Logic Design
‫التصميم المنطقي‬
Second Course
Syllabus
Combinational Logic
•
•
The NAND Gate as a Universal Logic Element.
The NOR Gate as a Universal Logic Element.
• Bit Parallel Adder.
• Decoders.
• Encoders.
• Multiplexers.
• De-multiplexers
Flip-Flop
• SR Flip-Flops.
• D Flip-Flops.
• JK Flip-Flops.
Shift Register
- Serial in \ Serial out shift Register
Binary Counter
- Asynchronous Binary Counter.
- Synchronous Binary Counter.
References
1-Computer System Architecture Third
Edition
M. Morris Mano
2- Digital Fundamentals Eight Edition
FLOYD
3- Digital Fundamentals Ninth Edition
FLOYD
4-Fundamentals of Digital Logic and
Microcomputer Design Fifth edition
M.RAFIQZZAMAN
Introduction
• We have learned all the prerequisite material:
– Truth tables and Boolean expressions describe functions
– Expressions can be converted into hardware circuits
– Boolean algebra and K-maps help simplify expressions and circuits
• Now, let us put all of these foundations to good use, to analyze and design
some larger circuits
• Logic circuits for digital systems may be
• A combinational circuit consists of logic gates whose outputs at any time
are determined by the current input values, i.e., it has no memory elements
• A sequential circuit consists of logic gates whose outputs at any time
are determined by the current input values as well as the past input
values, i.e., it has memory elements.
• Each input and output variable is a binary variable
• 2^n possible binary input combinations
• One possible binary value at the output for each input combination
• A truth table or m Boolean functions can be used to specify input-output relation
A combinational circuit consists of :
1- Input variables.
2- Logic gates
3- Output variables
Logic gates accepts signals ( Binary signals) from inputs and generate signals to the
outputs.
an example that converts binary coded decimal (BCD) to the excess-3 code
for the decimal digits.
The bit combinations assigned to the BCD and excess-3 codes are listed in
Table. Since each code uses four bits to represent a decimal digit, there mustbe
four input variables and four output variables.
z=D
y = CD + CD = CD + 1C + D2
x = BC + BD + BCD = B1C + D2 + BCD
= B1C + D2 + B1C + D2
w = A + BC + BD = A + B1C + D2
Half Adder
this circuit needs two binary inputs and two binary outputs. The input variables
designate the augend and addend bits; the output variables produce the sum and carry.
We assign symbols x and y to the two inputs and S (for sum) and C (for carry) to the
outputs. The truth table for the half adder is listed in Table. The C output is 1 only when
both inputs are 1. The simplified Boolean functions for the two outputs can be obtained
directly from the truth table. The simplified sum-of-products expressions are
S = xy + xy
C = xy
The logic diagram of the half adder implemented in sum of products is shown in Fig . It
can be also implemented with an exclusive-OR and an AND gate as shown in Fig . This
form is used to show that two half adders can be used to construct a full adder.
Full adder
A full adder is a combinational circuit that forms the arithmetic sum of three bits. It
consists of three inputs and two outputs. Two of the input variables, denoted by x and y ,
represent the two significant bits to be added. The third input, z , represents the carry
from the previous lower significant position. Two outputs are necessary because the
arithmetic sum of three binary digits ranges in value from 0 to 3, and binary
representation of 2 or 3 needs two bits. The two outputs are designated by the symbols S
for sum and C for carry.
S = xyz + xyz + xyz + xyz
C = xy + xz + yz