Digital Logic
Digital systems
A
digital system is a system whose
inputs and outputs fall within a
discrete, finite set of values
 Two main types

Combinational


Sequential


Outputs dependent only on current
input
Outputs dependent on both past and
present inputs
Combinational Logic
Aims

To express the inputs and
outputs of a system in binary
form
To develop the relationships
between these inputs and
outputs as a truth table
 To simplify the Boolean
expression using algebra or
Karnaugh maps
To select suitable electronic devices to
implement the required function

Example

Consider a buzzer which sounds
when :
The lights are on and
 The door is open and
 No key is in the ignition

Active
Alarm system
B
A
P
A
B
1
Variabl
Value Situatio
1
Lights are
0
Lights are off
1
Door is
0
Door is
Key is 0in ignition
Key is out of ignition
 Truth

C
Table
A Truth Table can be used to show the
relationships between :


the 3 inputs and
the single output
 Implementation
as a circuit using logic
gates.
0
0
1
0
P
1
Buzzer is on
0
Buzzer is off
0
0
0
0
A
B
P
C

*
Circuits with one input

Buffer P = A

Not
P=A
Buffer can be used to drive a device requiring more power than available, also for
delays and isolation of components
 Circuits with two Inputs
AND
P = A.B
OR
P=A+B
0
1
0
XOR
A
B
A
B
0
P = A ɵB
P
P