the function by combining the 0’s:
F’ = x’y + xy’ + z
The normal output for this function
F = (x’y + x’y + z)’
Which is in the AND-OR-INVERT form.
31
Example

The AND-NOR and NAND-AND implementations are
shown as follows.
32
Example
The OR-AND-INVERT forms require a simplified expression
of the complement of the function in POSs.
Combine the 1’s in the map
F = x’y’z’ + xyz’
Complement of the function
F’ = (x + y + z)(x’ + y’ + z)
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33
Example
The normal output F
F = (x + y + z)(x’ + y’ + z)]’
We can implement the function in the OR-NAND and NOR-OR
forms as follows.
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3-8. Exclusive-OR function
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The XOR symbol denote as ⊕, the Boolean
operation: x ⊕ y = xy’ + x’y
The X-NOR symbol denote as ⊙, the Boolean
operation: x ⊙ y = (x ⊕ y )’ = xy + x’y’
The identities of the XOR operation:
x⊕0=x
x ⊕ x’ = 1

x ⊕ 1 = x’ x ⊕ x = 0
x ⊕ y’ = x’ ⊕ y = (x ⊕ y)’
Commutative and associative:
A⊕B=B⊕A
(A ⊕ B) ⊕ C = A ⊕ (B ⊕ C) = A ⊕ B ⊕ C
35
Exclusive-OR implementations
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Fig.3-32(b), the first NAND gate performs the operation (xy)’ = (x’ + y’).
(x’ + y’)x + (x’ + y’)y = xy’ + x’y = x ⊕ y
36
Odd function
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Boolean expression of three-variable of the XOR:
A ⊕ B ⊕ C = (AB’ + A’B)C’ + (AB + A’B’)C
= AB’C’ + A’BC’ + ABC + A’B’C
=∑(1, 2, 4, 7)
Odd function defined as the logical sum of the 2n/2 minterms
whose binary numerical values have an odd number of 1’s.
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Odd and Even functions
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The 3-input odd function is implemented by means of 2input exclusive-OR gates.
38
Parity generation and checking
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Table 3-4, the P make the total number of 1’s
even(including P). P constitutes an odd function.
A parity bit is an extra bit included with a binary message to
make the number of 1’s either odd or even.
P=x ⊕ y ⊕ z
39
Parity generation and checking
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The circuit that generates the parity bit in the
transmitter(receiver) is called a parity generator(checker).
Transmitter
Receiver
40
Example of even parity
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Since the information was
transmitted with even parity,
the four bits received must
have an even number of 1’s.
An error occurs during the
transmission if the four bits
received have an odd number
of 1’s.
Error
The output of the parity checker,
denoted by C, will be equal to 1
if an error occurs, that is, if the
four bits received have an odd
number of 1’s.
C=x⊕y⊕z⊕P
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3-9. Hardware Description
Language (HDL)
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1.
2.
It resembles a programming language, but is
specifically oriented to describing hardware
structures and behavior.
There are two applications of HDL processing:
Logic simulation: allows the detection of functional errors
in a design without having to physically create the circuit.
Logic synthesis: the process of deriving a list of
components and their interconnections (called a netlist)
from the model of a digital system described in HDL.
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Module representation

Identifiers must start with an alphabetic character
or an underscore.
HDL Example 3-1
//Description of simple circuit Fig. 3-37
Module smpl_circuit(A,B,C,x,y);
input A,B,C;
output x,y;
wire e;
and g1(e,A,B);
not g2(y,C);
or g3(x,e,y);
endmodule
keywords
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Gate delays

The delay is specified in terms of time units and
the symbol #.
‘timescale 1ns/100ps
The first number specifies the unit of measurement for time
delays. The second number specifies the precision for which
the delays are rounded-off.

If no timescale is specified, the simulator defaults
to a certain time unit, usually 1 ns.
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Gate delays
45
Stimulus and circuit description
modules
46
Boolean expressions

Boolean expressions are specified in Verilog HDL with a
continuous assignment statement consisting of the keyword
assign followed by a Boolean expression.
assign x = (A & B) ∣ ~ C ;
47
User-Defined primitives (UDP)
48
User-Defined primitives (UDP)
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1.
2.
With keywords and, or, etc…,are referred to as
system primitives. The user can create additional
primitives by defining them in a tabular form, socalled UDP.
UDP use the keyword “primitive” instead of
keyword “module”.
HDL proceeds according to the following rules:
It is declared with the keyword primitive followed by a
name and port list.
There can be only one output and it must be listed first in
the ort list and declared with an output keyword.
49
User-Defined primitives (UDP)
3.
4.
5.
6.
There can be any number of inputs. The order in which
they are listed in the input declaration must conform to
the order in which they are given values in the table that
follows.
The truth table is enclosed within the keywords table and
endtable.
The values of the inputs are listed in order ending with a
colon(:). The output is always the last entry in a row
followed by a semicolon(;).
It ends with the keyword endprimitive.
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