INTRODUCTION
A
seven-segment display (SSD) is a vital form of electronic display device for
displaying decimal numerals. It is an alternative to the more complex dot
matrix displays. Today we often see Seven-Segment displays are widely used in
digital clocks, electronic meters and other electronic displays units.
Basically, SSD could be used to display single decimal digit i.e. from
zero to nine. It is generally driven by a binary coded decimal nibble. Thus in
order to find out how it operates, let us investigate the logic behind its input
and output signals.
CONTENTS
I.
Building
the truth table for SSD
II.
Boolean
expression for the logic function
III.
Simplification
using K-Map
IV.
The
circuit diagram of SSD
Building the truth table
- The seven segments are referred to by the letters A to G as shown below.
- Thus, in order to display a digit, we need to turn on all or some of the 7 segments of the SSD display by putting “1” to the appropriate SSD pins. In displaying decimal digits, due to the necessity of 10 distinct combinations of the 7 segments, the no. of binary inputs should be greater than or equal to 4. (Note: if “n” amount of binary inputs are given, maximum amount of combinations that could be obtained is 2n). Hence we supply 4 input signals to obtain 7 output signals, which is a practical application of a Decoder.
- Let the input signals be P, Q, R and S. Then the truth table for displaying digits could be given as follows.
Boolean Expressions
- Using the above truth table, we could express the Boolean expression for each output signal A to G as given below. Each expression is given as the sum of the combinations of input signals when a particular segment is 1.
In filling
K-Maps, considering a particular segment, all “1”s for P,Q,R & S are put
down from all the combinations. Then zeros are filled for all the other valid
BCD inputs.
After completion of a K-Map and looping the largest possible areas, simplified
Boolean expression could be given. It could be further minimized using Boolean
Algebra.
