COMBINATIONAL AND SEQUENTIAL CIRCUITS,DESIGN AND ANALYSIS PROCEDURE OF COMBINATIONAL CIRCUITS,

2. DIGITAL LOGIC CIRCUITS
~ INTRODUCTION TO COMBINATIONAL AND
SEQUENTIAL CIRCUITS.
~DESIGN PROCEDURE OF COMBINATIONAL
CIRCUITS
~ANALYSIS PROCEDURE OF COMBINATIONAL
CIRCUITS
PRESENTED BY:
QURATULAIN
BS(CS)-1
QUAID-E-AZAM UNIVERSITY
ISLAMABAD ,PAKISTAN.

3. TYPES OF DIGITAL LOGIC
CIRCUITS
 COMBINATIONAL CIRCUITS
 SEQUENTIAL CIRCUITS

4. COMBINATIONAL CIRCUITS
 Combinational Circuits are made of logic
gates.
 Doesn’t contain memory element , that’s
why they cant store any information.
 Value of present output is determined by
present input.
 Examples of combinational circuits are
half adders, full adders, sub tractors etc.

5. BLOCK DIAGRAM OF A
COMBINATIONAL CIRCUIT

6. SEQUENTIAL LOGIC CIRCUITS
 Made up of combinational circuits and
memory elements.
 These memory elements are devices
capable of storing ONE-BIT information.
 Output depend on input and previous
state.
 Examples of sequential circuits are flip
flops, counters, shift registers

7. BLOCK DIAGRAM OF A
SEQUENTIAL CIRCUIT

8. TYPES OF SEQUENTIAL
CIRCUITS
Sequential circuits are of two types:
1.SYNCHRONOUS CIRCUITS:
 In synchronous sequential circuits, the state
of the device changes only at discrete times
in response to a clock Pulse.
2.ASYNCHRONOUS CIRCUITS:
 Asynchronous circuit is not synchronized by
a clock signal; the outputs of the circuit
change directly in response to changes in
Inputs.

9. DESIGN PROCEDURE OF
COMBINATIONAL CIRCUITS
This procedure involves the following
steps:
 The problem is stated.
 The number of available input
variables and output variables is
determined.
 The input and output variables are
assigned letter symbols.
 Truth table is drawn
 Boolean function for output is obtained.

10. ANALYSIS PROCEDURE OF
COMBINATIONAL CIRCUIT
 TO DETERMINE THE OUTPUT FUNCTIONS
AS ALGEBRAIC EXPRESSIONS.
 It is the reverse process of design procedure.
 Logic diagram of the circuit is given.
 Obtain the truth table from the diagram.
 Obtain Boolean function from the Truth Table
for output.

11. LOGIC DIAGRAM FOR
ANALYSIS EXAMPLE:

12. THE CIRCUIT HAS THREE
INPUTS, A,B,C
AND TWO OUTPUTS F1 AND F2
 The Boolean function for outputs are:
F2=AB+AC+BC
T1=A+B+C
T2=ABC
 Outputs functions for gates are :
T3=F2’T1
F1=T3+T2

13. Substituting and simplifying, we get :

14. TRUTH TABLE DRAWN FROM
THE LOGIC DIAGRAM.

15. BOOLEAN FUNCTIONS
OBTAINED FOR OUTPUT ARE :
 F2=AB+AC+BC
 F1=A’BC’+A’B’C+AB’C’+ABC