lecture5

Logic Gates & Boolean Algebra
Binary Logic
*
1
0
Logic Gates, truth Tables, and Boolean
Expressions
input
The truth table is
Logic equation
Symbol
OR
AND
Gate name is
NOT
input
output
output
input output
x
0
y
0
z
0
x
0
y
0
z
0
x
0
z
1
0
1
0
0
1
1
1
0
1
0
0
1
0
1
1
1
1
1
1
1
z=x•y=xy
x
y
z
z=x=x
z=x+y
x
y
z
x
z
NAND (Not AND)
input
x
0
0
1
1
input
output
y
0
1
0
1
x
y
• NOR (Not OR)
Z
1
1
1
0
x•y
x
0
0
1
1
output
y
0
1
0
1
x
y
Buffer
z
1
0
0
0
x+y
input
output
x
0
1
z
0
1
x
x
 XOR (Exclusive-OR)
input
x
y
xÅ y
xy+xy
 XNOR (Exclusive-NOR)
x
0
0
1
1
output
y
0
1
0
1
input
x
y
xÅ y
x y
xy+xy
x
0
0
1
1
Z
0
1
1
0
output
y
0
1
0
1
Z
1
0
0
1
Gates Equivalence
Equivalence between OR and bubbled
AND
Equivalence between NAND and OR
Equivalence between AND and bubbled
AND
OR
Equivalence between NOR and AND
A timing diagram for some gates 
X
0
0
1
1
Y
0
1
0
1
X ·Y
0
0
0
1
(OR)
X + Y
0
1
1
1
(NOT)
X
1
1
0
0
(AND)
Practical design
The Design of a house alarm to protect it from a thief
The thief breaks in through window 3 
Boolean Algebra rules
1.
X + 0= X
2. X . 1 = X
3.
X + 1= 1
4. X
5.
X+X=X
6. X . X = X
7.
X+X=1
8. X . X = 0
9.
X=X
10. X + Y = Y + X
12. (X + Y) + Z = X + (Y + Z)
14. X(Y + Z) = XY + XZ
16. X + Y = X . Y
18
x+(x•y)=x
.
Identity
0= 0
Inverse (complement)
Double Complement
11. XY = YX
13. (XY) Z = X(Y Z)
Commutative
Associative
15. X + YZ = (X + Y) (X + Z) Distributive
17. X . Y = X + Y
DeMorgan
19. x • ( x + y ) = x
Absorption
Use DeMorgan’s theorem
F (a, b, c, d , e)  a [b  c (d  e )]
a  [b  c (d  e )]
a  b (c (d  e ))
a  b (c  (d  e ))
a  b (c  (d e))
a  b (c  d e)
Simplify the following Boolean expressions to the minimum
number of literals
then approve it using the truth table
logic circuits
Re-draw the equivalent of the following circuit using only
bubbled OR gates
H.W
1-
2-
Reduce the following Boolean expressions to the indicated
number of literals:
(a) A’C’ + ABC + AC’
to three literals
(b) (x’y’ + z)’ + z + xy + wz
to three literals
(c) A’B (D’ + C’D) + B (A + A’CD)
to one literal
(d) (A’ + C) (A’ + C’) (A + B + C’D)
to four literals
Find the complement of F = x + yz; then show that
FF’ = 0 and F + F’ = 1
3-
Find the complement of the following expressions:
(a) xy’ + x’y
(b) (AB’ + C)D’ + E
(c) (x + y’ + z) (x’ + z’) (x + y)
4-
List the truth table of the function:
F = xy + xy’ + y’z
5-
Logical operations can be performed on strings of bits by
considering each pair of corresponding bits separately (this
is called bitwise operation). Given two 8-bit strings
A = 10101101 and B = 10001110, evaluate the 8-bit result
after the following logical operations: (a) AND, (b) OR, (c)
XOR, (d) NOT A, (e) NOT B.
6-Draw the logic diagrams for the following Boolean
expressions:
(b) Y = BC + AC’
(a) Y = A’B’ + B (A + C)
(d) Y = (A + B) (C’ + D)
(c) Y = A + CD
7-Simplify the Boolean function T1 and T2 to a minimum
number of literals.
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
T1
1
1
1
0
0
0
0
0
T2
0
0
0
1
1
1
1
1
8-Given the Boolean function
F = xy’z + x’y’z + w’xy + wx’y + wxy
(a) Obtain the truth table of the function.
(b) Draw the logic diagram using the original Boolean
expression.
(c) Simplify the function to a minimum number of literals
using Boolean algebra.
(d) Obtain the truth table of the function from the
simplified expression and show that it is the same as
the one in part (a)
(e) Draw the logic diagram from the simplified expression
and compare the total number of gates with the
diagram of part (b).

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